     [file version.txi not included]

This manual documents the SCM Scheme implementation.  SCM version SCMVERSION was released SCMDATE.  The most recent information about SCM can be found on SCM's WWW home page:

http://swiss.csail.mit.edu/~jaffer/SCM

Copyright (C) 1990-1999 Free Software Foundation

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the author.

Overview
********

Scm is a portable Scheme implementation written in C.  Scm provides a machine independent platform for [JACAL], a symbolic algebra system.

Features
========

  * Conforms to Revised^5 Report on the Algorithmic Language Scheme [R5RS] and the [IEEE] P1178 specification.

  * Support for [SICP], [R2RS], [R3RS], and [R5RS] scheme code.

  * Runs under Amiga, Atari-ST, MacOS, MS-DOS, OS/2, NOS/VE, Unicos, VMS, Unix and similar systems.  Supports ASCII and EBCDIC character sets.

  * Is fully documented in TeXinfo form, allowing documentation to be generated in info, TeX, html, nroff, and troff formats.

  * Supports inexact real and complex numbers, 30 bit immediate integers and large precision integers.

  * Many Common Lisp functions: logand, logor, logxor, lognot, ash, logcount, integer-length, bit-extract, defmacro, macroexpand, macroexpand1, gentemp, defvar, force-output, software-type, get-decoded-time, get-internal-run-time, get-internal-real-time, delete-file, rename-file, copy-tree, acons, and eval.

  * Char-code-limit, most-positive-fixnum, most-negative-fixnum, and internal-time-units-per-second constants.  *Features* and *load-pathname* variables.

  * Arrays and bit-vectors.  String ports and software emulation ports. I/O extensions providing ANSI C and POSIX.1 facilities.

  * Interfaces to standard libraries including REGEX string regular expression matching and the CURSES screen management package.

  * Available add-on packages including an interactive debugger, database, X-window graphics, BGI graphics, Motif, and Open-Windows packages.

  * A compiler (HOBBIT, available separately) and dynamic linking of compiled modules.

  * User definable responses to interrupts and errors, Process-syncronization primitives.  Setable levels of monitoring and timing information printed interactively (the verbose function). Restart, quit, and exec.

Authors
=======

Aubrey Jaffer (agj @ alum.mit.edu)
     Most of SCM.

Radey Shouman
     Arrays, gsubrs, compiled closures, records, Ecache, syntax-rules macros, and safeports.

Jerry D. Hedden
     Real and Complex functions.  Fast mixed type arithmetics.

Hugh Secker-Walker
     Syntax checking and memoization of special forms by evaluator.  Storage allocation strategy and parameters.

George Carrette
     Siod, written by George Carrette, was the starting point for SCM. The major innovations taken from Siod are the evaluator's use of the C-stack and being able to garbage collect off the C-stack (see Garbage Collection).

There are many other contributors to SCM.  They are acknowledged in the file ChangeLog, a log of changes that have been made to scm.

Copyright
=========

Authors have assigned their SCM copyrights to:

Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111, USA

The SCM License
---------------

The license of SCM consists of the GNU GPL plus a special statement giving blanket permission to link with non-free software.  This is the license statement as found in any individual file that it applies to:

     This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version.

     This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.

     You should have received a copy of the GNU General Public License along with this software; see the file COPYING.  If not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

     As a special exception, the Free Software Foundation gives permission for additional uses of the text contained in its release of SCM.

     The exception is that, if you link the SCM library with other files to produce an executable, this does not by itself cause the resulting executable to be covered by the GNU General Public License.  Your use of that executable is in no way restricted on account of linking the SCM library code into it.

     This exception does not however invalidate any other reasons why the executable file might be covered by the GNU General Public License.

     This exception applies only to the code released by the Free Software Foundation under the name SCM.  If you copy code from other Free Software Foundation releases into a copy of SCM, as the General Public License permits, the exception does not apply to the code that you add in this way.  To avoid misleading anyone as to the status of such modified files, you must delete this exception notice from them.

     If you write modifications of your own for SCM, it is your choice whether to permit this exception to apply to your modifications.  If you do not wish that, delete this exception notice.

Permission to use, copy, modify, distribute, and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation.

NO WARRANTY

BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

SIOD copyright
--------------

COPYRIGHT (c) 1989 BY
PARADIGM ASSOCIATES INCORPORATED, CAMBRIDGE, MASSACHUSETTS.
ALL RIGHTS RESERVED

Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the name of Paradigm Associates Inc not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission.

PARADIGM DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL PARADIGM BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

gjc@paradigm.com

Phone: 617-492-6079

Paradigm Associates Inc
29 Putnam Ave, Suite 6
Cambridge, MA 02138

Bibliography
============

[IEEE]
     IEEE Standard 1178-1990.  IEEE Standard for the Scheme Programming Language.  IEEE, New York, 1991.

[R4RS]
     William Clinger and Jonathan Rees, Editors. Revised(4) Report on the Algorithmic Language Scheme. ACM Lisp Pointers Volume IV, Number 3 (July-September 1991), pp. 1-55.

     Revised(4) Report on the Algorithmic Language Scheme.

[R5RS]
     Richard Kelsey and William Clinger and Jonathan (Rees, editors) Revised(5) Report on the Algorithmic Language Scheme. Higher-Order and Symbolic Computation Volume 11, Number 1 (1998), pp. 7-105, and ACM SIGPLAN Notices 33(9), September 1998.

     Revised(5) Report on the Algorithmic Language Scheme.

[Exrename]
     William Clinger Hygienic Macros Through Explicit Renaming Lisp Pointers Volume IV, Number 4 (December 1991), pp 17-23.

[SICP]
     Harold Abelson and Gerald Jay Sussman with Julie Sussman. Structure and Interpretation of Computer Programs. MIT Press, Cambridge, 1985.

[Simply]
     Brian Harvey and Matthew Wright. Simply Scheme: Introducing Computer Science MIT Press, 1994 ISBN 0-262-08226-8

[SchemePrimer]
     $B8$;tBg(B(Dai Inukai) $BF~Lg(BScheme 1999$BG/(B12$B7n=iHG(B ISBN4-87966-954-7

[SLIB]
     Todd R. Eigenschink, Dave Love, and Aubrey Jaffer. SLIB, The Portable Scheme Library. Version 2c8, June 2000.

     SLIB.

[JACAL]
     Aubrey Jaffer. JACAL Symbolic Mathematics System. Version 1b0, Sep 1999.

     JACAL.

scm.texi
scm.info
     Documentation of scm extensions (beyond Scheme standards). Documentation on the internal representation and how to extend or include scm in other programs.

Xlibscm.texi
Xlibscm.info
     Documentation of the Xlib - SCM Language X Interface.

Installing SCM
**************

Making SCM
==========

The SCM distribution has Makefile which contains rules for making scmlit, a ``bare-bones'' version of SCM sufficient for running build.  build is used to compile (or create scripts to compile) full featured versions.

Makefiles are not portable to the majority of platforms.  If Makefile works for you, good; If not, I don't want to hear about it.  If you need to compile SCM without build, there are several ways to proceed:

  * Use the build (http://swiss.csail.mit.edu/~jaffer/buildscm.html) web page to create custom batch scripts for compiling SCM.

  * Use SCM on a different platform to run build to create a script to build SCM;

  * Use another implementation of Scheme to run build to create a script to build SCM;

  * Create your own script or Makefile.

SLIB
====

[SLIB] is a portable Scheme library meant to provide compatibility and utility functions for all standard Scheme implementations.  Although SLIB is not neccessary to run SCM, I strongly suggest you obtain and install it.  Bug reports about running SCM without SLIB have very low priority.  SLIB is available from the same sites as SCM:

  * swiss.csail.mit.edu:/pub/scm/slib3a2.tar.gz

  * ftp.gnu.org:/pub/gnu/jacal/slib3a2.tar.gz

  * ftp.cs.indiana.edu:/pub/scheme-repository/imp/slib3a2.tar.gz

Unpack SLIB (tar xzf slib3a2.tar.gz or unzip -ao slib3a2.zip) in an appropriate directory for your system; both tar and unzip will create the directory slib.

Then create a file require.scm in the SCM implementation-vicinity (this is the same directory as where the file InitSCMVERSION.scm is installed). require.scm should have the contents:

     (define (library-vicinity) "/usr/local/lib/slib/")

where the pathname string /usr/local/lib/slib/ is to be replaced by the pathname into which you installed SLIB.  Absolute pathnames are recommended here; if you use a relative pathname, SLIB can get confused when the working directory is changed (see chmod). The way to specify a relative pathname is to append it to the implementation-vicinity, which is absolute:

     (define library-vicinity
       (let ((lv (string-append (implementation-vicinity) "../slib/")))
         (lambda () lv)))

Alternatively, you can set the (shell) environment variable SCHEME_LIBRARY_PATH to the pathname of the SLIB directory (see Environment Variables).  If set, the environment variable overrides require.scm.  Again, absolute pathnames are recommended.

Building SCM
============

The file build loads the file build.scm, which constructs a relational database of how to compile and link SCM executables. build.scm has information for the platforms which SCM has been ported to (of which I have been notified).  Some of this information is old, incorrect, or incomplete.  Send corrections and additions to jaffer @ ai.mit.edu.

Invoking Build
--------------

The all method will also work for MS-DOS and unix.  Use the all method if you encounter problems with build.

MS-DOS
     From the SCM source directory, type build followed by up to 9 command line arguments.

unix
     From the SCM source directory, type ./build followed by command line arguments.

all
     From the SCM source directory, start scm or scmlit and type (load "build").  Alternatively, start scm or scmlit with the command line argument -ilbuild.

Invoking build without the -F option will build or create a shell script with the arrays, inexact, and bignums options as defaults.

     bash$ ./build
     -|
     #! /bin/sh
     # unix (linux) script created by SLIB/batch
     # ================ Write file with C defines
     rm -f scmflags.h
     echo '#define IMPLINIT "InitSCMVERSION.scm"'>>scmflags.h
     echo '#define BIGNUMS'>>scmflags.h
     echo '#define FLOATS'>>scmflags.h
     echo '#define ARRAYS'>>scmflags.h
     # ================ Compile C source files
     gcc -O2 -c continue.c scm.c scmmain.c findexec.c script.c time.c repl.c scl.c eval.c sys.c subr.c debug.c unif.c rope.c
     # ================ Link C object files
     gcc -rdynamic -o scm continue.o scm.o scmmain.o findexec.o script.o time.o repl.o scl.o eval.o sys.o subr.o debug.o unif.o rope.o -lm -lc

To cross compile for another platform, invoke build with the -p or --platform= option.  This will create a script for the platform named in the -p or --platform= option.

     bash$ ./build -o scmlit -p darwin -F lit
     -|
     #! /bin/sh
     # unix (darwin) script created by SLIB/batch
     # ================ Write file with C defines
     rm -f scmflags.h
     echo '#define IMPLINIT "InitSCMVERSION.scm"'>>scmflags.h
     # ================ Compile C source files
     cc -O3 -c continue.c scm.c scmmain.c findexec.c script.c time.c repl.c scl.c eval.c sys.c subr.c debug.c unif.c rope.c
     # ================ Link C object files
     mv -f scmlit scmlit~
     cc -o scmlit continue.o scm.o scmmain.o findexec.o script.o time.o repl.o scl.o eval.o sys.o subr.o debug.o unif.o rope.o

Build Options
-------------

The options to build specify what, where, and how to build a SCM program or dynamically linked module.  These options are unrelated to the SCM command line options.

 -- Build Option: -p PLATFORM-NAME
 -- Build Option: ---platform=PLATFORM-NAME
     specifies that the compilation should be for a computer/operating-system combination called PLATFORM-NAME.  Note The case of PLATFORM-NAME is distinguised.  The current PLATFORM-NAMEs are all lower-case.

     The platforms defined by table platform in build.scm are:

          [file platform.txi not included]

 -- Build Option: -f PATHNAME
     specifies that the build options contained in PATHNAME be spliced into the argument list at this point.  The use of option files can separate functional features from platform-specific ones.

     The Makefile calls out builds with the options in .opt files:

     dlls.opt
          Options for Makefile targets mydlls, myturtle, and x.so.

     gdb.opt
          Options for udgdbscm and gdbscm.

     libscm.opt
          Options for libscm.a.

     pg.opt
          Options for pgscm, which instruments C functions.

     udscm4.opt
          Options for targets udscm4 and dscm4 (scm).

     udscm5.opt
          Options for targets udscm5 and dscm5 (scm).

     The Makefile creates options files it depends on only if they do not already exist.

 -- Build Option: -o FILENAME
 -- Build Option: ---outname=FILENAME
     specifies that the compilation should produce an executable or object name of FILENAME.  The default is scm.  Executable suffixes will be added if neccessary, e.g. scm => scm.exe.

 -- Build Option: -l LIBNAME ...
 -- Build Option: ---libraries=LIBNAME
     specifies that the LIBNAME should be linked with the executable produced.  If compile flags or include directories (-I) are needed, they are automatically supplied for compilations.  The c library is always included.  SCM features specify any libraries they need; so you shouldn't need this option often.

 -- Build Option: -D DEFINITION ...
 -- Build Option: ---defines=DEFINITION
     specifies that the DEFINITION should be made in any C source compilations.  If compile flags or include directories (-I) are needed, they are automatically supplied for compilations.  SCM features specify any flags they need; so you shouldn't need this option often.

 -- Build Option: ---compiler-options=FLAG
     specifies that that FLAG will be put on compiler command-lines.

 -- Build Option: ---linker-options=FLAG
     specifies that that FLAG will be put on linker command-lines.

 -- Build Option: -s PATHNAME
 -- Build Option: ---scheme-initial=PATHNAME
     specifies that PATHNAME should be the default location of the SCM initialization file InitSCMVERSION.scm.  SCM tries several likely locations before resorting to PATHNAME (see File-System Habitat).  If not specified, the current directory (where build is building) is used.

 -- Build Option: -c PATHNAME ...
 -- Build Option: ---c-source-files=PATHNAME
     specifies that the C source files PATHNAME ... are to be compiled.

 -- Build Option: -j PATHNAME ...
 -- Build Option: ---object-files=PATHNAME
     specifies that the object files PATHNAME ... are to be linked.

 -- Build Option: -i CALL ...
 -- Build Option: ---initialization=CALL
     specifies that the C functions CALL ... are to be invoked during initialization.

 -- Build Option: -t BUILD-WHAT
 -- Build Option: ---type=BUILD-WHAT
     specifies in general terms what sort of thing to build.  The choices are:

     exe
          executable program.

     lib
          library module.

     dlls
          archived dynamically linked library object files.

     dll
          dynamically linked library object file.

     The default is to build an executable.

 -- Build Option: -h BATCH-SYNTAX
 -- Build Option: --batch-dialect=BATCH-SYNTAX
     specifies how to build.  The default is to create a batch file for the host system.  The SLIB file batch.scm knows how to create batch files for:

       * unix

       * dos

       * vms

       * amigaos (was amigados)

       * system

          This option executes the compilation and linking commands through the use of the system procedure.

       * *unknown*

          This option outputs Scheme code.

 -- Build Option: -w BATCH-FILENAME
 -- Build Option: --script-name=BATCH-FILENAME
     specifies where to write the build script.  The default is to display it on (current-output-port).

 -- Build Option: -F FEATURE ...
 -- Build Option: ---features=FEATURE
     specifies to build the given features into the executable.  The defined features are:

               [file features.txi not included]

Compiling and Linking Custom Files
----------------------------------

A correspondent asks:

     How can we link in our own c files to the SCM interpreter so that we can add our own functionality?  (e.g. we have a bunch of tcp functions we want access to).  Would this involve changing build.scm or the Makefile or both?

(see Changing Scm has instructions describing the C code format). Suppose a C file foo.c has functions you wish to add to SCM.  To compile and link your file at compile time, use the -c and -i options to build:

     bash$ ./build -c foo.c -i init_foo
     -|
     #! /bin/sh
     rm -f scmflags.h
     echo '#define IMPLINIT "/home/jaffer/scm/InitSCMVERSION.scm"'>>scmflags.h
     echo '#define COMPILED_INITS init_foo();'>>scmflags.h
     echo '#define BIGNUMS'>>scmflags.h
     echo '#define FLOATS'>>scmflags.h
     echo '#define ARRAYS'>>scmflags.h
     gcc -O2 -c continue.c scm.c findexec.c script.c time.c repl.c scl.c \
             eval.c sys.c subr.c unif.c rope.c foo.c
     gcc -rdynamic -o scm continue.o scm.o findexec.o script.o time.o \
             repl.o scl.o eval.o sys.o subr.o unif.o rope.o foo.o -lm -lc

To make a dynamically loadable object file use the -t dll option:

     bash$ ./build -t dll -c foo.c
     -|
     #! /bin/sh
     rm -f scmflags.h
     echo '#define IMPLINIT "/home/jaffer/scm/InitSCMVERSION.scm"'>>scmflags.h
     echo '#define BIGNUMS'>>scmflags.h
     echo '#define FLOATS'>>scmflags.h
     echo '#define ARRAYS'>>scmflags.h
     echo '#define DLL'>>scmflags.h
     gcc -O2 -fpic -c foo.c
     gcc -shared -o foo.so foo.o -lm -lc

Once foo.c compiles correctly (and your SCM build supports dynamic-loading), you can load the compiled file with the Scheme command (load "./foo.so").  See Configure Module Catalog for how to add a compiled dll file to SLIB's catalog.

Installing Dynamic Linking
==========================

Dynamic linking has not been ported to all platforms.  Operating systems in the BSD family (a.out binary format) can usually be ported to DLD.  The dl library (#define SUN_DL for SCM) was a proposed POSIX standard and may be available on other machines with COFF binary format.  For notes about porting to MS-Windows and finishing the port to VMS VMS Dynamic Linking.

DLD is a library package of C functions that performs dynamic link editing on Linux, VAX (Ultrix), Sun 3 (SunOS 3.4 and 4.0), SPARCstation (SunOS 4.0), Sequent Symmetry (Dynix), and Atari ST.  It is available from:

  * ftp.gnu.org:pub/gnu/dld-3.3.tar.gz

These notes about using libdl on SunOS are from gcc.info:

     On a Sun, linking using GNU CC fails to find a shared library and reports that the library doesn't exist at all.

     This happens if you are using the GNU linker, because it does only static linking and looks only for unshared libraries.  If you have a shared library with no unshared counterpart, the GNU linker won't find anything.

     We hope to make a linker which supports Sun shared libraries, but please don't ask when it will be finished--we don't know.

     Sun forgot to include a static version of libdl.a with some versions of SunOS (mainly 4.1).  This results in undefined symbols when linking static binaries (that is, if you use -static).  If you see undefined symbols _dlclose, _dlsym or _dlopen when linking, compile and link against the file mit/util/misc/dlsym.c from the MIT version of X windows.

Configure Module Catalog
========================

The SLIB module catalog can be extended to define other require-able packages by adding calls to the Scheme source file mkimpcat.scm.  Within mkimpcat.scm, the following procedures are defined.

 -- Function: add-link feature object-file lib1 ...
     FEATURE should be a symbol.  OBJECT-FILE should be a string naming a file containing compiled object-code.  Each LIBn argument should be either a string naming a library file or #f.

     If OBJECT-FILE exists, the add-link procedure registers symbol FEATURE so that the first time require is called with the symbol FEATURE as its argument, OBJECT-FILE and the LIB1 ... are dynamically linked into the executing SCM session.

     If OBJECT-FILE exists, add-link returns #t, otherwise it returns #f.

     For example, to install a compiled dll foo, add these lines to mkimpcat.scm:

                  (add-link 'foo
                            (in-vicinity (implementation-vicinity) "foo"
                                         link:able-suffix))

 -- Function: add-alias alias feature
     ALIAS and FEATURE are symbols.  The procedure add-alias registers ALIAS as an alias for FEATURE. An unspecified value is returned.

     add-alias causes (require 'ALIAS) to behave like (require 'FEATURE).

 -- Function: add-source feature filename
     FEATURE is a symbol.  FILENAME is a string naming a file containing Scheme source code.  The procedure add-source registers FEATURE so that the first time require is called with the symbol FEATURE as its argument, the file FILENAME will be loaded.  An unspecified value is returned.

Remember to delete the file slibcat after modifying the file mkimpcat.scm in order to force SLIB to rebuild its cache.

Saving Images
=============

In SCM, the ability to save running program images is called dump (see Dump).  In order to make dump available to SCM, build with feature dump.  dumped executables are compatible with dynamic linking.

Most of the code for dump is taken from emacs-19.34/src/unex*.c.  No modifications to the emacs source code were required to use unexelf.c.  Dump has not been ported to all platforms.  If unexec.c or unexelf.c don't work for you, try using the appropriate unex*.c file from emacs.

Automatic C Preprocessor Definitions
====================================

These #defines are automatically provided by preprocessors of various C compilers.  SCM uses the presence or absence of these definitions to configure include file locations and aliases for library functions.  If the definition(s) corresponding to your system type is missing as your system is configured, add -DFLAG to the compilation command lines or add a #define FLAG line to scmfig.h or the beginning of scmfig.h.

     #define         Platforms:
     -------         ----------
     ARM_ULIB        Huw Rogers free unix library for acorn archimedes
     AZTEC_C         Aztec_C 5.2a
     __CYGWIN__      Cygwin
     _DCC            Dice C on AMIGA
     __GNUC__        Gnu CC (and DJGPP)
     __EMX__         Gnu C port (gcc/emx 0.8e) to OS/2 2.0
     __HIGHC__       MetaWare High C
     __IBMC__        C-Set++ on OS/2 2.1
     _MSC_VER        MS VisualC++ 4.2
     MWC             Mark Williams C on COHERENT
     __MWERKS__      Metrowerks Compiler; Macintosh and WIN32 (?)
     _POSIX_SOURCE   ??
     _QC             Microsoft QuickC
     __STDC__        ANSI C compliant
     __TURBOC__      Turbo C and Borland C
     __USE_POSIX     ??
     __WATCOMC__     Watcom C on MS-DOS
     __ZTC__         Zortech C

     _AIX            AIX operating system
     __APPLE__       Apple Darwin
     AMIGA           SAS/C 5.10 or Dice C on AMIGA
     __amigaos__     Gnu CC on AMIGA
     atarist         ATARI-ST under Gnu CC
     __FreeBSD__     FreeBSD
     GNUDOS          DJGPP (obsolete in version 1.08)
     __GO32__        DJGPP (future?)
     hpux            HP-UX
     linux           Linux
     macintosh       Macintosh (THINK_C and __MWERKS__ define)
     MCH_AMIGA       Aztec_c 5.2a on AMIGA
     __MACH__        Apple Darwin
     MSDOS           Microsoft C 5.10 and 6.00A
     _MSDOS          Microsoft CLARM and CLTHUMB compilers.
     __MSDOS__       Turbo C, Borland C, and DJGPP
     __NetBSD__      NetBSD
     nosve           Control Data NOS/VE
     SVR2            System V Revision 2.
     sun             SunOS
     __SVR4          SunOS
     THINK_C         developement environment for the Macintosh
     ultrix          VAX with ULTRIX operating system.
     unix            most Unix and similar systems and DJGPP (!?)
     __unix__        Gnu CC and DJGPP
     _UNICOS         Cray operating system
     vaxc            VAX C compiler
     VAXC            VAX C compiler
     vax11c          VAX C compiler
     VAX11           VAX C compiler
     _Windows        Borland C 3.1 compiling for Windows
     _WIN32          MS VisualC++ 4.2 and Cygwin (Win32 API)
     _WIN32_WCE      MS Windows CE
     vms             (and VMS) VAX-11 C under VMS.

     __alpha         DEC Alpha processor
     __alpha__       DEC Alpha processor
     hp9000s800      HP RISC processor
     __ia64          GCC on IA64
     __ia64__        GCC on IA64
     _LONGLONG       GCC on IA64
     __i386__        DJGPP
     i386            DJGPP
     _M_ARM          Microsoft CLARM compiler defines as 4 for ARM.
     _M_ARMT         Microsoft CLTHUMB compiler defines as 4 for Thumb.
     MULTIMAX        Encore computer
     ppc             PowerPC
     __ppc__         PowerPC
     pyr             Pyramid 9810 processor
     __sgi__         Silicon Graphics Inc.
     sparc           SPARC processor
     sequent         Sequent computer
     tahoe           CCI Tahoe processor
     vax             VAX processor

Problems Compiling
==================

FILE
PROBLEM / MESSAGE
HOW TO FIX
*.c
include file not found.
Correct the status of STDC_HEADERS in scmfig.h.

fix #include statement or add #define for system type to scmfig.h.
*.c
Function should return a value.
Ignore.

Parameter is never used.

Condition is always false.

Unreachable code in function.

scm.c
assignment between incompatible types.
Change SIGRETTYPE in scm.c.
time.c
CLK_TCK redefined.
incompatablility between <stdlib.h> and <sys/types.h>.

Remove STDC_HEADERS in scmfig.h.

Edit <sys/types.h> to remove incompatability.
subr.c
Possibly incorrect assignment in function lgcd.
Ignore.
sys.c
statement not reached.
Ignore.

constant in conditional expression.

sys.c
undeclared, outside of functions.
#undef STDC_HEADERS in scmfig.h.
scl.c
syntax error.
#define SYSTNAME to your system type in scl.c (softtype).

Problems Linking
================

PROBLEM
HOW TO FIX
_sin etc. missing.
Uncomment LIBS in makefile.

Problems Running
================

PROBLEM
HOW TO FIX
Opening message and then machine crashes.
Change memory model option to C compiler (or makefile).

Make sure sizet definition is correct in scmfig.h.

Reduce the size of HEAP_SEG_SIZE in setjump.h.
Input hangs.
#define NOSETBUF
ERROR: heap: need larger initial.
Increase initial heap allocation using -a<kb> or INIT_HEAP_SIZE.
ERROR: Could not allocate.
Check sizet definition.

Use 32 bit compiler mode.

Don't try to run as subproccess.
remove <FLAG> in scmfig.h and recompile scm.
Do so and recompile files.
add <FLAG> in scmfig.h and recompile scm.

ERROR: InitSCMVERSION.scm not found.
Assign correct IMPLINIT in makefile or scmfig.h.

Define environment variable SCM_INIT_PATH to be the full pathname of InitSCMVERSION.scm.
WARNING: require.scm not found.
Define environment variable SCHEME_LIBRARY_PATH to be the full pathname of the scheme library [SLIB].

Change library-vicinity in InitSCMVERSION.scm to point to library or remove.

Make sure the value of (library-vicinity) has a trailing file separator (like / or \).

Testing
=======

Loading r4rstest.scm in the distribution will run an [R4RS] conformance test on scm.

     > (load "r4rstest.scm")
     -|
     ;loading "r4rstest.scm"
     SECTION(2 1)
     SECTION(3 4)
      #<primitive-procedure boolean?>
         #<primitive-procedure char?>
            #<primitive-procedure null?>
               #<primitive-procedure number?>
     ...

Loading pi.scm in the distribution will enable you to compute digits of pi.

     > (load "pi")
     ;loading "pi"
     ;done loading "pi.scm"
     ;Evaluation took 20 ms (0 in gc) 767 cells work, 233.B other
     #<unspecified>
     > (pi 100 5)
     00003 14159 26535 89793 23846 26433 83279 50288 41971 69399
     37510 58209 74944 59230 78164 06286 20899 86280 34825 34211
     70679
     ;Evaluation took 550 ms (60 in gc) 36976 cells work, 1548.B other
     #<unspecified>

Loading bench.scm will compute and display performance statistics of SCM running pi.scm.  make bench or make benchlit appends the performance report to the file BenchLog, facilitating tracking effects of changes to SCM on performance.

PROBLEM
HOW TO FIX
Runs some and then machine crashes.
See above under machine crashes.
Runs some and then ERROR: ... (after a GC has happened).
Remove optimization option to C compiler and recompile.

#define SHORT_ALIGN in scmfig.h.
Some symbol names print incorrectly.
Change memory model option to C compiler (or makefile).

Check that HEAP_SEG_SIZE fits within sizet.

Increase size of HEAP_SEG_SIZE (or INIT_HEAP_SIZE if it is smaller than HEAP_SEG_SIZE).
ERROR: Rogue pointer in Heap.
See above under machine crashes.
Newlines don't appear correctly in output files.
Check file mode (define OPEN_... in InitSCMVERSION.scm).
Spaces or control characters appear in symbol names.
Check character defines in scmfig.h.
Negative numbers turn positive.
Check SRS in scmfig.h.
VMS: Couldn't unwind stack.
#define CHEAP_CONTIUATIONS in scmfig.h.
VAX: botched longjmp.

Sparc(SUN-4) heap is growing out of control
     You are experiencing a GC problem peculiar to the Sparc.  The problem is that SCM doesn't know how to clear register windows.  Every location which is not reused still gets marked at GC time.  This causes lots of stuff which should be collected to not be.  This will be a problem with any conservative GC until we find what instruction will clear the register windows.  This problem is exacerbated by using lots of call-with-current-continuations. A possible fix for dynthrow() is commented out in continue.c.

Reporting Problems
==================

Reported problems and solutions are grouped under Compiling, Linking, Running, and Testing.  If you don't find your problem listed there, you can send a bug report to agj @ alum.mit.edu.  The bug report should include:

 1. The version of SCM (printed when SCM is invoked with no arguments).

 2. The type of computer you are using.

 3. The name and version of your computer's operating system.

 4. The values of the environment variables SCM_INIT_PATH and SCHEME_LIBRARY_PATH.

 5. The name and version of your C compiler.

 6. If you are using an executable from a distribution, the name, vendor, and date of that distribution.  In this case, corresponding with the vendor is recommended.

Operational Features
********************

Invoking SCM
============

      scm  [-a kbytes] [-muvbiq] [--version] [--help]
     [[-]-no-init-file] [-p int] [-r feature] [-h feature]
     [-d filename] [-f filename] [-l filename]
     [-c expression] [-e expression] [-o dumpname]
     [-- | - | -s] [filename] [arguments ...]

Upon startup scm loads the file specified by by the environment variable SCM_INIT_PATH.

If SCM_INIT_PATH is not defined or if the file it names is not present, scm tries to find the directory containing the executable file.  If it is able to locate the executable, scm looks for the initialization file (usually InitSCMVERSION.scm) in platform-dependent directories relative to this directory.  See File-System Habitat for a blow-by-blow description.

As a last resort (if initialization file cannot be located), the C compile parameter IMPLINIT (defined in the makefile or scmfig.h) is tried.

Unless the option -no-init-file or --no-init-file occurs in the command line, InitSCMVERSION.scm checks to see if there is file ScmInit.scm in the path specified by the environment variable HOME (or in the current directory if HOME is undefined).  If it finds such a file it is loaded.

InitSCMVERSION.scm then looks for command input from one of three sources: From an option on the command line, from a file named on the command line, or from standard input.

This explanation applies to SCMLIT or other builds of SCM.

Scheme-code files can also invoke SCM and its variants. See #!.

Options
=======

The options are processed in the order specified on the command line.

 -- Command Option: -a k
     specifies that scm should allocate an initial heapsize of K kilobytes.  This option, if present, must be the first on the command line.  If not specified, the default is INIT_HEAP_SIZE in source file setjump.h which the distribution sets at 25000*sizeof(cell).

 -- Command Option: -no-init-file
 -- Command Option: ---no-init-file
     Inhibits the loading of ScmInit.scm as described above.

 -- Command Option: ---help
     prints usage information and URI; then exit.

 -- Command Option: ---version
     prints version information and exit.

 -- Command Option: -r feature
     requires FEATURE.  This will load a file from [SLIB] if that FEATURE is not already provided.  If FEATURE is 2, 2rs, or r2rs; 3, 3rs, or r3rs; 4, 4rs, or r4rs; 5, 5rs, or r5rs; scm will require the features neccessary to support [R2RS]; [R3RS]; [R4RS]; or [R5RS], respectively.

 -- Command Option: -h feature
     provides FEATURE.

 -- Command Option: -l filename
 -- Command Option: -f filename
     loads FILENAME.  Scm will load the first (unoptioned) file named on the command line if no -c, -e, -f, -l, or -s option preceeds it.

 -- Command Option: -d filename
     Loads SLIB databases feature and opens FILENAME as a database.

 -- Command Option: -e expression
 -- Command Option: -c expression
     specifies that the scheme expression EXPRESSION is to be evaluated.  These options are inspired by perl and sh respectively.  On Amiga systems the entire option and argument need to be enclosed in quotes.  For instance "-e(newline)".

 -- Command Option: -o dumpname
     saves the current SCM session as the executable program dumpname. This option works only in SCM builds supporting dump (see Dump).

     If options appear on the command line after -o DUMPNAME, then the saved session will continue with processing those options when it is invoked.  Otherwise the (new) command line is processed as usual when the saved image is invoked.

 -- Command Option: -p level
     sets the prolixity (verboseness) to LEVEL.  This is the same as the scm command (verobse LEVEL).

 -- Command Option: -v
     (verbose mode) specifies that scm will print prompts, evaluation times, notice of loading files, and garbage collection statistics.  This is the same as -p3.

 -- Command Option: -q
     (quiet mode) specifies that scm will print no extra information.  This is the same as -p0.

 -- Command Option: -m
     specifies that subsequent loads, evaluations, and user interactions will be with syntax-rules macro capability.  To use a specific syntax-rules macro implementation from [SLIB] (instead of [SLIB]'s default) put -r MACROPACKAGE before -m on the command line.

 -- Command Option: -u
     specifies that subsequent loads, evaluations, and user interactions will be without syntax-rules macro capability.  Syntax-rules macro capability can be restored by a subsequent -m on the command line or from Scheme code.

 -- Command Option: -i
     specifies that scm should run interactively.  That means that scm will not terminate until the (quit) or (exit) command is given, even if there are errors.  It also sets the prolixity level to 2 if it is less than 2.  This will print prompts, evaluation times, and notice of loading files.  The prolixity level can be set by subsequent options.  If scm is started from a tty, it will assume that it should be interactive unless given a subsequent -b option.

 -- Command Option: -b
     specifies that scm should run non-interactively.  That means that scm will terminate after processing the command line or if there are errors.

 -- Command Option: -s
     specifies, by analogy with sh, that scm should run interactively and that further options are to be treated as program aguments.

 -- Command Option: -
 -- Command Option: ---
     specifies that further options are to be treated as program aguments.

Invocation Examples
===================

% scm foo.scm
     Loads and executes the contents of foo.scm and then enters interactive session.

% scm -f foo.scm arg1 arg2 arg3
     Parameters arg1, arg2, and arg3 are stored in the global list *argv*; Loads and executes the contents of foo.scm and exits.

% scm -s foo.scm arg1 arg2
     Sets *argv* to ("foo.scm" "arg1" "arg2") and enters interactive session.

% scm -e `(write (list-ref *argv* *optind*))' bar
     Prints "bar".

% scm -rpretty-print -r format -i
     Loads pretty-print and format and enters interactive session.

% scm -r5
     Loads dynamic-wind, values, and syntax-rules macros and enters interactive (with macros) session.

% scm -r5 -r4
     Like above but rev4-optional-procedures are also loaded.

Environment Variables
=====================

 -- Environment Variable: SCM_INIT_PATH
     is the pathname where scm will look for its initialization code.  The default is the file InitSCMVERSION.scm in the source directory.

 -- Environment Variable: SCHEME_LIBRARY_PATH
     is the [SLIB] Scheme library directory.

 -- Environment Variable: HOME
     is the directory where InitSCMVERSION.scm will look for the user initialization file ScmInit.scm.

 -- Environment Variable: EDITOR
     is the name of the program which ed will call.  If EDITOR is not defined, the default is ed.

Scheme Variables
================

 -- Variable: *argv*
     contains the list of arguments to the program.  *argv* can change during argument processing.  This list is suitable for use as an argument to [SLIB] getopt.

 -- Variable: *syntax-rules*
     controls whether loading and interaction support syntax-rules macros.  Define this in ScmInit.scm or files specified on the command line.  This can be overridden by subsequent -m and -u options.

 -- Variable: *interactive*
     controls interactivity as explained for the -i and -b options.  Define this in ScmInit.scm or files specified on the command line.  This can be overridden by subsequent -i and -b options.

SCM Session
===========

  * Options, file loading and features can be specified from the command line.  See System interface (SCM).  See Require (SLIB).

  * Typing the end-of-file character at the top level session (while SCM is not waiting for parenthesis closure) causes SCM to exit.

  * Typing the interrupt character aborts evaluation of the current form and resumes the top level read-eval-print loop.

 -- Function: quit
 -- Function: quit n
 -- Function: exit
 -- Function: exit n
     Aliases for exit (see exit (SLIB)).  On many systems, SCM can also tail-call another program.  See execp.

 -- Callback procedure: boot-tail dumped?
     boot-tail is called by scm_top_level just before entering interactive top-level.  If boot-tail calls quit, then interactive top-level is not entered.

 -- Function: program-arguments
     Returns a list of strings of the arguments scm was called with.

 -- Function: getlogin
     Returns the (login) name of the user logged in on the controlling terminal of the process, or #f if this information cannot be determined.

For documentation of the procedures getenv and system see System Interface (SLIB).

 -- Function: vms-debug
     If SCM is compiled under VMS this vms-debug will invoke the VMS debugger.

Editing Scheme Code
===================

 -- Function: ed arg1 ...
     The value of the environment variable EDITOR (or just ed if it isn't defined) is invoked as a command with arguments ARG1 ....

 -- Function: ed filename
     If SCM is compiled under VMS ed will invoke the editor with a single the single argument FILENAME.

Gnu Emacs:
     Editing of Scheme code is supported by emacs.  Buffers holding files ending in .scm are automatically put into scheme-mode.

     If your Emacs can run a process in a buffer you can use the Emacs command M-x run-scheme with SCM.  Otherwise, use the emacs command M-x suspend-emacs; or see ``other systems'' below.

Epsilon (MS-DOS):
     There is lisp (and scheme) mode available by use of the package LISP.E.  It offers several different indentation formats.  With this package, buffers holding files ending in .L, .LSP, .S, and .SCM (my modification) are automatically put into lisp-mode.

     It is possible to run a process in a buffer under Epsilon.  With Epsilon 5.0 the command line options -e512 -m0 are neccessary to manage RAM properly.  It has been reported that when compiling SCM with Turbo C, you need to #define NOSETBUF for proper operation in a process buffer with Epsilon 5.0.

     One can also call out to an editor from SCM if RAM is at a premium; See ``under other systems'' below.

other systems:
     Define the environment variable EDITOR to be the name of the editing program you use.  The SCM procedure (ed arg1 ...) will invoke your editor and return to SCM when you exit the editor.  The following definition is convenient:

          (define (e) (ed "work.scm") (load "work.scm"))

     Typing (e) will invoke the editor with the file of interest. After editing, the modified file will be loaded.

Debugging Scheme Code
=====================

The cautious and stack-limit options of build (see Build Options) support debugging in Scheme.

CAUTIOUS
     If SCM is built with the CAUTIOUS flag, then when an error occurs, a stack trace of certain pending calls are printed as part of the default error response.  A (memoized) expression and newline are printed for each partially evaluated combination whose procedure is not builtin.  See Memoized Expressions for how to read memoized expressions.

     Also as the result of the CAUTIOUS flag, both error and user-interrupt (invoked by C-c) to print stack traces and conclude by calling breakpoint (see Breakpoints (SLIB)) instead of aborting to top level.  Under either condition, program execution can be resumed by (continue).

     In this configuration one can interrupt a running Scheme program with C-c, inspect or modify top-level values, trace or untrace procedures, and continue execution with (continue).

STACK_LIMIT
     If SCM is built with the STACK_LIMIT flag, the interpreter will check stack size periodically.  If the size of stack exceeds a certain amount (default is HEAP_SEG_SIZE/2), SCM generates a segment violation interrupt.

     The usefulness of STACK_LIMIT depends on the user.  I don't use it; but the user I added this feature for got primarily this type of error.

There are several SLIB macros which so useful that SCM automatically loads the appropriate module from SLIB if they are invoked.

 -- Macro: trace proc1 ...
     Traces the top-level named procedures given as arguments.
 -- Macro: trace
     With no arguments, makes sure that all the currently traced identifiers are traced (even if those identifiers have been redefined) and returns a list of the traced identifiers.

 -- Macro: untrace proc1 ...
     Turns tracing off for its arguments.
 -- Macro: untrace
     With no arguments, untraces all currently traced identifiers and returns a list of these formerly traced identifiers.

The routines I use most frequently for debugging are:

 -- Function: print arg1 ...
     Print writes all its arguments, separated by spaces. Print outputs a newline at the end and returns the value of the last argument.

     One can just insert (print '<label> and ) around an expression in order to see its values as a program operates.

 -- Function: pprint arg1 ...
     Pprint pretty-prints (see Pretty-Print (SLIB)) all its arguments, separated by newlines.  Pprint returns the value of the last argument.

     One can just insert (pprint '<label> and ) around an expression in order to see its values as a program operates. Note pretty-print does not format procedures.

When typing at top level, pprint is not a good way to see nested structure because it will return the last object pretty-printed, which could be large.  pp is a better choice.

 -- Procedure: pp arg1 ...
     Pprint pretty-prints (see Pretty-Print (SLIB)) all its arguments, separated by newlines.  pp returns #<unspecified>.

 -- Syntax: print-args name
 -- Syntax: print-args
     Writes NAME if supplied; then writes the names and values of the closest lexical bindings enclosing the call to Print-args.

          (define (foo a b) (print-args foo) (+ a b))
          (foo 3 6)
          -| In foo: a = 3; b = 6;
          => 9

Sometimes more elaborate measures are needed to print values in a useful manner.  When the values to be printed may have very large (or infinite) external representations, Quick Print (SLIB), can be used.

When trace is not sufficient to find program flow problems, SLIB-PSD, the Portable Scheme Debugger offers source code debugging from GNU Emacs.  PSD runs slowly, so start by instrumenting only a few functions at a time.

     http://swiss.csail.mit.edu/ftpdir/scm/slib-psd1-3.tar.gz
     swiss.csail.mit.edu:/pub/scm/slib-psd1-3.tar.gz
     ftp.maths.tcd.ie:pub/bosullvn/jacal/slib-psd1-3.tar.gz
     ftp.cs.indiana.edu:/pub/scheme-repository/utl/slib-psd1-3.tar.gz

Debugging Continuations
=======================

These functions are defined in debug.c, all operate on captured continuations:

 -- Procedure: frame-trace cont n
     Prints information about the code being executed and the environment scopes active for continuation frame N of continuation CONT.  A "continuation frame" is an entry in the environment stack; a new frame is pushed when the environment is replaced or extended in a non-tail call context.  Frame 0 is the top of the stack.

 -- Procedure: frame->environment cont n
     Prints the environment for continuation frame N of continuation CONT.  This contains just the names, not the values, of the environment.

 -- Procedure: scope-trace env
     will print information about active lexical scopes for environment ENV.

 -- Procedure: frame-eval cont n expr
     Evaluates EXPR in the environment defined by continuation frame N of continuation CONT and returns the result.  Values in the environment may be returned or SET!.

stack-trace also now accepts an optional continuation argument.  stack-trace differs from frame-trace in that it truncates long output using safeports and prints code from all available frames.

     (define k #f)
     (define (foo x y)
        (set! k (call-with-current-continuation identity))
        #f)
     (let ((a 3) (b 4))
       (foo a b)
       #f)
     (stack-trace k)
     -|
     ;STACK TRACE
     1; ((#@set! #@k (#@call-with-current-continuation #@identity)) #f ...
     2; (#@let ((a 3) (b 4))  (#@foo #@a #@b) #f)
     ...
     #t

     (frame-trace k 0)
     -|
     (#@call-with-current-continuation #@identity)
     ; in scope:
     ;   (x y)  procedure foo#<unspecified>

     (frame-trace k 1)
     -|
     ((#@set! #@k (#@call-with-current-continuation #@identity)) #f)
     ; in scope:
     ;   (x y)  procedure foo#<unspecified>

     (frame-trace k 2)
     -|
     (#@let ((a 3) (b 4))  (#@foo #@a #@b) #f)
     ; in scope:
     ;   (a b . #@let)#<unspecified>

     (frame-trace k 3)
     -|
     (#@let ((a 3) (b 4))  (#@foo #@a #@b) #f)
     ; in top level environment.

     (frame->environment k 0)
     -|
     ((x y) 2 foo)

     (scope-trace (frame->environment k 0))
     -|
     ; in scope:
     ;   (x y)  procedure foo#<unspecified>

     (frame-eval k 0 'x) => 3

     (frame-eval k 0 '(set! x 8))
     (frame-eval k 0 'x) => 8

Errors
======

A computer-language implementation designer faces choices of how reflexive to make the implementation in handling exceptions and errors; that is, how much of the error and exception routines should be written in the language itself.  The design of a portable implementation is further constrained by the need to have (almost) all errors print meaningful messages, even when the implementation itself is not functioning correctly.  Therefore, SCM implements much of its error response code in C.

The following common error and conditions are handled by C code.  Those with callback names after them can also be handled by Scheme code (see Interrupts).  If the callback identifier is not defined at top level, the default error handler (C code) is invoked.  There are many other error messages which are not treated specially.

ARGn
     Wrong type in argument

ARG1
     Wrong type in argument 1

ARG2
     Wrong type in argument 2

ARG3
     Wrong type in argument 3

ARG4
     Wrong type in argument 4

ARG5
     Wrong type in argument 5

WNA
     Wrong number of args

OVFLOW
     numerical overflow

OUTOFRANGE
     Argument out of range

NALLOC
     (out-of-storage)

THRASH
     GC is (thrashing)

EXIT
     (end-of-program)

HUP_SIGNAL
     (hang-up)

INT_SIGNAL
     (user-interrupt)

FPE_SIGNAL
     (arithmetic-error)

BUS_SIGNAL
     bus error

SEGV_SIGNAL
     segment violation

ALRM_SIGNAL
     (alarm-interrupt)

VTALRM_SIGNAL
     (virtual-alarm-interrupt)

PROF_SIGNAL
     (profile-alarm-interrupt)

 -- Variable: errobj
     When SCM encounters a non-fatal error, it aborts evaluation of the current form, prints a message explaining the error, and resumes the top level read-eval-print loop.  The value of ERROBJ is the offending object if appropriate.  The builtin procedure error does not set ERROBJ.

errno and perror report ANSI C errors encountered during a call to a system or library function.

 -- Function: errno
 -- Function: errno n
     With no argument returns the current value of the system variable errno.  When given an argument, errno sets the system variable errno to N and returns the previous value of errno.  (errno 0) will clear outstanding errors.  This is recommended after try-load returns #f since this occurs when the file could not be opened.

 -- Function: perror string
     Prints on standard error output the argument STRING, a colon, followed by a space, the error message corresponding to the current value of errno and a newline.  The value returned is unspecified.

warn and error provide a uniform way for Scheme code to signal warnings and errors.

 -- Function: warn arg1 arg2 arg3 ...
     Alias for slib:warn (SLIB).  Outputs an error message containing the arguments.  warn is defined in InitSCMVERSION.scm.

 -- Function: error arg1 arg2 arg3 ...
     Alias for slib:error (SLIB).  Outputs an error message containing the arguments, aborts evaluation of the current form and resumes the top level read-eval-print loop.  Error is defined in InitSCMVERSION.scm.

If SCM is built with the CAUTIOUS flag, then when an error occurs, a stack trace of certain pending calls are printed as part of the default error response.  A (memoized) expression and newline are printed for each partially evaluated combination whose procedure is not builtin.  See Memoized Expressions for how to read memoized expressions.

Also as the result of the CAUTIOUS flag, both error and user-interrupt (invoked by C-c) are defined to print stack traces and conclude by calling breakpoint (see Breakpoints (SLIB)).  This allows the user to interract with SCM as with Lisp systems.

 -- Function: stack-trace
     Prints information describing the stack of partially evaluated expressions.  stack-trace returns #t if any lines were printed and #f otherwise.  See InitSCMVERSION.scm for an example of the use of stack-trace.

Memoized Expressions
====================

SCM memoizes the address of each occurence of an identifier's value when first encountering it in a source expression.  Subsequent executions of that memoized expression is faster because the memoized reference encodes where in the top-level or local environment its value is.

When procedures are displayed, the memoized locations appear in a format different from references which have not yet been executed.  I find this a convenient aid to locating bugs and untested expressions.

  * The names of memoized lexically bound identifiers are replaced with #@<m>-<n>, where <m> is the number of binding contours back and <n> is the index of the value in that binding countour.

  * The names of identifiers which are not lexiallly bound but defined at top-level have #@ prepended.

For instance, open-input-file is defined as follows in InitSCMVERSION.scm:

     (define (open-input-file str)
       (or (open-file str OPEN_READ)
           (and (procedure? could-not-open) (could-not-open) #f)
           (error "OPEN-INPUT-FILE couldn't open file " str)))

If open-input-file has not yet been used, the displayed procedure is similar to the original definition (lines wrapped for readability):

     open-input-file =>
     #<CLOSURE (str) (or (open-file str open_read)
      (and (procedure? could-not-open) (could-not-open) #f)
      (error "OPEN-INPUT-FILE couldn't open file " str))>

If we open a file using open-input-file, the sections of code used become memoized:

     (open-input-file "r4rstest.scm") => #<input-port 3>
     open-input-file =>
     #<CLOSURE (str) (#@or (#@open-file #@0+0 #@open_read)
      (and (procedure? could-not-open) (could-not-open) #f)
      (error "OPEN-INPUT-FILE couldn't open file " str))>

If we cause open-input-file to execute other sections of code, they too become memoized:

     (open-input-file "foo.scm") =>

     ERROR: No such file or directory
     ERROR: OPEN-INPUT-FILE couldn't open file  "foo.scm"

     open-input-file =>
     #<CLOSURE (str) (#@or (#@open-file #@0+0 #@open_read)
      (#@and (#@procedure? #@could-not-open) (could-not-open) #f)
      (#@error "OPEN-INPUT-FILE couldn't open file " #@0+0))>

Internal State
==============

 -- Variable: *interactive*
     The variable *INTERACTIVE* determines whether the SCM session is interactive, or should quit after the command line is processed. *INTERACTIVE* is controlled directly by the command-line options -b, -i, and -s (see Invoking SCM).  If none of these options are specified, the rules to determine interactivity are more complicated; see InitSCMVERSION.scm for details.

 -- Function: abort
     Resumes the top level Read-Eval-Print loop.

 -- Function: restart
     Restarts the SCM program with the same arguments as it was originally invoked.  All -l loaded files are loaded again; If those files have changed, those changes will be reflected in the new session.

     Note When running a saved executable (see Dump), restart is redefined to be exec-self.

 -- Function: exec-self
     Exits and immediately re-invokes the same executable with the same arguments.  If the executable file has been changed or replaced since the beginning of the current session, the new executable will be invoked.  This differentiates exec-self from restart.

 -- Function: verbose n
     Controls how much monitoring information is printed. If N is:

     0
          no prompt or information is printed.

     >= 1
          a prompt is printed.

     >= 2
          messages bracketing file loading are printed.

     >= 3
          the CPU time is printed after each top level form evaluated; notifications of heap growth printed.

     >= 4
          a garbage collection summary is printed after each top level form evaluated;

     >= 5
          a message for each GC (see Garbage Collection) is printed; warnings issued for top-level symbols redefined.

 -- Function: gc
     Scans all of SCM objects and reclaims for further use those that are no longer accessible.

 -- Function: room
 -- Function: room #t
     Prints out statistics about SCM's current use of storage.  (room #t) also gives the hexadecimal heap segment and stack bounds.

 -- Constant: *scm-version*
     Contains the version string (e.g. SCMVERSION) of SCM.

Executable path
---------------

In order to dump a saved executable or to dynamically-link using DLD, SCM must know where its executable file is.  Sometimes SCM (see Executable Pathname) guesses incorrectly the location of the currently running executable.  In that case, the correct path can be set by calling execpath with the pathname.

 -- Function: execpath
     Returns the path (string) which SCM uses to find the executable file whose invocation the currently running session is, or #f if the path is not set.

 -- Function: execpath #f
 -- Function: execpath newpath
     Sets the path to #f or NEWPATH, respectively.  The old path is returned.

For other configuration constants and procedures see Configuration (SLIB).

Scripting
=========

Unix Scheme Scripts
-------------------

In reading this section, keep in mind that the first line of a script file has (different) meanings to SCM and the operating system (execve).

 -- file: #! interpreter \ ...

     On unix systems, a Shell-Script is a file (with execute permissions) whose first two characters are #!.  The INTERPRETER argument must be the pathname of the program to process the rest of the file.  The directories named by environment variable PATH are not searched to find INTERPRETER.

     When executing a shell-script, the operating system invokes INTERPRETER with a single argument encapsulating the rest of the first line's contents (if not just whitespace), the pathname of the Scheme Script file, and then any arguments which the shell-script was invoked with.

     Put one space character between #! and the first character of INTERPRETER (/).  The INTERPRETER name is followed by \; SCM substitutes the second line of FILE for \ (and the rest of the line), then appends any arguments given on the command line invoking this Scheme-Script.

     When SCM executes the script, the Scheme variable *SCRIPT* will be set to the script pathname.  The last argument before !# on the second line should be -; SCM will load the script file, preserve the unprocessed arguments, and set *ARGV* to a list of the script pathname and the unprocessed arguments.

     Note that the interpreter, not the operating system, provides the \ substitution; this will only take place if INTERPRETER is a SCM or SCSH interpreter.

 -- Read syntax: #! ignored !#
     When the first two characters of the file being loaded are #! and a \ is present before a newline in the file, all characters up to !# will be ignored by SCM read.

This combination of interpretatons allows SCM source files to be used as POSIX shell-scripts if the first line is:

     #! /usr/local/bin/scm \

The following Scheme-Script prints factorial of its argument:

     #! /usr/local/bin/scm \ %0 %*
     - !#

     (define (fact.script args)
       (cond ((and (= 1 (length args))
                   (string->number (car args)))
              => (lambda (n) (print (fact n)) #t))
             (else (fact.usage))))

     (define (fact.usage)
       (print *argv*)
       (display "\
     Usage: fact N
       Returns the factorial of N.
     "
                (current-error-port))
       #f)

     (define (fact n) (if (< n 2) 1 (* n (fact (+ -1 n)))))

     (if *script* (exit (fact.script (list-tail *argv* *optind*))))

     ./fact 32
     =>
     263130836933693530167218012160000000

If the wrong number of arguments is given, fact prints its ARGV with usage information.

     ./fact 3 2
     -|
     ("./fact" "3" "2")
     Usage: fact N
       Returns the factorial of N.

MS-DOS Compatible Scripts
-------------------------

It turns out that we can create scheme-scripts which run both under unix and MS-DOS.  To implement this, I have written the MS-DOS programs: #!.bat and !#.exe, which are available from: http://swiss.csail.mit.edu/ftpdir/scm/sharpbang.zip

With these two programs installed in a PATH directory, we have the following syntax for <PROGRAM>.BAT files.

 -- file: #! interpreter \ %0 %*

     The first two characters of the Scheme-Script are #!.  The INTERPRETER can be either a unix style program path (using / between filename components) or a DOS program name or path. The rest of the first line of the Scheme-Script should be literally \ %0 %*, as shown.

     If INTERPRETER has / in it, INTERPRETER is converted to a DOS style filename (/ => \).

     In looking for an executable named INTERPRETER, #! first checks this (converted) filename; if INTERPRETER doesn't exist, it then tries to find a program named like the string starting after the last \ (or /) in INTERPRETER.  When searching for executables, #! tries all directories named by environment variable PATH.

     Once the INTERPRETER executable path is found, arguments are processed in the manner of scheme-shell, with all the text after the \ taken as part of the meta-argument.  More precisely, #! calls INTERPRETER with any options on the second line of the Scheme-Script up to !#, the name of the Scheme-Script file, and then any of at most 8 arguments given on the command line invoking this Scheme-Script.

The previous example Scheme-Script works in both MS-DOS and unix systems.

Unix Shell Scripts
------------------

Scheme-scripts suffer from two drawbacks:

  * Some Unixes limit the length of the #! interpreter line to the size of an object file header, which can be as small as 32 bytes.

  * A full, explicit pathname must be specified, perhaps requiring more than 32 bytes and making scripts vulnerable to breakage when programs are moved.

The following approach solves these problems at the expense of slower startup.  Make #! /bin/sh the first line and prepend every subsequent line to be executed by the shell with :;.  The last line to be executed by the shell should contain an exec command; exec tail-calls its argument.

/bin/sh is thus invoked with the name of the script file, which it executes as a *sh script.  Usually the second line starts :;exec scm -f$0, which executes scm, which in turn loads the script file.  When SCM loads the script file, it ignores the first and second lines, and evaluates the rest of the file as Scheme source code.

The second line of the script file does not have the length restriction mentioned above.  Also, /bin/sh searches the directories listed in the `PATH' environment variable for scm, eliminating the need to use absolute locations in order to invoke a program.

The following example additionally sets *SCRIPT* to the script argument, making it compatible with the scheme code of the previous example.

     #! /bin/sh
     :;exec scm -e"(set! *script* \"$0\")" -l$0 "$@"

     (define (fact.script args)
       (cond ((and (= 1 (length args))
                   (string->number (car args)))
              => (lambda (n) (print (fact n)) #t))
             (else (fact.usage))))

     (define (fact.usage)
       (print *argv*)
       (display "\
     Usage: fact N
       Returns the factorial of N.
     "
                (current-error-port))
       #f)

     (define (fact n) (if (< n 2) 1 (* n (fact (+ -1 n)))))

     (if *script* (exit (fact.script (list-tail *argv* *optind*))))

     ./fact 6
     => 720

The Language
************

Standards Compliance
====================

Scm conforms to the IEEE Standard 1178-1990.  IEEE Standard for the Scheme Programming Language. (see Bibliography), and Revised(5) Report on the Algorithmic Language Scheme. Revised(5) Report on the Algorithmic Language Scheme. All the required features of these specifications are supported. Many of the optional features are supported as well.

Optionals of [R5RS] Supported by SCM
------------------------------------

- and / of more than 2 arguments
exp
log
sin
cos
tan
asin
acos
atan
sqrt
expt
make-rectangular
make-polar
real-part
imag-part
magnitude
angle
exact->inexact
inexact->exact
     See Numerical operations (Revised(5) Scheme).
with-input-from-file
with-output-to-file
     See Ports (Revised(5) Scheme).
load
transcript-on
transcript-off
     See System interface (Revised(5) Scheme).

Optionals of [R5RS] not Supported by SCM
----------------------------------------

numerator
denominator
rationalize
     See Numerical operations (Revised(5) Scheme).

[SLIB] Features of SCM and SCMLIT
---------------------------------

delay
full-continuation
ieee-p1178
object-hash
rev4-report
source
     See SLIB file Template.scm.

current-time
     See Time and Date (SLIB).

defmacro
     See Defmacro (SLIB).

getenv
system
     See System Interface (SLIB).

hash
     See Hashing (SLIB).

logical
     See Bit-Twiddling (SLIB).

multiarg-apply
     See Multi-argument Apply (SLIB).

multiarg/and-
     See Multi-argument / and - (SLIB).

rev4-optional-procedures
     See Rev4 Optional Procedures (SLIB).

string-port
     See String Ports (SLIB).

tmpnam
     See Input/Output (SLIB).

transcript
     See Transcripts (SLIB).

vicinity
     See Vicinity (SLIB).

with-file
     See With-File (SLIB).

[SLIB] Features of SCM
----------------------

array
     See Arrays (SLIB).

array-for-each
     See Array Mapping (SLIB).

bignum
complex
inexact
rational
real
     See Require (SLIB).

Storage
=======

 -- Function: vector-set-length! object length
     Change the length of string, vector, bit-vector, or uniform-array OBJECT to LENGTH.  If this shortens OBJECT then the remaining contents are lost.  If it enlarges OBJECT then the contents of the extended part are undefined but the original part is unchanged.  It is an error to change the length of literal datums.  The new object is returned.

 -- Function: copy-tree obj
 -- Function: @copy-tree obj
     See copy-tree (SLIB).  This extends the SLIB version by also copying vectors.  Use @copy-tree if you depend on this feature; copy-tree could get redefined.

 -- Function: acons obj1 obj2 obj3
     Returns (cons (cons obj1 obj2) obj3).

          (set! a-list (acons key datum a-list))

     Adds a new association to a-list.

 -- Callback procedure: gc-hook ...
     Allows a Scheme procedure to be run shortly after each garbage collection. This procedure will not be run recursively.  If it runs long enough to cause a garbage collection before returning a warning will be printed.

     To remove the gc-hook, (set! gc-hook #f).

 -- Function: add-finalizer object finalizer
     OBJECT may be any garbage collected object, that is, any object other than an immediate integer, character, or special token such as #f or #t, see Immediates.  FINALIZER is a thunk, or procedure taking no arguments.

     FINALIZER will be invoked asynchronously exactly once some time after OBJECT becomes eligible for garbage collection.  A reference to OBJECT in the environment of FINALIZER will not prevent finalization, but will delay the reclamation of OBJECT at least until the next garbage collection.  A reference to OBJECT in some other object's finalizer will necessarily prevent finalization until both objects are eligible for garbage collection.

     Finalizers are not run in any predictable order.  All finalizers will be run by the time the program ends.

     This facility was based on the paper by Simon Peyton Jones, et al, ``Stretching the storage manager: weak pointers and stable names in Haskell'', Proc. 11th International Workshop on the Implementation of Functional Languages, The Netherlands, September 7-10 1999, Springer-Verlag LNCS.

Time
====

 -- Constant: internal-time-units-per-second

     Is the integer number of internal time units in a second.

 -- Function: get-internal-run-time
     Returns the integer run time in internal time units from an unspecified starting time.  The difference of two calls to get-internal-run-time divided by internal-time-units-per-second will give elapsed run time in seconds.

 -- Function: get-internal-real-time
     Returns the integer time in internal time units from an unspecified starting time.  The difference of two calls to get-internal-real-time divided by interal-time-units-per-second will give elapsed real time in seconds.

 -- Function: current-time
     Returns the time since 00:00:00 GMT, January 1, 1970, measured in seconds.  See current-time (SLIB).  current-time is used in Time and Date (SLIB).

Interrupts
==========

 -- Function: ticks n
     Returns the number of ticks remaining till the next tick interrupt. Ticks are an arbitrary unit of evaluation.  Ticks can vary greatly in the amount of time they represent.

     If N is 0, any ticks request is canceled.  Otherwise a ticks-interrupt will be signaled N from the current time. ticks is supported if SCM is compiled with the ticks flag defined.

 -- Callback procedure: ticks-interrupt ...
     Establishes a response for tick interrupts.  Another tick interrupt will not occur unless ticks is called again.  Program execution will resume if the handler returns.  This procedure should (abort) or some other action which does not return if it does not want processing to continue.

 -- Function: alarm secs
     Returns the number of seconds remaining till the next alarm interrupt. If SECS is 0, any alarm request is canceled.  Otherwise an alarm-interrupt will be signaled SECS from the current time.  ALARM is not supported on all systems.

 -- Function: milli-alarm millisecs interval
 -- Function: virtual-alarm millisecs interval
 -- Function: profile-alarm millisecs interval
     milli-alarm is similar to alarm, except that the first argument MILLISECS, and the return value are measured in milliseconds rather than seconds.  If the optional argument INTERVAL is supplied then alarm interrupts will be scheduled every INTERVAL milliseconds until turned off by a call to milli-alarm or alarm.

     virtual-alarm and profile-alarm are similar. virtual-alarm decrements process execution time rather than real time, and causes SIGVTALRM to be signaled. profile-alarm decrements both process execution time  and system execution time on behalf of the process, and causes SIGPROF to be signaled.

     milli-alarm, virtual-alarm, and profile-alarm are supported only on systems providing the setitimer system call.

 -- Callback procedure: user-interrupt ...
 -- Callback procedure: alarm-interrupt ...
 -- Callback procedure: virtual-alarm-interrupt ...
 -- Callback procedure: profile-alarm-interrupt ...
     Establishes a response for SIGINT (control-C interrupt) and SIGALRM, SIGVTALRM, and SIGPROF interrupts. Program execution will resume if the handler returns.  This procedure should (abort) or some other action which does not return if it does not want processing to continue after it returns.

     Interrupt handlers are disabled during execution system and ed procedures.

     To unestablish a response for an interrupt set the handler symbol to #f.  For instance, (set! user-interrupt #f).

 -- Callback procedure: out-of-storage ...
 -- Callback procedure: could-not-open ...
 -- Callback procedure: end-of-program ...
 -- Callback procedure: hang-up ...
 -- Callback procedure: arithmetic-error ...
     Establishes a response for storage allocation error, file opening error, end of program, SIGHUP (hang up interrupt) and arithmetic errors respectively.  This procedure should (abort) or some other action which does not return if it does not want the default error message to also be displayed.  If no procedure is defined for HANG-UP then END-OF-PROGRAM (if defined) will be called.

     To unestablish a response for an error set the handler symbol to #f.  For instance, (set! could-not-open #f).

Process Synchronization
=======================

An exchanger is a procedure of one argument regulating mutually exclusive access to a resource.  When a exchanger is called, its current content is returned, while being replaced by its argument in an atomic operation.

 -- Function: make-exchanger obj

     Returns a new exchanger with the argument OBJ as its initial content.

          (define queue (make-exchanger (list a)))

     A queue implemented as an exchanger holding a list can be protected from reentrant execution thus:

          (define (pop queue)
            (let ((lst #f))
              (dynamic-wind
                  (lambda () (set! lst (queue #f)))
                  (lambda () (and lst (not (null? lst))
                                  (let ((ret (car lst)))
                                    (set! lst (cdr lst))
                                    ret)))
                  (lambda () (and lst (queue lst))))))

          (pop queue)         => a

          (pop queue)         => #f

 -- Function: make-arbiter name

     Returns an object of type arbiter and name NAME.  Its state is initially unlocked.

 -- Function: try-arbiter arbiter

     Returns #t and locks ARBITER if ARBITER was unlocked. Otherwise, returns #f.

 -- Function: release-arbiter arbiter

     Returns #t and unlocks ARBITER if ARBITER was locked. Otherwise, returns #f.

Files and Ports
===============

These procedures generalize and extend the standard capabilities in Ports (Revised(5) Scheme).

Opening and Closing
-------------------

 -- Function: open-file string modes
 -- Function: try-open-file string modes
     Returns a port capable of receiving or delivering characters as specified by the MODES string.  If a file cannot be opened #f is returned.

     Internal functions opening files callback to the SCM function open-file.  You can extend open-file by redefining it. try-open-file is the primitive procedure; Do not redefine try-open-file!

 -- Constant: open_read
 -- Constant: open_write
 -- Constant: open_both
     Contain modes strings specifying that a file is to be opened for reading, writing, and both reading and writing respectively.

     Both input and output functions can be used with io-ports.  An end of file must be read or a file-set-position done on the port between a read operation and a write operation or vice-versa.

 -- Function: _ionbf modestr
     Returns a version of MODESTR which when open-file is called with it as the second argument will return an unbuffered port.  An input-port must be unbuffered in order for char-ready? and wait-for-input to work correctly on it.  The initial value of (current-input-port) is unbuffered if the platform supports it.

 -- Function: _tracked modestr
     Returns a version of MODESTR which when open-file is called with it as the second argument will return a tracked port.  A tracked port maintains current line and column numbers, which may be queried with port-line and port-column.

 -- Function: _exclusive modestr
     Returns a version of MODESTR which when open-file is called with it as the second argument will return a port only if the named file does not already exist.  This functionality is provided by calling try-create-file see I/O-Extensions, which is not available for all platforms.

 -- Function: open-ports
     Returns a list of all currently open ports, excluding string ports, see String Ports (SLIB).  This may be useful after a fork see Posix Extensions, or for debugging.  Bear in mind that ports that would be closed by gc will be kept open by a reference to this list.

 -- Function: close-port port
     Closes PORT.  The same as close-input-port and close-output-port.

Port Properties
---------------

 -- Function: port-closed? port
     Returns #t if PORT is closed.

 -- Function: port-type obj
     If OBJ is not a port returns false, otherwise returns a symbol describing the port type, for example string or pipe.

 -- Function: port-filename port
     Returns the filename PORT was opened with.  If PORT is not open to a file the result is unspecified.

 -- Function: port-line port
 -- Function: port-column port
     If PORT is a tracked port, return the current line (column) number, otherwise return #f.  Line and column numbers begin with 1. The column number applies to the next character to be read; if that character is a newline, then the column number will be one more than the length of the line.

 -- Function: freshline port
     Outputs a newline to optional argument PORT unless the current output column number of PORT is known to be zero, ie output will start at the beginning of a new line.  PORT defaults to current-output-port.  If PORT is not a tracked port freshline is equivalent to newline.

 -- Function: isatty? port
     Returns #t if PORT is input or output to a serial non-file device.

 -- procedure: char-ready?
 -- procedure: char-ready? port

     Returns #t if a character is ready on the input PORT and returns #f otherwise.  If char-ready? returns #t then the next read-char operation on the given PORT is guaranteed not to hang.  If the PORT is at end of file then char-ready? returns #t. PORT may be omitted, in which case it defaults to the value returned by current-input-port.

     Rationale Char-ready? exists to make it possible for a program to accept characters from interactive ports without getting stuck waiting for input.  Any input editors associated with such ports must ensure that characters whose existence has been asserted by char-ready? cannot be rubbed out.  If char-ready? were to return #f at end of file, a port at end of file would be indistinguishable from an interactive port that has no ready characters.

 -- procedure: wait-for-input x
 -- procedure: wait-for-input x port1 ...
     Returns a list those ports PORT1 ... which are char-ready?. If none of PORT1 ... become char-ready? within the time interval of X seconds, then #f is returned.  The PORT1 ... arguments may be omitted, in which case they default to the list of the value returned by current-input-port.

Port Redirection
----------------

 -- Function: current-error-port
     Returns the current port to which diagnostic output is directed.

 -- Function: with-error-to-file string thunk
     THUNK must be a procedure of no arguments, and string must be a string naming a file.  The file is opened for output, an output port connected to it is made the default value returned by current-error-port, and the THUNK is called with no arguments.  When the thunk returns, the port is closed and the previous default is restored.  With-error-to-file returns the value yielded by THUNK.

 -- Function: with-input-from-port port thunk
 -- Function: with-output-to-port port thunk
 -- Function: with-error-to-port port thunk
     These routines differ from with-input-from-file, with-output-to-file, and with-error-to-file in that the first argument is a port, rather than a string naming a file.

 -- Function: call-with-outputs thunk proc
     Calls the THUNK procedure while the current-output-port and current-error-port are directed to string-ports.  If THUNK returns, the PROC procedure is called with the output-string, the error-string, and the value returned by THUNK.  If THUNK does not return a value (perhaps because of error), PROC is called with just the output-string and the error-string as arguments.

Soft Ports
----------

A soft-port is a port based on a vector of procedures capable of accepting or delivering characters.  It allows emulation of I/O ports.

 -- Function: make-soft-port vector modes
     Returns a port capable of receiving or delivering characters as specified by the MODES string (see open-file). VECTOR must be a vector of length 5.  Its components are as follows:

      0. procedure accepting one character for output

      1. procedure accepting a string for output

      2. thunk for flushing output

      3. thunk for getting one character

      4. thunk for closing port (not by garbage collection)

     For an output-only port only elements 0, 1, 2, and 4 need be procedures.  For an input-only port only elements 3 and 4 need be procedures.  Thunks 2 and 4 can instead be #f if there is no useful operation for them to perform.

     If thunk 3 returns #f or an eof-object (see eof-object? (Revised(5) Scheme)) it indicates that the port has reached end-of-file.  For example:

     If it is necessary to explicitly close the port when it is garbage collected, (see add-finalizer).

          (define stdout (current-output-port))
          (define p (make-soft-port
                     (vector
                      (lambda (c) (write c stdout))
                      (lambda (s) (display s stdout))
                      (lambda () (display "." stdout))
                      (lambda () (char-upcase (read-char)))
                      (lambda () (display "@" stdout)))
                     "rw"))

          (write p p) => #<input-output-soft#\space45d10#\>

Eval and Load
=============

 -- Function: try-load filename
     If the string FILENAME names an existing file, the try-load procedure reads Scheme source code expressions and definitions from the file and evaluates them sequentially and returns #t.  If not, try-load returns #f.  The try-load procedure does not affect the values returned by current-input-port and current-output-port.

 -- Variable: *load-pathname*
     Is set to the pathname given as argument to load, try-load, and dyn:link (see Compiling And Linking (Hobbit)). *load-pathname* is used to compute the value of program-vicinity (SLIB).

 -- Function: eval obj
     Alias for eval (SLIB).

 -- Function: eval-string str
     Returns the result of reading an expression from STR and evaluating it.  eval-string does not change *load-pathname* or  line-number.

 -- Function: load-string str
     Reads and evaluates all the expressions from STR.  As with load, the value returned is unspecified.  load-string does not change *load-pathname* or line-number.

 -- Function: line-number
     Returns the current line number of the file currently being loaded.

Line Numbers
------------

Scheme code defined by load may optionally contain line number information.  Currently this information is used only for reporting expansion time errors, but in the future run-time error messages may also include line number information.

 -- Function: try-load pathname reader
     This is the primitive for loading, PATHNAME is the name of a file containing Scheme code, and optional argument READER is a function of one argument, a port.  READER should read and return Scheme code as list structure.  The default value is read, which is used if READER is not supplied or is false.

Line number objects are disjoint from integers or other Scheme types. When evaluated or loaded as Scheme code, an s-expression containing a line-number in the car is equivalent to the cdr of the s-expression.  A pair consisting of a line-number in the car and a vector in the cdr is equivalent to the vector.  The meaning of s-expressions with line-numbers in other positions is undefined.

 -- Function: read-numbered port
     Behaves like read, except that

       bullet Load (read) sytnaxes are enabled.

       bullet every s-expression read will be replaced with a cons of a line-number object and the sexp actually read. This replacement is done only if PORT is a tracked port See Files and Ports.

 -- Function: integer->line-number int
     Returns a line-number object with value INT.  INT should be an exact non-negative integer.

 -- Function: line-number->integer linum
     Returns the value of line-number object LINUM as an integer.

 -- Function: line-number? obj
     Returns true if and only if OBJ is a line-number object.

 -- Function: read-for-load port
     Behaves like read, except that load syntaxes are enabled.

 -- Variable: *load-reader*
 -- Variable: *slib-load-reader*
     The value of *load-reader* should be a value acceptable as the second argument to try-load (note that #f is acceptable). This value will be used to read code during calls to scm:load. The value of *slib-load-reader* will similarly be used during calls to slib:load and require.

     In order to disable all line-numbering, it is sufficient to set! *load-reader* and *slib-load-reader* to #f.

Lexical Conventions
===================

Common-Lisp Read Syntax
-----------------------

 -- Read syntax: #\token
     If TOKEN is a sequence of two or more digits, then this syntax is equivalent to #.(integer->char (string->number token 8)).

     If TOKEN is C-, c-, or ^ followed by a character, then this syntax is read as a control character.  If TOKEN is M- or m- followed by a character, then a meta character is read.  c- and m- prefixes may be combined.

 -- Read syntax: #+ feature form
     If feature is provided? (by *features*) then FORM is read as a scheme expression.  If not, then FORM is treated as whitespace.

     Feature is a boolean expression composed of symbols and and, or, and not of boolean expressions.

     For more information on provided? and *features*, see Require (SLIB).

 -- Read syntax: #- feature form
     is equivalent to #+(not feature) expression.

 -- Read syntax: #| any thing |#
     Is a balanced comment.  Everything up to the matching |# is ignored by the read.  Nested #|...|# can occur inside ANY THING.

Load sytax is Read syntax enabled for read only when that read is part of loading a file or string.  This distinction was made so that reading from a datafile would not be able to corrupt a scheme program using #..

 -- Load syntax: #. expression
     Is read as the object resulting from the evaluation of EXPRESSION. This substitution occurs even inside quoted structure.

     In order to allow compiled code to work with #. it is good practice to define those symbols used inside of EXPRESSION with #.(define ...).  For example:

          #.(define foo 9)                        => #<unspecified>
          '(#.foo #.(+ foo foo))                  => (9 18)

 -- Load syntax: #' form
     is equivalent to FORM (for compatibility with common-lisp).

Load Syntax
-----------

#! is the unix mechanism for executing scripts. See Unix Scheme Scripts for the full description of how this comment supports scripting.

 -- Load syntax: #?line
 -- Load syntax: #?column
     Return integers for the current line and column being read during a load.

 -- Load syntax: #?file
     Returns the string naming the file currently being loaded.  This path is the string passed to load, possibly with .scm appended.

Documentation and Comments
--------------------------

 -- procedure: procedure-documentation proc
     Returns the documentation string of PROC if it exists, or #f if not.

     If the body of a lambda (or the definition of a procedure) has more than one expression, and the first expression (preceeding any internal definitions) is a string, then that string is the documentation string of that procedure.

          (procedure-documentation (lambda (x) "Identity" x)) => "Identity"
          (define (square x)
              "Return the square of X."
              (* x x))
          => #<unspecified>
          (procedure-documentation square) => "Return the square of X."

 -- Function: comment string1 ...
     Appends STRING1 ... to the strings given as arguments to previous calls comment.

 -- Function: comment
     Returns the (appended) strings given as arguments to previous calls comment and empties the current string collection.

 -- Load syntax: #;text-till-end-of-line
     Behaves as (comment "TEXT-TILL-END-OF-LINE").

Modifying Read Syntax
---------------------

 -- Callback procedure: read:sharp c port
     If a # followed by a character (for a non-standard syntax) is encountered by read, read will call the value of the symbol read:sharp with arguments the character and the port being read from.  The value returned by this function will be the value of read for this expression unless the function returns #<unspecified> in which case the expression will be treated as whitespace.  #<unspecified> is the value returned by the expression (if #f #f).

 -- Callback procedure: load:sharp c port
     Dispatches like read:sharp, but only during loads.  The read-syntaxes handled by load:sharp are a superset of those handled by read:sharp.  load:sharp calls read:sharp if none of its syntaxes match C.

 -- Callback procedure: char:sharp token
     If the sequence #\ followed by a non-standard character name is encountered by read, read will call the value of the symbol char:sharp with the token (a string of length at least two) as argument.  If the value returned is a character, then that will be the value of read for this expression, otherwise an error will be signaled.

Note When adding new # syntaxes, have your code save the previous value of load:sharp, read:sharp, or char:sharp when defining it.  Call this saved value if an invocation's syntax is not recognized.  This will allow #+, #-, and Uniform Arrays to still be supported (as they dispatch from read:sharp).

Syntax
======

SCM provides a native implementation of defmacro. See Defmacro (SLIB).

When built with -F macro build option (see Build Options) and *syntax-rules* is non-false, SCM also supports [R5RS] syntax-rules macros.  See Macros (Revised(5) Scheme).

Other Scheme Syntax Extension Packages from SLIB can be employed through the use of macro:eval and macro:load; Or by using the SLIB read-eval-print-loop:

     (require 'repl)
     (repl:top-level macro:eval)

With the appropriate catalog entries (see Library Catalogs (SLIB)), files using macro packages will automatically use the correct macro loader when required.

Define and Set
--------------

 -- Special Form: defined? symbol
     Equivalent to #t if SYMBOL is a syntactic keyword (such as if) or a symbol with a value in the top level environment (see Variables and regions (Revised(5) Scheme)).  Otherwise equivalent to #f.

 -- Special Form: defvar identifier initial-value
     If IDENTIFIER is unbound in the top level environment, then IDENTIFIER is defined to the result of evaluating the form INITIAL-VALUE as if the defvar form were instead the form (define identifier initial-value) .  If IDENTIFIER already has a value, then INITIAL-VALUE is not evaluated and IDENTIFIER's value is not changed.  defvar is valid only when used at top-level.

 -- Special Form: defconst identifier value
     If IDENTIFIER is unbound in the top level environment, then IDENTIFIER is defined to the result of evaluating the form VALUE as if the defconst form were instead the form (define identifier value) .  If IDENTIFIER already has a value, then VALUE is not evaluated, IDENTIFIER's value is not changed, and an error is signaled.  defconst is valid only when used at top-level.

 -- Special Form: set! (variable1 variable2 ...) <expression>

     The identifiers VARIABLE1, VARIABLE2, ... must be bound either in some region enclosing the set! expression or at top level.

     <Expression> is evaluated, and the elements of the resulting list are stored in the locations to which each corresponding VARIABLE is bound.  The result of the set! expression is unspecified.

          (define x 2)
          (define y 3)
          (+ x y)                              => 5
          (set! (x y) (list 4 5))              => unspecified
          (+ x y)                              => 9

 -- Special Form: qase key clause1 clause2 ...
     qase is an extension of standard Scheme case: Each CLAUSE of a qase statement must have as first element a list containing elements which are:

       * literal datums, or

       * a comma followed by the name of a symbolic constant, or

       * a comma followed by an at-sign (@) followed by the name of a symbolic constant whose value is a list.

     A qase statement is equivalent to a case statement in which these symbolic constants preceded by commas have been replaced by the values of the constants, and all symbolic constants preceded by comma-at-signs have been replaced by the elements of the list values of the constants.  This use of comma, (or, equivalently, unquote) is similar to that of quasiquote except that the unquoted expressions must be symbolic constants.

     Symbolic constants are defined using defconst, their values are substituted in the head of each qase clause during macro expansion.  defconst constants should be defined before use. qase can be substituted for any correct use of case.

          (defconst unit '1)
          (defconst semivowels '(w y))
          (qase (* 2 3)
            ((2 3 5 7) 'prime)
            ((,unit 4 6 8 9) 'composite))        ==>  composite
          (qase (car '(c d))
            ((a) 'a)
            ((b) 'b))                            ==>  unspecified
          (qase (car '(c d))
            ((a e i o u) 'vowel)
            ((,@semivowels) 'semivowel)
            (else 'consonant))                   ==>  consonant

Defmacro
--------

SCM supports the following constructs from Common Lisp: defmacro, macroexpand, macroexpand-1, and gentemp.  See Defmacro (SLIB).

SCM defmacro is extended over that described for SLIB:

     (defmacro (macro-name . arguments) body)

is equivalent to

     (defmacro macro-name arguments body)

As in Common Lisp, an element of the formal argument list for defmacro may be a possibly nested list, in which case the corresponding actual argument must be a list with as many members as the formal argument.  Rest arguments are indicated by improper lists, as in Scheme.  It is an error if the actual argument list does not have the tree structure required by the formal argument list.

For example:

     (defmacro (let1 ((name value)) . body)
         `((lambda (,name) ,@body) ,value))

     (let1 ((x (foo))) (print x) x) == ((lambda (x) (print x) x) (foo))

     (let1 not legal syntax) error--> not "does not match" ((name value))

Syntax-Rules
------------

SCM supports [R5RS] syntax-rules macros see Macros (Revised(5) Scheme).

The pattern language is extended by the syntax (... <obj>), which is identical to <obj> except that ellipses in <obj> are treated as ordinary identifiers in a template, or as literals in a pattern.  In particular, (... ...) quotes the ellipsis token ... in a pattern or template.

For example:

     (define-syntax check-tree
       (syntax-rules ()
         ((_ (?pattern (... ...)) ?obj)
          (let loop ((obj ?obj))
            (or (null? obj)
                (and (pair? obj)
                     (check-tree ?pattern (car obj))
                     (loop (cdr obj))))))
         ((_ (?first . ?rest) ?obj)
          (let ((obj ?obj))
            (and (pair? obj)
                 (check-tree ?first (car obj))
                 (check-tree ?rest (cdr obj)))))
         ((_ ?atom ?obj) #t)))

     (check-tree ((a b) ...) '((1 2) (3 4) (5 6))) => #t

     (check-tree ((a b) ...) '((1 2) (3 4) not-a-2list) => #f

Note that although the ellipsis is matched as a literal token in the defined macro it is not included in the literals list for syntax-rules.

The pattern language is also extended to support identifier macros.  A reference to an identifier macro keyword that is not the first identifier in a form may expand into Scheme code, rather than raising a ``keyword as variable'' error.  The pattern for expansion of such a bare macro keyword is a single identifier, as in other syntax rules the identifier is ignored.

For example:

     (define-syntax eight
         (syntax-rules ()
           (_ 8)))

     (+ 3 eight) => 11
     (eight) => ERROR
     (set! eight 9) => ERROR

Macro Primitives
----------------

 -- Function: procedure->syntax proc
     Returns a macro which, when a symbol defined to this value appears as the first symbol in an expression, returns the result of applying PROC to the expression and the environment.

 -- Function: procedure->macro proc
 -- Function: procedure->memoizing-macro proc
 -- Function: procedure->identifier-macro
     Returns a macro which, when a symbol defined to this value appears as the first symbol in an expression, evaluates the result of applying PROC to the expression and the environment.  The value returned from PROC which has been passed to PROCEDURE->MEMOIZING-MACRO replaces the form passed to PROC.  For example:

          (defsyntax trace
            (procedure->macro
             (lambda (x env) `(set! ,(cadr x) (tracef ,(cadr x) ',(cadr x))))))

          (trace foo) == (set! foo (tracef foo 'foo)).

     PROCEDURE->IDENTIFIER-MACRO is similar to PROCEDURE->MEMOIZING-MACRO except that PROC is also called in case the symbol bound to the macro appears in an expression but not as the first symbol, that is, when it looks like a variable reference.  In that case, the form passed to PROC is a single identifier.

 -- Special Form: defsyntax name expr
     Defines NAME as a macro keyword bound to the result of evaluating EXPR, which should be a macro.  Using define for this purpose may not result in NAME being interpreted as a macro keyword.

Environment Frames
------------------

An environment is a list of frames representing lexical bindings. Only the names and scope of the bindings are included in environments passed to macro expanders -- run-time values are not included.

There are several types of environment frames:

((lambda (variable1 ...) ...) value1 ...)
(let ((variable1 value1) (variable2 value2) ...) ...)
(letrec ((variable1 value1)  ...) ...)
     result in a single enviroment frame:

          (variable1 variable2 ...)

(let ((variable1 value1)) ...)
(let* ((variable1 value1) ...) ...)
     result in an environment frame for each variable:

          variable1 variable2 ...

(let-syntax ((key1 macro1) (key2 macro2)) ...)
(letrec-syntax ((key1 value1) (key2 value2)) ...)
     Lexically bound macros result in environment frames consisting of a marker and an alist of keywords and macro objects:

          (<env-syntax-marker> (key1 . value1) (key2 . value2))

     Currently <env-syntax-marker> is the integer 6.

line numbers
     Line numbers (see Line Numbers) may be included in the environment as frame entries to indicate the line number on which a function is defined.  They are ignored for variable lookup.

          #<line 8>

miscellaneous
     Debugging information is stored in environments in a plist format: Any exact integer stored as an environment frame may be followed by any value.  The two frame entries are ignored when doing variable lookup. Load file names, procedure names, and closure documentation strings are stored in this format.

          <env-filename-marker> "foo.scm" <env-procedure-name-marker> foo ...

     Currently <env-filename-marker> is the integer 1 and <env-procedure-name-marker> the integer 2.

 -- Special Form: @apply procedure argument-list
     Returns the result of applying PROCEDURE to ARGUMENT-LIST. @apply differs from apply when the identifiers bound by the closure being applied are set!; setting affects ARGUMENT-LIST.

          (define lst (list 'a 'b 'c))
          (@apply (lambda (v1 v2 v3) (set! v1 (cons v2 v3))) lst)
          lst           => ((b . c) b c)

     Thus a mutable environment can be treated as both a list and local bindings.

Syntactic Hooks for Hygienic Macros
-----------------------------------

SCM provides a synthetic identifier type for efficient implementation of hygienic macros (for example, syntax-rules see Macros (Revised(5) Scheme)) A synthetic identifier may be inserted in Scheme code by a macro expander in any context where a symbol would normally be used.  Collectively, symbols and synthetic identifiers are identifiers.

 -- Function: identifier? obj
     Returns #t if OBJ is a symbol or a synthetic identifier, and #f otherwise.

If it is necessary to distinguish between symbols and synthetic identifiers, use the predicate symbol?.

A synthetic identifier includes two data: a parent, which is an identifier, and an environment, which is either #f or a lexical environment which has been passed to a macro expander (a procedure passed as an argument to procedure->macro, procedure->memoizing-macro, or procedure->syntax).

 -- Function: renamed-identifier parent env
     Returns a synthetic identifier.  PARENT must be an identifier, and ENV must either be #f or a lexical environment passed to a macro expander.  renamed-identifier returns a distinct object for each call, even if passed identical arguments.

There is no direct way to access all of the data internal to a synthetic identifier, those data are used during variable lookup.  If a synthetic identifier is inserted as quoted data then during macro expansion it will be repeatedly replaced by its parent, until a symbol is obtained.

 -- Function: identifier->symbol id
     Returns the symbol obtained by recursively extracting the parent of ID, which must be an identifier.

Use of Synthetic Identifiers
----------------------------

renamed-identifier may be used as a replacement for gentemp:

     (define gentemp
       (let ((name (string->symbol "An unlikely variable")))
         (lambda ()
           (renamed-identifier name #f))))

If an identifier returned by this version of gentemp is inserted in a binding position as the name of a variable then it is guaranteed that no other identifier (except one produced by passing the first to renamed-identifier) may denote that variable.  If an identifier returned by gentemp is inserted free, then it will denote the top-level value bound to its parent, the symbol named ``An unlikely variable''.  This behavior, of course, is meant to be put to good use:

     (define top-level-foo
       (procedure->memoizing-macro
        (lambda (exp env)
          (renamed-identifier 'foo #f))))

Defines a macro which may always be used to refer to the top-level binding of foo.

     (define foo 'top-level)
     (let ((foo 'local))
       (top-level-foo))  => top-level

In other words, we can avoid capturing foo.

If a lexical environment is passed as the second argument to renamed-identifier then if the identifier is inserted free its parent will be looked up in that environment, rather than in the top-level environment.  The use of such an identifier must be restricted to the lexical scope of its environment.

There is another restriction imposed for implementation convenience: Macros passing their lexical environments to renamed-identifier may be lexically bound only by the special forms let-syntax or letrec-syntax.  No error is signaled if this restriction is not met, but synthetic identifier lookup will not work properly.

In order to maintain referential transparency it is necessary to determine whether two identifiers have the same denotation.  With synthetic identifiers it is not necessary that two identifiers be eq? in order to denote the same binding.

 -- Function: identifier-equal? id1 id2 env
     Returns #t if identifiers ID1 and ID2 denote the same binding in lexical environment ENV, and #f otherwise. ENV must either be a lexical environment passed to a macro transformer during macro expansion or the empty list.

     For example,

          (define top-level-foo?
            (procedure->memoizing-macro
             (let ((foo-name (renamed-identifier 'foo #f)))
               (lambda (exp env)
                 (identifier-equal? (cadr exp) foo-name env)))))

          (top-level-foo? foo)  => #t

          (let ((foo 'local))
            (top-level-foo? foo))  => #f

 -- Function: @macroexpand1 expr env
     If the car of EXPR denotes a macro in ENV, then if that macro is a primitive, EXPR will be returned, if the macro was defined in Scheme, then a macro expansion will be returned. If the car of EXPR does not denote a macro, the #f is returned.

 -- Function: extended-environment names values env
     Returns a new environment object, equivalent to ENV, which must either be an environment object or null, extended by one frame. NAMES must be an identifier, or an improper list of identifiers, usable as a formals list in a lambda expression.  VALUES must be a list of objects long enough to provide a binding for each of the identifiers in NAMES.  If NAMES is an identifier or an improper list then VALS may be, respectively, any object or an improper list of objects.

 -- Special Form: syntax-quote obj
     Synthetic identifiers are converted to their parent symbols by quote and quasiquote so that literal data in macro definitions will be properly transcribed.  syntax-quote behaves like quote, but preserves synthetic identifier intact.

 -- Special Form: the-macro mac
     the-macro is the simplest of all possible macro transformers: MAC may be a syntactic keyword (macro name) or an expression evaluating to a macro, otherwise an error is signaled.  MAC is evaluated and returned once only, after which the same memoizied value is returned.

     the-macro may be used to protect local copies of macros against redefinition, for example:

          (@let-syntax ((let (the-macro let)))
             ;; code that will continue to work even if LET is redefined.
                  ...)

 -- Special Form: renaming-transformer proc
     A low-level ``explicit renaming'' macro facility very similar to that proposed by W. Clinger [Exrename] is supported.  Syntax may be defined in define-syntax, let-syntax, and letrec-syntax using renaming-transformer instead of syntax-rules. PROC should evaluate to a procedure accepting three arguments: EXPR, RENAME, and COMPARE.  EXPR is a representation of Scheme code to be expanded, as list structure. RENAME is a procedure accepting an identifier and returning an identifier renamed in the definition environment of the new syntax. COMPARE accepts two identifiers and returns true if and only if both denote the same binding in the usage environment of the new syntax.

Packages
********

Dynamic Linking
===============

If SCM has been compiled with dynl.c then the additional properties of load and ([SLIB]) require specified here are supported. The require form is preferred.

 -- Function: require feature
     If the symbol FEATURE has not already been given as an argument to require, then the object and library files associated with FEATURE will be dynamically-linked, and an unspecified value returned.  If FEATURE is not found in *catalog*, then an error is signaled.

 -- Function: usr:lib lib
     Returns the pathname of the C library named LIB.  For example: (usr:lib "m") returns "/usr/lib/libm.a", the path of the C math library.

 -- Function: x:lib lib
     Returns the pathname of the X library named LIB.  For example: (x:lib "X11") returns "/usr/X11/lib/libX11.sa", the path of the X11 library.

 -- Function: load filename lib1 ...
     In addition to the [R5RS] requirement of loading Scheme expressions if FILENAME is a Scheme source file, load will also dynamically load/link object files (produced by compile-file, for instance).  The object-suffix need not be given to load.  For example,

          (load (in-vicinity (implementation-vicinity) "sc2"))
          or (load (in-vicinity (implementation-vicinity) "sc2.o"))
          or (require 'rev2-procedures)
          or (require 'rev3-procedures)

     will load/link sc2.o if it exists.

     The LIB1 ... pathnames specify additional libraries which may be needed for object files not produced by the Hobbit compiler.  For instance, crs is linked on Linux by

          (load (in-vicinity (implementation-vicinity) "crs.o")
                (usr:lib "ncurses") (usr:lib "c"))
          or (require 'curses)

     Turtlegr graphics library is linked by:

          (load (in-vicinity (implementation-vicinity) "turtlegr")
                (usr:lib "X11") (usr:lib "c") (usr:lib "m"))
          or (require 'turtle-graphics)

     And the string regular expression (see Regular Expression Pattern Matching) package is linked by:

          (load (in-vicinity (implementation-vicinity) "rgx") (usr:lib "c"))

     or

          (require 'regex)

The following functions comprise the low-level Scheme interface to dynamic linking.  See the file Link.scm in the SCM distribution for an example of their use.

 -- Function: dyn:link filename
     FILENAME should be a string naming an object or archive file, the result of C-compiling.  The dyn:link procedure links and loads FILENAME into the current SCM session. If successfull, dyn:link returns a link-token suitable for passing as the second argument to dyn:call.  If not successful, #f is returned.

 -- Function: dyn:call name link-token
     LINK-TOKEN should be the value returned by a call to dyn:link.  NAME should be the name of C function of no arguments defined in the file named FILENAME which was succesfully dyn:linked in the current SCM session.  The dyn:call procedure calls the C function corresponding to NAME.  If successful, dyn:call returns #t; If not successful, #f is returned.

     dyn:call is used to call the init_... function after loading SCM object files.  The init_... function then makes the identifiers defined in the file accessible as Scheme procedures.

 -- Function: dyn:main-call name link-token arg1 ...
     LINK-TOKEN should be the value returned by a call to dyn:link.  NAME should be the name of C function of 2 arguments, (int argc, char **argv), defined in the file named FILENAME which was succesfully dyn:linked in the current SCM session.  The dyn:main-call procedure calls the C function corresponding to NAME with argv style arguments, such as are given to C main functions.  If successful, dyn:main-call returns the integer returned from the call to NAME.

     dyn:main-call can be used to call a main procedure from SCM.  For example, I link in and dyn:main-call a large C program, the low level routines of which callback (see Callbacks) into SCM (which emulates PCI hardware).

 -- Function: dyn:unlink link-token
     LINK-TOKEN should be the value returned by a call to dyn:link.  The dyn:unlink procedure removes the previously loaded file from the current SCM session.  If successful, dyn:unlink returns #t; If not successful, #f is returned.

Dump
====

Dump, (also known as unexec), saves the continuation of an entire SCM session to an executable file, which can then be invoked as a program.  Dumped executables start very quickly, since no Scheme code has to be loaded.

There are constraints on which sessions are savable using dump

  * Saved continuations are invalid in subsequent invocations; they cause segmentation faults and other unpleasant side effects.

  * Although DLD (see Dynamic Linking) can be used to load compiled modules both before and after dumping, SUN_DL ELF systems can load compiled modules only after dumping.  This can be worked around by compiling in those features you wish to dump.

  * Ports (other than current-input-port, current-output-port, current-error-port), X windows, etc. are invalid in subsequent invocations.

     This restriction could be removed; see Improvements To Make.

  * Dump should only be called from a loading file when the call to dump is the last expression in that file.

  * Dump can be called from the command line.

 -- Function: dump newpath
 -- Function: dump newpath #f
 -- Function: dump newpath #t
 -- Function: dump newpath thunk

       * Calls gc.

       * Creates an executable program named NEWPATH which continues the state of the current SCM session when invoked.  The optional argument THUNK, if provided, should be a procedure of no arguments; BOOT-TAIL will be set to this procedure, causing it to be called in the restored executable.

          If the optional argument is missing or a boolean, SCM's standard command line processing will be called in the restored executable.

          If the second argument to dump is #t, argument processing will continue from the command line passed to the dumping session.  If the second argument is missing or #f then the command line arguments of the restoring invocation will be processed.

       * Resumes the top level Read-Eval-Print loop.  This is done instead of continuing normally to avoid creating a saved continuation in the dumped executable.

     dump may set the values of boot-tail, *argv*, restart, and *INTERACTIVE*.  dump returns an unspecified value.

When a dumped executable is invoked, the variable *INTERACTIVE* (see Internal State) has the value it possessed when dump created it.  Calling dump with a single argument sets *INTERACTIVE* to #f, which is the state it has at the beginning of command line processing.

The procedure program-arguments returns the command line arguments for the curent invocation.  More specifically, program-arguments for the restored session are not saved from the dumping session.  Command line processing is done on the value of the identifier *argv*.

The following example shows how to create rscm, which is like regular scm, but which loads faster and has the random package alreadly provided.

     bash$ scm -rrandom
     > (dump "rscm")
     #<unspecified>
     > (quit)
     bash$ ./rscm -lpi.scm -e"(pi (random 200) 5)"
     00003 14159 26535 89793 23846 26433 83279 50288 41971 69399
     37510 58209 74944 59230 78164 06286 20899 86280 34825 34211
     70679 82148 08651 32823 06647 09384 46095 50582 23172 53594
     08128 48111 74502 84102 70193 85211 05559 64462 29489
     bash$

This task can also be accomplished using the -o command line option (see SCM Options).

     bash$ scm -rrandom -o rscm
     > (quit)
     bash$ ./rscm -lpi.scm -e"(pi (random 200) 5)"
     00003 14159 26535 89793 23846 26433 83279 50288 41971 69399
     37510 58209 74944 59230 78164 06286 20899 86280 34825 34211
     70679 82148 08651 32823 06647 09384 46095 50582 23172 53594
     08128 48111 74502 84102 70193 85211 05559 64462 29489
     bash$

Numeric
=======

 -- Constant: most-positive-fixnum
     The immediate integer closest to positive infinity. See Configuration (SLIB).

 -- Constant: most-negative-fixnum
     The immediate integer closest to negative infinity.

 -- Constant: $pi
 -- Constant: pi
     The ratio of the circumference to the diameter of a circle.

These procedures augment the standard capabilities in Numerical operations (Revised(5) Scheme).

 -- Function: pi* z
     (* pi Z)

 -- Function: pi/ z
     (/ pi Z)

 -- Function: sinh z
 -- Function: cosh z
 -- Function: tanh z
     Return the hyperbolic sine, cosine, and tangent of Z

 -- Function: asinh z
 -- Function: acosh z
 -- Function: atanh z
     Return the inverse hyperbolic sine, cosine, and tangent of Z

 -- Function: $sqrt x
 -- Function: $abs x
 -- Function: $exp x
 -- Function: $log x
 -- Function: $sin x
 -- Function: $cos x
 -- Function: $tan x
 -- Function: $asin x
 -- Function: $acos x
 -- Function: $atan x

 -- Function: $sinh x
 -- Function: $cosh x
 -- Function: $tanh x
 -- Function: $asinh x
 -- Function: $acosh x
 -- Function: $atanh x
     Real-only versions of these popular functions.  The argument X must be a real number.  It is an error if the value which should be returned by a call to these procedures is not real.

 -- Function: $log10 x
     Real-only base 10 logarithm.

 -- Function: $atan2 y x
     Computes (angle (make-rectangular x y)) for real numbers Y and X.

 -- Function: $expt x1 x2
     Returns real number X1 raised to the real power X2.  It is an error if the value which should be returned by a call to $expt is not real.

Arrays
======

Conventional Arrays
-------------------

The following syntax and procedures are SCM extensions to feature array in Arrays (SLIB).

Arrays read and write as a # followed by the rank (number of dimensions) followed by the character #\a or #\A and what appear as lists (of lists) of elements.  The lists must be nested to the depth of the rank.  For each depth, all lists must be the same length.

     (make-array '#(ho) 4 3) =>
     #2A((ho ho ho) (ho ho ho) (ho ho ho) (ho ho ho))

Unshared, conventional (not uniform) 0-based arrays of rank 1 are equivalent to (and can't be distinguished from) scheme vectors.

     (make-array '#(ho) 3) => #(ho ho ho)

 -- Function: transpose-array array dim0 dim1 ...
     Returns an array sharing contents with ARRAY, but with dimensions arranged in a different order.  There must be one DIM argument for each dimension of ARRAY.  DIM0, DIM1, ... should be integers between 0 and the rank of the array to be returned.  Each integer in that range must appear at least once in the argument list.

     The values of DIM0, DIM1, ... correspond to dimensions in the array to be returned, their positions in the argument list to dimensions of ARRAY.  Several DIMs may have the same value, in which case the returned array will have smaller rank than ARRAY.

     examples:

          (transpose-array '#2A((a b) (c d)) 1 0) => #2A((a c) (b d))
          (transpose-array '#2A((a b) (c d)) 0 0) => #1A(a d)
          (transpose-array '#3A(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) =>
                          #2A((a 4) (b 5) (c 6))

 -- Function: enclose-array array dim0 dim1 ...
     DIM0, DIM1 ... should be nonnegative integers less than the rank of ARRAY.  ENCLOSE-ARRAY returns an array resembling an array of shared arrays.  The dimensions of each shared array are the same as the DIMth dimensions of the original array, the dimensions of the outer array are the same as those of the original array that did not match a DIM.

     An enclosed array is not a general Scheme array.  Its elements may not be set using array-set!.  Two references to the same element of an enclosed array will be equal? but will not in general be eq?.  The value returned by ARRAY-PROTOTYPE when given an enclosed array is unspecified.

     examples:

          (enclose-array '#3A(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1) =>
             #<enclosed-array (#1A(a d) #1A(b e) #1A(c f)) (#1A(1 4) #1A(2 5) #1A(3 6))>

          (enclose-array '#3A(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 0) =>
             #<enclosed-array #2A((a 1) (d 4)) #2A((b 2) (e 5)) #2A((c 3) (f 6))>

 -- Function: array->list array
     Returns a list consisting of all the elements, in order, of ARRAY. In the case of a rank-0 array, returns the single element.

 -- Function: array-contents array
 -- Function: array-contents array strict
     If ARRAY may be unrolled into a one dimensional shared array without changing their order (last subscript changing fastest), then array-contents returns that shared array, otherwise it returns #f.  All arrays made by MAKE-ARRAY may be unrolled, some arrays made by MAKE-SHARED-ARRAY may not be.

     If the optional argument STRICT is provided, a shared array will be returned only if its elements are stored internally contiguous in memory.

Uniform Array
-------------

Uniform Arrays and vectors are arrays whose elements are all of the same type.  Uniform vectors occupy less storage than conventional vectors.  Uniform Array procedures also work on vectors, uniform-vectors, bit-vectors, and strings.

SLIB now supports uniform arrys.  The primary array creation procedure is make-array, detailed in see Arrays (SLIB).

Unshared uniform character 0-based arrays of rank 1 (dimension) are equivalent to (and can't be distinguished from) strings.

     (make-array "" 3) => "$q2"

Unshared uniform boolean 0-based arrays of rank 1 (dimension) are equivalent to (and can't be distinguished from) bit-vectors.

     (make-array '#1at() 3) => #*000
     ==
     #1At(#f #f #f) => #*000

PROTOTYPE arguments in the following procedures are interpreted according to the table:

     prototype       type                              display prefix

     ()              conventional vector                     #A
     +64i            complex (double precision)              #A:floC64b
     64.0            double (double precision)               #A:floR64b
     32.0            float (single precision)                #A:floR32b
     32              unsigned integer (32-bit)               #A:fixN32b
     -32             signed integer (32-bit)                 #A:fixZ32b
     -16             signed integer (16-bit)                 #A:fixZ16b
     #\a             char (string)                           #A:char
     #t              boolean (bit-vector)                    #A:bool

Other uniform vectors are written in a form similar to that of general arrays, except that one or more modifying characters are put between the #\A character and the contents list.  For example, '#1A:fixZ32b(3 5 9) returns a uniform vector of signed integers.

 -- Function: array? obj prototype
     Returns #t if the OBJ is an array of type corresponding to PROTOTYPE, and #f if not.

 -- Function: array-prototype array
     Returns an object that would produce an array of the same type as ARRAY, if used as the PROTOTYPE for list->uniform-array.

 -- Function: list->uniform-array rank prot lst
     Returns a uniform array of the type indicated by prototype PROT with elements the same as those of LST.  Elements must be of the appropriate type, no coercions are done.

     In, for example, the case of a rank-2 array, LST must be a list of lists, all of the same length.  The length of LST will be the first dimension of the result array, and the length of each element the second dimension.

     If RANK is zero, LST, which need not be a list, is the single element of the returned array.

 -- Function: uniform-array-read! ura
 -- Function: uniform-array-read! ura port
     Attempts to read all elements of URA, in lexicographic order, as binary objects from PORT.  If an end of file is encountered during uniform-array-read! the objects up to that point only are put into URA (starting at the beginning) and the remainder of the array is unchanged.

     uniform-array-read! returns the number of objects read. PORT may be omitted, in which case it defaults to the value returned by (current-input-port).

 -- Function: uniform-array-write ura
 -- Function: uniform-array-write ura port
     Writes all elements of URA as binary objects to PORT.  The number of of objects actually written is returned.  PORT may be omitted, in which case it defaults to the value returned by (current-output-port).

 -- Function: logaref array index1 index2 ...
     If an INDEX is provided for each dimension of ARRAY returns the INDEX1, INDEX2, ...'th element of ARRAY.  If one more INDEX is provided, then the last index specifies bit position of the twos-complement representation of the array element indexed by the other INDEXs returning #t if the bit is 1, and #f if 0.  It is an error if this element is not an exact integer.

          (logaref '#(#b1101 #b0010) 0)       => #b1101
          (logaref '#(#b1101 #b0010) 0 1)     => #f
          (logaref '#2((#b1101 #b0010)) 0 0)  => #b1101

 -- Function: logaset! array val index1 index2 ...
     If an INDEX is provided for each dimension of ARRAY sets the INDEX1, INDEX2, ...'th element of ARRAY to VAL.  If one more INDEX is provided, then the last index specifies bit position of the twos-complement representation of an exact integer array element, setting the bit to 1 if VAL is #t and to 0 if VAL is #f.  In this case it is an error if the array element is not an exact integer or if VAL is not boolean.

Bit Vectors
-----------

Bit vectors can be written and read as a sequence of 0s and 1s prefixed by #*.

     #1At(#f #f #f #t #f #t #f) => #*0001010

Some of these operations will eventually be generalized to other uniform-arrays.

 -- Function: bit-count bool bv
     Returns the number of occurrences of BOOL in BV.

 -- Function: bit-position bool bv k
     Returns the minimum index of an occurrence of BOOL in BV which is at least K.  If no BOOL occurs within the specified range #f is returned.

 -- Function: bit-invert! bv
     Modifies BV by replacing each element with its negation.

 -- Function: bit-set*! bv uve bool
     If uve is a bit-vector BV and uve must be of the same length.  If BOOL is #t, uve is OR'ed into BV; If BOOL is #f, the inversion of uve is AND'ed into BV.

     If uve is a unsigned integer vector all the elements of uve must be between 0 and the LENGTH of BV.  The bits of BV corresponding to the indexes in uve are set to BOOL.

     The return value is unspecified.

 -- Function: bit-count* bv uve bool
     Returns

          (bit-count (bit-set*! (if bool bv (bit-invert! bv)) uve #t) #t).

     BV is not modified.

Array Mapping
-------------

(require 'array-for-each)

SCM has some extra functions in feature array-for-each:

 -- Function: array-fill! array fill
     Stores FILL in every element of ARRAY.  The value returned is unspecified.

 -- Function: serial-array:copy! destination source
     Same as array:copy! but guaranteed to copy in row-major order.

 -- Function: array-equal? array0 array1 ...
     Returns #t iff all arguments are arrays with the same shape, the same type, and have corresponding elements which are either equal?  or array-equal?.  This function differs from equal? in that a one dimensional shared array may be ARRAY-EQUAL? but not EQUAL? to a vector or uniform vector.

 -- Function: array-map! array0 proc array1 ...
     If ARRAY1, ... are arrays, they must have the same number of dimensions as ARRAY0 and have a range for each index which includes the range for the corresponding index in ARRAY0.  If they are scalars, that is, not arrays, vectors, or strings, then they will be converted internally to arrays of the appropriate shape. PROC is applied to each tuple of elements of ARRAY1 ... and the result is stored as the corresponding element in ARRAY0.  The value returned is unspecified.  The order of application is unspecified.

     Handling non-array arguments is a SCM extension of array-map! (SLIB)

 -- Function: serial-array-map! array0 proc array1 ...
     Same as ARRAY-MAP!, but guaranteed to apply PROC in row-major order.

 -- Function: array-map prototype proc array1 array2 ...
     ARRAY2, ... must have the same number of dimensions as ARRAY1 and have a range for each index which includes the range for the corresponding index in ARRAY1.  PROC is applied to each tuple of elements of ARRAY1, ARRAY2, ... and the result is stored as the corresponding element in a new array of type PROTOTYPE.  The new array is returned.  The order of application is unspecified.

 -- Function: scalar->array scalar array prototype
 -- Function: scalar->array scalar array
     Returns a uniform array of the same shape as ARRAY, having only one shared element, which is eqv? to SCALAR. If the optional argument PROTOTYPE is supplied it will be used as the prototype for the returned array.  Otherwise the returned array will be of the same type as array if that is possible, and a conventional array if it is not.  This function is used internally by array-map! and friends to handle scalar arguments.

Records
=======

SCM provides user-definable datatypes with the same interface as SLIB, see Records (SLIB), with the following extension.

 -- Function: record-printer-set! rtd printer
     Causes records of type RTD to be printed in a user-specified format. RTD must be a record type descriptor returned by make-record-type, PRINTER a procedure accepting three arguments:  the record to be printed, the port to print to, and a boolean which is true if the record is being written on behalf of write and false if for display. If PRINTER returns #f, the default record printer will be called.

     A PRINTER value of #f means use the default printer.

     Only the default printer will be used when printing error messages.

I/O-Extensions
==============

If 'i/o-extensions is provided (by linking in ioext.o), Line I/O (SLIB), and the following functions are defined:

 -- Function: stat <port-or-string>
     Returns a vector of integers describing the argument.  The argument can be either a string or an open input port.  If the argument is an open port then the returned vector describes the file to which the port is opened; If the argument is a string then the returned vector describes the file named by that string.  If there exists no file with the name string, or if the file cannot be accessed #f is returned. The elements of the returned vector are as follows:

     0  st_dev
          ID of device containing a directory entry for this file

     1  st_ino
          Inode number

     2  st_mode
          File type, attributes, and access control summary

     3  st_nlink
          Number of links

     4  st_uid
          User ID of file owner

     5  st_gid
          Group ID of file group

     6  st_rdev
          Device ID; this entry defined only for char or blk spec files

     7  st_size
          File size (bytes)

     8  st_atime
          Time of last access

     9  st_mtime
          Last modification time

     10 st_ctime
          Last file status change time

 -- Function: getpid
     Returns the process ID of the current process.

 -- Function: file-position port
     Returns the current position of the character in PORT which will next be read or written.  If PORT is not open to a file the result is unspecified.

 -- Function: file-set-position port integer
     Sets the current position in PORT which will next be read or written.  If PORT is not open to a file the action of file-set-position is unspecified.  The result of file-set-position is unspecified.

 -- Function: try-create-file name modes perms
     If the file with name NAME already exists, return #f, otherwise try to create and open the file like try-open-file, see Files and Ports.  If the optional integer argument PERMS is provided, it is used as the permissions of the new file (modified by the current umask).

 -- Function: reopen-file filename modes port
     Closes port PORT and reopens it with FILENAME and MODES.  reopen-file returns #t if successful, #f if not.

 -- Function: duplicate-port port modes
     Creates and returns a duplicate port from PORT.  Duplicate unbuffered ports share one file position.  MODES are as for open-file.

 -- Function: redirect-port! from-port to-port
     Closes TO-PORT and makes TO-PORT be a duplicate of FROM-PORT.  redirect-port! returns TO-PORT if successful, #f if not.  If unsuccessful, TO-PORT is not closed.

 -- Function: opendir dirname
     Returns a directory object corresponding to the file system directory named DIRNAME.  If unsuccessful, returns #f.

 -- Function: readdir dir
     Returns the string name of the next entry from the directory DIR. If there are no more entries in the directory, readdir returns a #f.

 -- Function: rewinddir dir
     Reinitializes DIR so that the next call to readdir with DIR will return the first entry in the directory again.

 -- Function: closedir dir
     Closes DIR and returns #t.  If DIR is already closed,, closedir returns a #f.

 -- Function: directory-for-each proc directory
     PROC must be a procedure taking one argument. Directory-For-Each applies PROC to the (string) name of each file in DIRECTORY.  The dynamic order in which PROC is applied to the filenames is unspecified.  The value returned by directory-for-each is unspecified.

 -- Function: directory-for-each proc directory pred
     Applies PROC only to those filenames for which the procedure PRED returns a non-false value.

 -- Function: directory-for-each proc directory match
     Applies PROC only to those filenames for which (filename:match?? MATCH) would return a non-false value (see Filenames (SLIB)).

          (require 'directory)
          (directory-for-each print "." "[A-Z]*.scm")
          -|
          "Init.scm"
          "Iedline.scm"
          "Link.scm"
          "Macro.scm"
          "Transcen.scm"
          "InitSCMVERSION.scm"

 -- Function: mkdir path mode
     The mkdir function creates a new, empty directory whose name is PATH.  The integer argument MODE specifies the file permissions for the new directory.  See The Mode Bits for Access Permission (Gnu C Library), for more information about this.

     mkdir returns if successful, #f if not.

 -- Function: rmdir path
     The rmdir function deletes the directory PATH.  The directory must be empty before it can be removed.  rmdir returns if successful, #f if not.

 -- Function: chdir filename
     Changes the current directory to FILENAME.  If FILENAME does not exist or is not a directory, #f is returned.  Otherwise, #t is returned.

 -- Function: getcwd
     The function getcwd returns a string containing the absolute file name representing the current working directory.  If this string cannot be obtained, #f is returned.

 -- Function: rename-file oldfilename newfilename
     Renames the file specified by OLDFILENAME to NEWFILENAME. If the renaming is successful, #t is returned.  Otherwise, #f is returned.

 -- Function: chmod file mode
     The function chmod sets the access permission bits for the file named by FILE to MODE.  The FILE argument may be a string containing the filename or a port open to the file.

     chmod returns if successful, #f if not.

 -- Function: utime pathname acctime modtime
     Sets the file times associated with the file named PATHNAME to have access time ACCTIME and modification time MODTIME. utime returns if successful, #f if not.

 -- Function: umask mode
     The function umask sets the file creation mask of the current process to MASK, and returns the previous value of the file creation mask.

 -- Function: fileno port
     Returns the integer file descriptor associated with the port PORT. If an error is detected, #f is returned.

 -- Function: access pathname how
     Returns #t if the file named by PATHNAME can be accessed in the way specified by the HOW argument.  The HOW argument can be the logior of the flags:

      0. File-exists?

      1. File-is-executable?

      2. File-is-writable?

      4. File-is-readable?

     Or the HOW argument can be a string of 0 to 3 of the following characters in any order.  The test performed is the and of the associated tests and file-exists?.

     x
          File-is-executable?

     w
          File-is-writable?

     r
          File-is-readable?

 -- Function: execl command arg0 ...
 -- Function: execlp command arg0 ...
     Transfers control to program COMMAND called with arguments ARG0 ....  For execl, COMMAND must be an exact pathname of an executable file.  execlp searches for COMMAND in the list of directories specified by the environment variable PATH.  The convention is that ARG0 is the same name as COMMAND.

     If successful, this procedure does not return.  Otherwise an error message is printed and the integer errno is returned.

 -- Function: execv command arglist
 -- Function: execvp command arglist
     Like execl and execlp except that the set of arguments to COMMAND is ARGLIST.

 -- Function: putenv string
     adds or removes definitions from the environment.  If the STRING is of the form NAME=VALUE, the definition is added to the environment.  Otherwise, the STRING is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed.

     Names of environment variables are case-sensitive and must not contain the character =.  System-defined environment variables are invariably uppercase.

     Putenv is used to set up the environment before calls to execl, execlp, execv, execvp, system, or open-pipe (see open-pipe).

     To access environment variables, use getenv (see getenv (SLIB)).

Posix Extensions
================

If 'posix is provided (by linking in posix.o), the following functions are defined:

 -- Function: open-pipe string modes
     If the string MODES contains an r, returns an input port capable of delivering characters from the standard output of the system command STRING.  Otherwise, returns an output port capable of receiving characters which become the standard input of the system command STRING.  If a pipe cannot be created #f is returned.

 -- Function: open-input-pipe string
     Returns an input port capable of delivering characters from the standard output of the system command STRING.  If a pipe cannot be created #f is returned.

 -- Function: open-output-pipe string
     Returns an output port capable of receiving characters which become the standard input of the system command STRING.  If a pipe cannot be created #f is returned.

 -- Function: broken-pipe port
     If this function is defined at top level, it will be called when an output pipe is closed from the other side (this is the condition under which a SIGPIPE is sent).  The already closed PORT will be passed so that any necessary cleanup may be done.  An error is not signaled when output to a pipe fails in this way, but any further output to the closed pipe will cause an error to be signaled.

 -- Function: close-port pipe
     Closes the PIPE, rendering it incapable of delivering or accepting characters.  This routine has no effect if the pipe has already been closed.  The value returned is unspecified.

 -- Function: pipe
     Returns (cons RD WD) where RD and WD are the read and write (port) ends of a pipe respectively.

 -- Function: fork
     Creates a copy of the process calling fork.  Both processes return from fork, but the calling (parent) process's fork returns the child process's ID whereas the child process's fork returns 0.

For a discussion of IDs see Process Persona (libc).

 -- Function: getppid
     Returns the process ID of the parent of the current process. For a process's own ID see getpid.

 -- Function: getuid
     Returns the real user ID of this process.

 -- Function: getgid
     Returns the real group ID of this process.

 -- Function: getegid
     Returns the effective group ID of this process.

 -- Function: geteuid
     Returns the effective user ID of this process.

 -- Function: setuid id
     Sets the real user ID of this process to ID. Returns #t if successful, #f if not.

 -- Function: setgid id
     Sets the real group ID of this process to ID. Returns #t if successful, #f if not.

 -- Function: setegid id
     Sets the effective group ID of this process to ID. Returns #t if successful, #f if not.

 -- Function: seteuid id
     Sets the effective user ID of this process to ID. Returns #t if successful, #f if not.

 -- Function: kill pid sig
     The kill function sends the signal SIGNUM to the process or process group specified by PID.  Besides the signals listed in Standard Signals (GNU C Library), SIGNUM can also have a value of zero to check the validity of the PID.

     The PID specifies the process or process group to receive the signal:

     > 0
          The process whose identifier is PID.

     0
          All processes in the same process group as the sender.  The sender itself does not receive the signal.

     -1
          If the process is privileged, send the signal to all processes except for some special system processes. Otherwise, send the signal to all processes with the same effective user ID.

     < -1
          The process group whose identifier is (abs PID).

     A process can send a signal to itself with (kill (getpid) SIGNUM).  If kill is used by a process to send a signal to itself, and the signal is not blocked, then kill delivers at least one signal (which might be some other pending unblocked signal instead of the signal SIGNUM) to that process before it returns.

     The return value from kill is zero if the signal can be sent successfully.  Otherwise, no signal is sent, and a value of -1 is returned.  If PID specifies sending a signal to several processes, kill succeeds if it can send the signal to at least one of them. There's no way you can tell which of the processes got the signal or whether all of them did.

 -- Function: waitpid pid options

     The waitpid function suspends execution of the current process until a child as specified by the PID argument has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function.  If a child as requested by PID has already exited by the time of the call (a so-called zombie process), the function returns immediately.  Any system resources used by the child are freed.

     The value of PID can be:

     < -1
          which means to wait for any child process whose process group ID is equal to the absolute value of PID.

     -1
          which means to wait for any child process; this is the same behaviour which wait exhibits.

     0
          which means to wait for any child process whose process group ID is equal to that of the calling process.

     > 0
          which means to wait for the child whose process ID is equal to the value of PID.

     The value of OPTIONS is one of the following:

      0. Nothing special.

      1. (WNOHANG) which means to return immediately if no child is there to be waited for.

      2. (WUNTRACED) which means to also return for children which are stopped, and whose status has not been reported.

      3. Which means both of the above.

     The return value normally is the exit status of the child process, including the exit value along with flags indicating whether a coredump was generated or the child terminated as a result of a signal.  If the WNOHANG option was specified and no child process is waiting to be noticed, the value is zero.  A value of #f is returned in case of error and errno is set.  For information about the errno codes see Process Completion (libc).

 -- Function: uname
     You can use the uname procedure to find out some information about the type of computer your program is running on.

     Returns a vector of strings.  These strings are:

      0. The name of the operating system in use.

      1. The network name of this particular computer.

      2. The current release level of the operating system implementation.

      3. The current version level within the release of the operating system.

      4. Description of the type of hardware that is in use.

          Some examples are "i386-ANYTHING", "m68k-hp", "sparc-sun", "m68k-sun", "m68k-sony" and "mips-dec".

 -- Function: getpw name
 -- Function: getpw uid
 -- Function: getpw
     Returns a vector of information for the entry for NAME, UID, or the next entry if no argument is given.  The information is:

      0. The user's login name.

      1. The encrypted password string.

      2. The user ID number.

      3. The user's default group ID number.

      4. A string typically containing the user's real name, and possibly other information such as a phone number.

      5. The user's home directory, initial working directory, or #f, in which case the interpretation is system-dependent.

      6. The user's default shell, the initial program run when the user logs in, or #f, indicating that the system default should be used.

 -- Function: setpwent #t
     Rewinds the pw entry table back to the begining.

 -- Function: setpwent #f
 -- Function: setpwent
     Closes the pw table.

 -- Function: getgr name
 -- Function: getgr uid
 -- Function: getgr
     Returns a vector of information for the entry for NAME, UID, or the next entry if no argument is given.  The information is:

      0. The name of the group.

      1. The encrypted password string.

      2. The group ID number.

      3. A list of (string) names of users in the group.

 -- Function: setgrent #t
     Rewinds the group entry table back to the begining.

 -- Function: setgrent #f
 -- Function: setgrent
     Closes the group table.

 -- Function: getgroups
     Returns a vector of all the supplementary group IDs of the process.

 -- Function: link oldname newname
     The link function makes a new link to the existing file named by OLDNAME, under the new name NEWNAME.

     link returns a value of #t if it is successful and #f on failure.

 -- Function: chown filename owner group
     The chown function changes the owner of the file FILENAME to OWNER, and its group owner to GROUP.

     chown returns a value of #t if it is successful and #f on failure.

 -- Function: ttyname port
     If port PORT is associated with a terminal device, returns a string containing the file name of termainal device; otherwise #f.

Unix Extensions
===============

If 'unix is provided (by linking in unix.o), the following functions are defined:

These privileged and symbolic link functions are not in Posix:

 -- Function: symlink oldname newname
     The symlink function makes a symbolic link to OLDNAME named NEWNAME.

     symlink returns a value of #t if it is successful and #f on failure.

 -- Function: readlink filename
     Returns the value of the symbolic link FILENAME or #f for failure.

 -- Function: lstat filename
     The lstat function is like stat, except that it does not follow symbolic links.  If FILENAME is the name of a symbolic link, lstat returns information about the link itself; otherwise, lstat works like stat.  See I/O-Extensions.

 -- Function: nice increment
     Increment the priority of the current process by INCREMENT. chown returns a value of #t if it is successful and #f on failure.

 -- Function: acct filename
     When called with the name of an exisitng file as argument, accounting is turned on, records for each terminating process are appended to FILENAME as it terminates.  An argument of #f causes accounting to be turned off.

     acct returns a value of #t if it is successful and #f on failure.

 -- Function: mknod filename mode dev
     The mknod function makes a special file with name FILENAME and modes MODE for device number DEV.

     mknod returns a value of #t if it is successful and #f on failure.

 -- Function: sync
     sync first commits inodes to buffers, and then buffers to disk. sync() only schedules the writes, so it may return before the actual writing is done.  The value returned is unspecified.

Sequence Comparison
===================

(require 'diff)

A blazing fast implementation of the sequence-comparison module in SLIB, see Sequence Comparison (SLIB).

Regular Expression Pattern Matching
===================================

These functions are defined in rgx.c using a POSIX or GNU regex library.  If your computer does not support regex, a package is available via ftp from ftp.gnu.org:/pub/gnu/regex-0.12.tar.gz.  For a description of regular expressions, see syntax ("regex" regular expression matching library).

 -- Function: regcomp PATTERN [FLAGS]
     Compile a regular expression.  Return a compiled regular expression, or an integer error code suitable as an argument to regerror.

     FLAGS in regcomp is a string of option letters used to control the compilation of the regular expression.  The letters may consist of:

     n
          newlines won't be matched by . or hat lists; ( [^...] )

     i
          ignore case.
          only when compiled with _GNU_SOURCE:

     0
          allows dot to match a null character.

     f
          enable GNU fastmaps.

 -- Function: regerror ERRNO
     Returns a string describing the integer ERRNO returned when regcomp fails.

 -- Function: regexec RE STRING
     Returns #f or a vector of integers.  These integers are in doublets.  The first of each doublet is the index of STRING of the start of the matching expression or sub-expression (delimited by parentheses in the pattern).  The last of each doublet is index of STRING of the end of that expression.  #f is returned if the string does not match.

 -- Function: regmatch? RE STRING
     Returns #t if the PATTERN such that REGEXP = (regcomp PATTERN) matches STRING as a POSIX extended regular expressions.  Returns #f otherwise.

 -- Function: regsearch RE STRING [START [LEN]]
 -- Function: regsearchv RE STRING [START [LEN]]
 -- Function: regmatch RE STRING [START [LEN]]
 -- Function: regmatchv RE STRING [START [LEN]]
     Regsearch searches for the pattern within the string.

     Regmatch anchors the pattern and begins matching it against string.

     Regsearch returns the character position where RE starts, or #f if not found.

     Regmatch returns the number of characters matched, #f if not matched.

     Regsearchv and regmatchv return the match vector is returned if RE is found, #f otherwise.

     re
          may be either:

           1. a compiled regular expression returned by regcomp;

           2. a string representing a regular expression;

           3. a list of a string and a set of option letters.

     string
          The string to be operated upon.

     start
          The character position at which to begin the search or match.  If absent, the default is zero.

          Compiled _GNU_SOURCE and using GNU libregex only

          When searching, if START is negative, the absolute value of START will be used as the start location and reverse searching will be performed.

     len
          The search is allowed to examine only the first LEN characters of STRING.  If absent, the entire string may be examined.

 -- Function: string-split RE STRING
 -- Function: string-splitv RE STRING
     String-split splits a string into substrings that are separated by RE, returning a vector of substrings.

     String-splitv returns a vector of string positions that indicate where the substrings are located.

 -- Function: string-edit RE EDIT-SPEC STRING [COUNT]
     Returns the edited string.

     edit-spec
          Is a string used to replace occurances of RE.  Backquoted integers in the range of 1-9 may be used to insert subexpressions in RE, as in sed.

     count
          The number of substitutions for string-edit to perform.  If #t, all occurances of RE will be replaced.  The default is to perform one substitution.

Line Editing
============

These procedures provide input line editing and recall.

These functions are defined in edline.c and Iedline.scm using the editline or GNU readline (see Overview (GNU Readline Library)) libraries available from:

  * ftp.sys.toronto.edu:/pub/rc/editline.shar

  * ftp.gnu.org:/pub/gnu/readline-2.0.tar.gz

When Iedline.scm is loaded, if the current input port is the default input port and the environment variable EMACS is not defined, line-editing mode will be entered.

 -- Function: default-input-port
     Returns the initial current-input-port SCM was invoked with (stdin).

 -- Function: default-output-port
     Returns the initial current-output-port SCM was invoked with (stdout).

 -- Function: make-edited-line-port
     Returns an input/output port that allows command line editing and retrieval of history.

 -- Function: line-editing
     Returns the current edited line port or #f.

 -- Function: line-editing bool
     If BOOL is false, exits line-editing mode and returns the previous value of (line-editing).  If BOOL is true, sets the current input and output ports to an edited line port and returns the previous value of (line-editing).

Curses
======

These functions are defined in crs.c using the curses library.  Unless otherwise noted these routines return #t for successful completion and #f for failure.

 -- Function: initscr
     Returns a port for a full screen window.  This routine must be called to initialize curses.

 -- Function: endwin
     A program should call endwin before exiting or escaping from curses mode temporarily, to do a system call, for example.  This routine will restore termio modes, move the cursor to the lower left corner of the screen and reset the terminal into the proper non-visual mode.  To resume after a temporary escape, call refresh.

Output Options Setting
----------------------

These routines set options within curses that deal with output.  All options are initially #f, unless otherwise stated.  It is not necessary to turn these options off before calling endwin.

 -- Function: clearok win bf
     If enabled (BF is #t), the next call to force-output or refresh with WIN will clear the screen completely and redraw the entire screen from scratch.  This is useful when the contents of the screen are uncertain, or in some cases for a more pleasing visual effect.

 -- Function: idlok win bf
     If enabled (BF is #t), curses will consider using the hardware ``insert/delete-line'' feature of terminals so equipped.  If disabled (BF is #f), curses will very seldom use this feature.  The ``insert/delete-character'' feature is always considered. This option should be enabled only if your application needs ``insert/delete-line'', for example, for a screen editor.  It is disabled by default because

     ``insert/delete-line'' tends to be visually annoying when used in applications where it is not really needed.  If ``insert/delete-line'' cannot be used, curses will redraw the changed portions of all lines.

 -- Function: leaveok win bf
     Normally, the hardware cursor is left at the location of the window cursor being refreshed.  This option allows the cursor to be left wherever the update happens to leave it.  It is useful for applications where the cursor is not used, since it reduces the need for cursor motions.  If possible, the cursor is made invisible when this option is enabled.

 -- Function: scrollok win bf
     This option controls what happens when the cursor of window WIN is moved off the edge of the window or scrolling region, either from a newline on the bottom line, or typing the last character of the last line.  If disabled (BF is #f), the cursor is left on the bottom line at the location where the offending character was entered. If enabled (BF is #t), force-output is called on the window WIN, and then the physical terminal and window WIN are scrolled up one line.

     Note in order to get the physical scrolling effect on the terminal, it is also necessary to call idlok.

 -- Function: nodelay win bf
     This option causes wgetch to be a non-blocking call.  If no input is ready, wgetch will return an eof-object.  If disabled, wgetch will hang until a key is pressed.

Terminal Mode Setting
---------------------

These routines set options within curses that deal with input.  The options involve using ioctl(2) and therefore interact with curses routines.  It is not necessary to turn these options off before calling endwin.  The routines in this section all return an unspecified value.

 -- Function: cbreak
 -- Function: nocbreak
     These two routines put the terminal into and out of CBREAK mode, respectively.  In CBREAK mode, characters typed by the user are immediately available to the program and erase/kill character processing is not performed.  When in NOCBREAK mode, the tty driver will buffer characters typed until a LFD or RET is typed. Interrupt and flowcontrol characters are unaffected by this mode. Initially the terminal may or may not be in CBREAK mode, as it is inherited, therefore, a program should call cbreak or nocbreak explicitly.  Most interactive programs using curses will set CBREAK mode.

     Note cbreak overrides raw.  For a discussion of how these routines interact with echo and noecho see read-char.

 -- Function: raw
 -- Function: noraw
     The terminal is placed into or out of RAW mode.  RAW mode is similar to CBREAK mode, in that characters typed are immediately passed through to the user program.  The differences are that in RAW mode, the interrupt, quit, suspend, and flow control characters are passed through uninterpreted, instead of generating a signal.  RAW mode also causes 8-bit input and output.  The behavior of the BREAK key depends on other bits in the terminal driver that are not set by curses.

 -- Function: echo
 -- Function: noecho
     These routines control whether characters typed by the user are echoed by read-char as they are typed.  Echoing by the tty driver is always disabled, but initially read-char is in ECHO mode, so characters typed are echoed.  Authors of most interactive programs prefer to do their own echoing in a controlled area of the screen, or not to echo at all, so they disable echoing by calling noecho. For a discussion of how these routines interact with echo and noecho see read-char.

 -- Function: nl
 -- Function: nonl
     These routines control whether LFD is translated into RET and LFD on output, and whether RET is translated into LFD on input.  Initially, the translations do occur.  By disabling these translations using nonl, curses is able to make better use of the linefeed capability, resulting in faster cursor motion.

 -- Function: resetty
 -- Function: savetty
     These routines save and restore the state of the terminal modes. savetty saves the current state of the terminal in a buffer and resetty restores the state to what it was at the last call to savetty.

Window Manipulation
-------------------

 -- Function: newwin nlines ncols begy begx
     Create and return a new window with the given number of lines (or rows), NLINES, and columns, NCOLS.  The upper left corner of the window is at line BEGY, column BEGX.  If either NLINES or NCOLS is 0, they will be set to the value of LINES-BEGY and COLS-BEGX.  A new full-screen window is created by calling newwin(0,0,0,0).

 -- Function: subwin orig nlines ncols begy begx
     Create and return a pointer to a new window with the given number of lines (or rows), NLINES, and columns, NCOLS.  The window is at position (BEGY, BEGX) on the screen.  This position is relative to the screen, and not to the window ORIG.  The window is made in the middle of the window ORIG, so that changes made to one window will affect both windows.  When using this routine, often it will be necessary to call touchwin or touchline on ORIG before calling force-output.

 -- Function: close-port win
     Deletes the window WIN, freeing up all memory associated with it. In the case of sub-windows, they should be deleted before the main window WIN.

 -- Function: refresh
 -- Function: force-output win
     These routines are called to write output to the terminal, as most other routines merely manipulate data structures.  force-output copies the window WIN to the physical terminal screen, taking into account what is already there in order to minimize the amount of information that's sent to the terminal (called optimization).  Unless leaveok has been enabled, the physical cursor of the terminal is left at the location of window WIN's cursor.  With refresh, the number of characters output to the terminal is returned.

 -- Function: mvwin win y x
     Move the window WIN so that the upper left corner will be at position (Y, X).  If the move would cause the window WIN to be off the screen, it is an error and the window WIN is not moved.

 -- Function: overlay srcwin dstwin
 -- Function: overwrite srcwin dstwin

     These routines overlay SRCWIN on top of DSTWIN; that is, all text in SRCWIN is copied into DSTWIN.  SRCWIN and DSTWIN need not be the same size; only text where the two windows overlap is copied.  The difference is that overlay is non-destructive (blanks are not copied), while overwrite is destructive.

 -- Function: touchwin win
 -- Function: touchline win start count
     Throw away all optimization information about which parts of the window WIN have been touched, by pretending that the entire window WIN has been drawn on.  This is sometimes necessary when using overlapping windows, since a change to one window will affect the other window, but the records of which lines have been changed in the other window will not reflect the change.  touchline only pretends that COUNT lines have been changed, beginning with line START.

 -- Function: wmove win y x
     The cursor associated with the window WIN is moved to line (row) Y, column X.  This does not move the physical cursor of the terminal until refresh (or force-output) is called.  The position specified is relative to the upper left corner of the window WIN, which is (0, 0).

Output
------

These routines are used to draw text on windows

 -- Function: display ch win
 -- Function: display str win
 -- Function: wadd win ch
 -- Function: wadd win str
     The character CH or characters in STR are put into the window WIN at the current cursor position of the window and the position of WIN's cursor is advanced.  At the right margin, an automatic newline is performed.  At the bottom of the scrolling region, if scrollok is enabled, the scrolling region will be scrolled up one line.

     If CH is a TAB, LFD, or backspace, the cursor will be moved appropriately within the window WIN.  A LFD also does a wclrtoeol before moving.  TAB characters are considered to be at every eighth column.  If CH is another control character, it will be drawn in the C-x notation.  (Calling winch after adding a control character will not return the control character, but instead will return the representation of the control character.)

     Video attributes can be combined with a character by or-ing them into the parameter.  This will result in these attributes also being set. The intent here is that text, including attributes, can be copied from one place to another using inch and display.  See standout, below.

     Note For wadd CH can be an integer and will insert the character of the corresponding value.

 -- Function: werase win
     This routine copies blanks to every position in the window WIN.

 -- Function: wclear win
     This routine is like werase, but it also calls clearok, arranging that the screen will be cleared completely on the next call to refresh or force-output for window WIN, and repainted from scratch.

 -- Function: wclrtobot win
     All lines below the cursor in window WIN are erased.  Also, the current line to the right of the cursor, inclusive, is erased.

 -- Function: wclrtoeol win
     The current line to the right of the cursor, inclusive, is erased.

 -- Function: wdelch win
     The character under the cursor in the window WIN is deleted.  All characters to the right on the same line are moved to the left one position and the last character on the line is filled with a blank.  The cursor position does not change.  This does not imply use of the hardware ``delete-character'' feature.

 -- Function: wdeleteln win
     The line under the cursor in the window WIN is deleted.  All lines below the current line are moved up one line.  The bottom line WIN is cleared.  The cursor position does not change.  This does not imply use of the hardware ``deleteline'' feature.

 -- Function: winsch win ch
     The character CH is inserted before the character under the cursor.  All characters to the right are moved one SPC to the right, possibly losing the rightmost character of the line.  The cursor position does not change .  This does not imply use of the hardware ``insertcharacter'' feature.

 -- Function: winsertln win
     A blank line is inserted above the current line and the bottom line is lost.  This does not imply use of the hardware ``insert-line'' feature.

 -- Function: scroll win
     The window WIN is scrolled up one line.  This involves moving the lines in WIN's data structure.  As an optimization, if WIN is stdscr and the scrolling region is the entire window, the physical screen will be scrolled at the same time.

Input
-----

 -- Function: read-char win
     A character is read from the terminal associated with the window WIN.  Depending on the setting of cbreak, this will be after one character (CBREAK mode), or after the first newline (NOCBREAK mode).  Unless noecho has been set, the character will also be echoed into WIN.

     When using read-char, do not set both NOCBREAK mode (nocbreak) and ECHO mode (echo) at the same time. Depending on the state of the terminal driver when each character is typed, the program may produce undesirable results.

 -- Function: winch win
     The character, of type chtype, at the current position in window WIN is returned.  If any attributes are set for that position, their values will be OR'ed into the value returned.

 -- Function: getyx win
     A list of the y and x coordinates of the cursor position of the window WIN is returned

Curses Miscellany
-----------------

 -- Function: wstandout win
 -- Function: wstandend win

     These functions set the current attributes of the window WIN.  The current attributes of WIN are applied to all characters that are written into it.  Attributes are a property of the character, and move with the character through any scrolling and insert/delete line/character operations.  To the extent possible on the particular terminal, they will be displayed as the graphic rendition of characters put on the screen.

     wstandout sets the current attributes of the window WIN to be visibly different from other text.  wstandend turns off the attributes.

 -- Function: box win vertch horch
     A box is drawn around the edge of the window WIN.  VERTCH and HORCH are the characters the box is to be drawn with.  If VERTCH and HORCH are 0, then appropriate default characters, ACS_VLINE and ACS_HLINE, will be used.

     Note VERTCH and HORCH can be an integers and will insert the character (with attributes) of the corresponding values.

 -- Function: unctrl c
     This macro expands to a character string which is a printable representation of the character C.  Control characters are displayed in the C-x notation.  Printing characters are displayed as is.

Sockets
=======

These procedures (defined in socket.c) provide a Scheme interface to most of the C socket library.  For more information on sockets, see Sockets (The GNU C Library Reference Manual).

Host and Other Inquiries
------------------------

 -- Constant: af_inet
 -- Constant: af_unix
     Integer family codes for Internet and Unix sockets, respectively.

 -- Function: gethost host-spec
 -- Function: gethost
     Returns a vector of information for the entry for HOST-SPEC or the next entry if HOST-SPEC isn't given.  The information is:

      0. host name string

      1. list of host aliases strings

      2. integer address type (AF_INET)

      3. integer size of address entries (in bytes)

      4. list of integer addresses

 -- Function: sethostent stay-open
 -- Function: sethostent
     Rewinds the host entry table back to the begining if given an argument. If the argument STAY-OPEN is #f queries will be be done using UDP datagrams.  Otherwise, a connected TCP socket will be used.  When called without an argument, the host table is closed.

 -- Function: getnet name-or-number
 -- Function: getnet
     Returns a vector of information for the entry for NAME-OR-NUMBER or the next entry if an argument isn't given.  The information is:

      0. official network name string

      1. list of network aliases strings

      2. integer network address type (AF_INET)

      3. integer network number

 -- Function: setnetent stay-open
 -- Function: setnetent
     Rewinds the network entry table back to the begining if given an argument.  If the argument STAY-OPEN is #f the table will be closed between calls to getnet.  Otherwise, the table stays open.  When called without an argument, the network table is closed.

 -- Function: getproto name-or-number
 -- Function: getproto
     Returns a vector of information for the entry for NAME-OR-NUMBER or the next entry if an argument isn't given.  The information is:

      1. official protocol name string

      2. list of protocol aliases strings

      3. integer protocol number

 -- Function: setprotoent stay-open
 -- Function: setprotoent
     Rewinds the protocol entry table back to the begining if given an argument.  If the argument STAY-OPEN is #f the table will be closed between calls to getproto.  Otherwise, the table stays open.  When called without an argument, the protocol table is closed.

 -- Function: getserv name-or-port-number protocol
 -- Function: getserv
     Returns a vector of information for the entry for NAME-OR-PORT-NUMBER and PROTOCOL or the next entry if arguments aren't given.  The information is:

      0. official service name string

      1. list of service aliases strings

      2. integer port number

      3. protocol

 -- Function: setservent stay-open
 -- Function: setservent
     Rewinds the service entry table back to the begining if given an argument.  If the argument STAY-OPEN is #f the table will be closed between calls to getserv.  Otherwise, the table stays open.  When called without an argument, the service table is closed.

Internet Addresses and Socket Names
-----------------------------------

 -- Function: inet:string->address string
     Returns the host address number (integer) for host STRING or #f if not found.

 -- Function: inet:address->string address
     Converts an internet (integer) address to a string in numbers and dots notation.

 -- Function: inet:network address
     Returns the network number (integer) specified from ADDRESS or #f if not found.

 -- Function: inet:local-network-address address
     Returns the integer for the address of ADDRESS within its local network or #f if not found.

 -- Function: inet:make-address network local-address
     Returns the Internet address of LOCAL-ADDRESS in NETWORK.

The type socket-name is used for inquiries about open sockets in the following procedures:

 -- Function: getsockname socket
     Returns the socket-name of SOCKET.  Returns #f if unsuccessful or SOCKET is closed.

 -- Function: getpeername socket
     Returns the socket-name of the socket connected to SOCKET. Returns #f if unsuccessful or SOCKET is closed.

 -- Function: socket-name:family socket-name
     Returns the integer code for the family of SOCKET-NAME.

 -- Function: socket-name:port-number socket-name
     Returns the integer port number of SOCKET-NAME.

 -- Function: socket-name:address socket-name
     Returns the integer Internet address for SOCKET-NAME.

Socket
------

When a port is returned from one of these calls it is unbuffered. This allows both reading and writing to the same port to work.  If you want buffered ports you can (assuming sock-port is a socket i/o port):

     (require 'i/o-extensions)
     (define i-port (duplicate-port sock-port "r"))
     (define o-port (duplicate-port sock-port "w"))

 -- Function: make-stream-socket family
 -- Function: make-stream-socket family protocol

     Returns a SOCK_STREAM socket of type FAMILY using PROTOCOL.  If FAMILY has the value AF_INET, SO_REUSEADDR will be set.  The integer argument PROTOCOL corresponds to the integer protocol numbers returned (as vector elements) from (getproto).  If the PROTOCOL argument is not supplied, the default (0) for the specified FAMILY is used.  SCM sockets look like ports opened for neither reading nor writing.

 -- Function: make-stream-socketpair family
 -- Function: make-stream-socketpair family protocol

     Returns a pair (cons) of connected SOCK_STREAM (socket) ports of type FAMILY using PROTOCOL.  Many systems support only socketpairs of the af-unix FAMILY.  The integer argument PROTOCOL corresponds to the integer protocol numbers returned (as vector elements) from (getproto).  If the PROTOCOL argument is not supplied, the default (0) for the specified FAMILY is used.

 -- Function: socket:shutdown socket how
     Makes SOCKET no longer respond to some or all operations depending on the integer argument HOW:

      0. Further input is disallowed.

      1. Further output is disallowed.

      2. Further input or output is disallowed.

     Socket:shutdown returns SOCKET if successful, #f if not.

 -- Function: socket:connect inet-socket host-number port-number
 -- Function: socket:connect unix-socket pathname
     Returns SOCKET (changed to a read/write port) connected to the Internet socket on host HOST-NUMBER, port PORT-NUMBER or the Unix socket specified by PATHNAME.  Returns #f if not successful.

 -- Function: socket:bind inet-socket port-number
 -- Function: socket:bind unix-socket pathname
     Returns INET-SOCKET bound to the integer PORT-NUMBER or the UNIX-SOCKET bound to new socket in the file system at location PATHNAME.  Returns #f if not successful.  Binding a UNIX-SOCKET creates a socket in the file system that must be deleted by the caller when it is no longer needed (using delete-file).

 -- Function: socket:listen socket backlog
     The bound (see bind) SOCKET is readied to accept connections.  The positive integer BACKLOG specifies how many pending connections will be allowed before further connection requests are refused.  Returns SOCKET (changed to a read-only port) if successful, #f if not.

 -- Function: char-ready? listen-socket
     The input port returned by a successful call to socket:listen can be polled for connections by char-ready? (see char-ready?).  This avoids blocking on connections by socket:accept.

 -- Function: socket:accept socket
     Accepts a connection on a bound, listening SOCKET.  Returns an input/output port for the connection.

The following example is not too complicated, yet shows the use of sockets for multiple connections without input blocking.

     ;;;; Scheme chat server

     ;;; This program implements a simple `chat' server which accepts
     ;;; connections from multiple clients, and sends to all clients any
     ;;; characters received from any client.

     ;;; To connect to chat `telnet localhost 8001'

     (require 'socket)
     (require 'i/o-extensions)

     (let ((listener-socket (socket:bind (make-stream-socket af_inet) 8001))
           (connections '()))
       (socket:listen listener-socket 5)
       (do () (#f)
         (let ((actives (or (apply wait-for-input 5 listener-socket connections)
                            '())))
           (cond ((null? actives))
                 ((memq listener-socket actives)
                  (set! actives (cdr (memq listener-socket actives)))
                  (let ((con (socket:accept listener-socket)))
                    (display "accepting connection from ")
                    (display (getpeername con))
                    (newline)
                    (set! connections (cons con connections))
                    (display "connected" con)
                    (newline con))))
           (set! connections
                 (let next ((con-list connections))
                   (cond ((null? con-list) '())
                         (else
                          (let ((con (car con-list)))
                            (cond ((memq con actives)
                                   (let ((c (read-char con)))
                                     (cond ((eof-object? c)
                                            (display "closing connection from ")
                                            (display (getpeername con))
                                            (newline)
                                            (close-port con)
                                            (next (cdr con-list)))
                                           (else
                                            (for-each (lambda (con)
                                                        (file-set-position con 0)
                                                        (write-char c con)
                                                        (file-set-position con 0))
                                                      connections)
                                            (cons con (next (cdr con-list)))))))
                                  (else (cons con (next (cdr con-list)))))))))))))

You can use telnet localhost 8001 to connect to the chat server, or you can use a client written in scheme:

     ;;;; Scheme chat client

     ;;; this program connects to socket 8001.  It then sends all
     ;;; characters from current-input-port to the socket and sends all
     ;;; characters from the socket to current-output-port.

     (require 'socket)
     (require 'i/o-extensions)

     (define con (make-stream-socket af_inet))
     (set! con (socket:connect con (inet:string->address "localhost") 8001))

     (define (go)
       (define actives (wait-for-input (* 30 60) con (current-input-port)))
       (let ((cs (and actives (memq con actives) (read-char con)))
             (ct (and actives (memq (current-input-port) actives) (read-char))))
         (cond ((or (eof-object? cs) (eof-object? ct)) (close-port con))
               (else (cond (cs (display cs)))
                     (cond (ct (file-set-position con 0)
                               (display ct con)
                               (file-set-position con 0)))
                     (go)))))
     (cond (con (display "Connecting to ")
                (display (getpeername con))
                (newline)
                (go))
           (else (display "Server not listening on port 8001")
                 (newline)))

SCMDB
=====

(require 'mysql)

SCMDB is an add-on for SCM that ports the MySQL C-library to SCM.

It is available from: http://www.dedecker.net/jessie/scmdb/

The Implementation
******************

Data Types
==========

In the descriptions below it is assumed that long ints are 32 bits in length.  Acutally, SCM is written to work with any long int size larger than 31 bits.  With some modification, SCM could work with word sizes as small as 24 bits.

All SCM objects are represented by type SCM.  Type SCM come in 2 basic flavors, Immediates and Cells:

Immediates
----------

An immediate is a data type contained in type SCM (long int).  The type codes distinguishing immediate types from each other vary in length, but reside in the low order bits.

 -- Macro: IMP x
 -- Macro: NIMP x
     Return non-zero if the SCM object X is an immediate or non-immediate type, respectively.

 -- Immediate: inum
     immediate 30 bit signed integer.  An INUM is flagged by a 1 in the second to low order bit position.  The high order 30 bits are used for the integer's value.

      -- Macro: INUMP x
      -- Macro: NINUMP x
          Return non-zero if the SCM X is an immediate integer or not an immediate integer, respectively.

      -- Macro: INUM x
          Returns the C long integer corresponding to SCM X.

      -- Macro: MAKINUM x
          Returns the SCM inum corresponding to C long integer x.

      -- Immediate Constant: INUM0
          is equivalent to MAKINUM(0).

     Computations on INUMs are performed by converting the arguments to C integers (by a shift), operating on the integers, and converting the result to an inum.  The result is checked for overflow by converting back to integer and checking the reverse operation.

     The shifts used for conversion need to be signed shifts.  If the C implementation does not support signed right shift this fact is detected in a #if statement in scmfig.h and a signed right shift, SRS, is constructed in terms of unsigned right shift.

 -- Immediate: ichr
     characters.

      -- Macro: ICHRP x
          Return non-zero if the SCM object X is a character.

      -- Macro: ICHR x
          Returns corresponding unsigned char.

      -- Macro: MAKICHR x
          Given char X, returns SCM character.

 -- Immediate: iflags
     These are frequently used immediate constants.
      -- Immediate Constant: SCM BOOL_T
          #t

      -- Immediate Constant: SCM BOOL_F
          #f

      -- Immediate Constant: SCM EOL
          ().  If SICP is #defined, EOL is #defined to be identical with BOOL_F.  In this case, both print as #f.

      -- Immediate Constant: SCM EOF_VAL
          end of file token, #<eof>.

      -- Immediate Constant: SCM UNDEFINED
          #<undefined> used for variables which have not been defined and absent optional arguments.

      -- Immediate Constant: SCM UNSPECIFIED
          #<unspecified> is returned for those procedures whose return values are not specified.

 -- Macro: IFLAGP n
     Returns non-zero if N is an ispcsym, isym or iflag.

 -- Macro: ISYMP n
     Returns non-zero if N is an ispcsym or isym.

 -- Macro: ISYMNUM n
     Given ispcsym, isym, or iflag N, returns its index in the C array isymnames[].

 -- Macro: ISYMCHARS n
     Given ispcsym, isym, or iflag N, returns its char * representation (from isymnames[]).

 -- Macro: MAKSPCSYM n
     Returns SCM ispcsym N.

 -- Macro: MAKISYM n
     Returns SCM iisym N.

 -- Macro: MAKIFLAG n
     Returns SCM iflag N.

 -- Variable: isymnames
     An array of strings containing the external representations of all the ispcsym, isym, and iflag immediates.  Defined in repl.c.

 -- Constant: NUM_ISPCSYM
 -- Constant: NUM_ISYMS
     The number of ispcsyms and ispcsyms+isyms, respectively.  Defined in scm.h.

 -- Immediate: isym
     and, begin, case, cond, define, do, if, lambda, let, let*, letrec, or, quote, set!, #f, #t, #<undefined>, #<eof>, (), and #<unspecified>.

 -- CAR Immediate: ispcsym
     special symbols: syntax-checked versions of first 14 isyms

 -- CAR Immediate: iloc
     indexes to a variable's location in environment

 -- CAR Immediate: gloc
     pointer to a symbol's value cell

 -- Immediate: CELLPTR
     pointer to a cell (not really an immediate type, but here for completeness).  Since cells are always 8 byte aligned, a pointer to a cell has the low order 3 bits 0.

     There is one exception to this rule, CAR Immediates, described next.

A CAR Immediate is an Immediate point which can only occur in the CARs of evaluated code (as a result of ceval's memoization process).

Cells
-----

Cells represent all SCM objects other than immediates.  A cell has a CAR and a CDR.  Low-order bits in CAR identify the type of object.  The rest of CAR and CDR hold object data.  The number after tc specifies how many bits are in the type code.  For instance, tc7 indicates that the type code is 7 bits.

 -- Macro: NEWCELL x
     Allocates a new cell and stores a pointer to it in SCM local variable X.

     Care needs to be taken that stores into the new cell pointed to by X do not create an inconsistent object.  See Signals.

All of the C macros decribed in this section assume that their argument is of type SCM and points to a cell (CELLPTR).

 -- Macro: CAR x
 -- Macro: CDR x
     Returns the car and cdr of cell X, respectively.

 -- Macro: TYP3 x
 -- Macro: TYP7 x
 -- Macro: TYP16 x
     Returns the 3, 7, and 16 bit type code of a cell.

 -- Cell: tc3_cons
     scheme cons-cell returned by (cons arg1 arg2).

      -- Macro: CONSP x
      -- Macro: NCONSP x
          Returns non-zero if X is a tc3_cons or isn't, respectively.

 -- Cell: tc3_closure
     applicable object returned by (lambda (args) ...). tc3_closures have a pointer to the body of the procedure in the CAR and a pointer to the environment in the CDR.  Bits 1 and 2 (zero-based) in the CDR indicate a lower bound on the number of required arguments to the closure, which is used to avoid allocating rest argument lists in the environment cache.  This encoding precludes an immediate value for the CDR:  In the case of an empty environment all bits above 2 in the CDR are zero.

      -- Macro: CLOSUREP x
          Returns non-zero if X is a tc3_closure.

      -- Macro: CODE x
      -- Macro: ENV x
          Returns the code body or environment of closure X, respectively.

      -- Macro: ARGC x
          Returns the a lower bound on the number of required arguments to closure X, it cannot exceed 3.

Header Cells
------------

Headers are Cells whose CDRs point elsewhere in memory, such as to memory allocated by malloc.

 -- Header: spare
     spare tc7 type code

 -- Header: tc7_vector
     scheme vector.

      -- Macro: VECTORP x
      -- Macro: NVECTORP x
          Returns non-zero if X is a tc7_vector or if not, respectively.

      -- Macro: VELTS x
      -- Macro: LENGTH x
          Returns the C array of SCMs holding the elements of vector X or its length, respectively.

 -- Header: tc7_ssymbol
     static scheme symbol (part of initial system)

 -- Header: tc7_msymbol
     malloced scheme symbol (can be GCed)

      -- Macro: SYMBOLP x
          Returns non-zero if X is a tc7_ssymbol or tc7_msymbol.

      -- Macro: CHARS x
      -- Macro: UCHARS x
      -- Macro: LENGTH x
          Returns the C array of chars or as unsigned chars holding the elements of symbol X or its length, respectively.

 -- Header: tc7_string
     scheme string

      -- Macro: STRINGP x
      -- Macro: NSTRINGP x
          Returns non-zero if X is a tc7_string or isn't, respectively.

      -- Macro: CHARS x
      -- Macro: UCHARS x
      -- Macro: LENGTH x
          Returns the C array of chars or as unsigned chars holding the elements of string X or its length, respectively.

 -- Header: tc7_bvect
     uniform vector of booleans (bit-vector)

 -- Header: tc7_ivect
     uniform vector of integers

 -- Header: tc7_uvect
     uniform vector of non-negative integers

 -- Header: tc7_svect
     uniform vector of short integers

 -- Header: tc7_fvect
     uniform vector of short inexact real numbers

 -- Header: tc7_dvect
     uniform vector of double precision inexact real numbers

 -- Header: tc7_cvect
     uniform vector of double precision inexact complex numbers

 -- Header: tc7_contin
     applicable object produced by call-with-current-continuation

 -- Header: tc7_specfun
     subr that is treated specially within the evaluator

     apply and call-with-current-continuation are denoted by these objects.  Their behavior as functions is built into the evaluator; they are not directly associated with C functions.  This is necessary in order to make them properly tail recursive.

     tc16_cclo is a subtype of tc7_specfun, a cclo is similar to a vector (and is GCed like one), but can be applied as a function:

      1. the cclo itself is consed onto the head of the argument list

      2. the first element of the cclo is applied to that list.  Cclo invocation is currently not tail recursive when given 2 or more arguments.

      -- Function: makcclo proc len
          makes a closure from the subr PROC with LEN-1 extra locations for SCM data.  Elements of a CCLO are referenced using VELTS(cclo)[n] just as for vectors.

      -- Macro: CCLO_LENGTH cclo
          Expands to the length of CCLO.

Subr Cells
----------

A Subr is a header whose CDR points to a C code procedure. Scheme primitive procedures are subrs.  Except for the arithmetic tc7_cxrs, the C code procedures will be passed arguments (and return results) of type SCM.

 -- Subr: tc7_asubr
     associative C function of 2 arguments.  Examples are +, -, *, /, max, and min.

 -- Subr: tc7_subr_0
     C function of no arguments.

 -- Subr: tc7_subr_1
     C function of one argument.

 -- Subr: tc7_cxr
     These subrs are handled specially.  If inexact numbers are enabled, the CDR should be a function which takes and returns type double.  Conversions are handled in the interpreter.

     floor, ceiling, truncate, round, $sqrt, $abs, $exp, $log, $sin, $cos, $tan, $asin, $acos, $atan, $sinh, $cosh, $tanh, $asinh, $acosh, $atanh, and exact->inexact are defined this way.

     If the CDR is 0 (NULL), the name string of the procedure is used to control traversal of its list structure argument.

     car, cdr, caar, cadr, cdar, cddr, caaar, caadr, cadar, caddr, cdaar, cdadr, cddar, cdddr, caaaar, caaadr, caadar, caaddr, cadaar, cadadr, caddar, cadddr, cdaaar, cdaadr, cdadar, cdaddr, cddaar, cddadr, cdddar, and cddddr are defined this way.

 -- Subr: tc7_subr_3
     C function of 3 arguments.

 -- Subr: tc7_subr_2
     C function of 2 arguments.

 -- Subr: tc7_rpsubr
     transitive relational predicate C function of 2 arguments.  The C function should return either BOOL_T or BOOL_F.

 -- Subr: tc7_subr_1o
     C function of one optional argument.  If the optional argument is not present, UNDEFINED is passed in its place.

 -- Subr: tc7_subr_2o
     C function of 1 required and 1 optional argument.  If the optional argument is not present, UNDEFINED is passed in its place.

 -- Subr: tc7_lsubr_2
     C function of 2 arguments and a list of (rest of) SCM arguments.

 -- Subr: tc7_lsubr
     C function of list of SCM arguments.

Ptob Cells
----------

A ptob is a port object, capable of delivering or accepting characters.  See Ports (Revised(5) Report on the Algorithmic Language Scheme).  Unlike the types described so far, new varieties of ptobs can be defined dynamically (see Defining Ptobs).  These are the initial ptobs:

 -- ptob: tc16_inport
     input port.

 -- ptob: tc16_outport
     output port.

 -- ptob: tc16_ioport
     input-output port.

 -- ptob: tc16_inpipe
     input pipe created by popen().

 -- ptob: tc16_outpipe
     output pipe created by popen().

 -- ptob: tc16_strport
     String port created by cwos() or cwis().

 -- ptob: tc16_sfport
     Software (virtual) port created by mksfpt() (see Soft Ports).

 -- Macro: PORTP x
 -- Macro: OPPORTP x
 -- Macro: OPINPORTP x
 -- Macro: OPOUTPORTP x
 -- Macro: INPORTP x
 -- Macro: OUTPORTP x
     Returns non-zero if X is a port, open port, open input-port, open output-port, input-port, or output-port, respectively.

 -- Macro: OPENP x
 -- Macro: CLOSEDP x
     Returns non-zero if port X is open or closed, respectively.

 -- Macro: STREAM x
     Returns the FILE * stream for port X.

Ports which are particularly well behaved are called fports. Advanced operations like file-position and reopen-file only work for fports.

 -- Macro: FPORTP x
 -- Macro: OPFPORTP x
 -- Macro: OPINFPORTP x
 -- Macro: OPOUTFPORTP x
     Returns non-zero if X is a port, open port, open input-port, or open output-port, respectively.

Smob Cells
----------

A smob is a miscellaneous datatype.  The type code and GCMARK bit occupy the lower order 16 bits of the CAR half of the cell.  The rest of the CAR can be used for sub-type or other information. The CDR contains data of size long and is often a pointer to allocated memory.

Like ptobs, new varieties of smobs can be defined dynamically (see Defining Smobs).  These are the initial smobs:

 -- smob: tc_free_cell
     unused cell on the freelist.

 -- smob: tc16_flo
     single-precision float.

     Inexact number data types are subtypes of type tc16_flo.  If the sub-type is:

      0. a single precision float is contained in the CDR.

      1. CDR is a pointer to a malloced double.

      3. CDR is a pointer to a malloced pair of doubles.

      -- smob: tc_dblr
          double-precision float.

      -- smob: tc_dblc
          double-precision complex.

 -- smob: tc16_bigpos
 -- smob: tc16_bigneg
     positive and negative bignums, respectively.

     Scm has large precision integers called bignums.  They are stored in sign-magnitude form with the sign occuring in the type code of the SMOBs bigpos and bigneg.  The magnitude is stored as a malloced array of type BIGDIG which must be an unsigned integral type with size smaller than long.  BIGRAD is the radix associated with BIGDIG.

     NUMDIGS_MAX (defined in scmfig.h) limits the number of digits of a bignum to 1000.  These digits are base BIGRAD, which is typically 65536, giving 4816 decimal digits.

     Why only 4800 digits?  The simple multiplication algorithm SCM uses is O(n^2); this means the number of processor instructions required to perform a multiplication is some multiple of the product of the number of digits of the two multiplicands.

          digits * digits  ==> operations
           5                    x
           50                   100 * x
           500                  10000 * x
           5000                 1000000 * x

     To calculate numbers larger than this, FFT multiplication [O(n*log(n))] and other specialized algorithms are required.  You should obtain a package which specializes in number-theoretical calculations:

     ftp://megrez.math.u-bordeaux.fr/pub/pari/

 -- smob: tc16_promise
     made by DELAY.  See Control features (Revised(5) Scheme).

 -- smob: tc16_arbiter
     synchronization object.  See Process Synchronization.

 -- smob: tc16_macro
     macro expanding function.  See Macro Primitives.

 -- smob: tc16_array
     multi-dimensional array.  See Arrays.

     This type implements both conventional arrays (those with arbitrary data as elements see Conventional Arrays) and uniform arrays (those with elements of a uniform type see Uniform Array).

     Conventional Arrays have a pointer to a vector for their CDR. Uniform Arrays have a pointer to a Uniform Vector type (string, bvect, ivect, uvect, fvect, dvect, or cvect) in their CDR.

Data Type Representations
-------------------------

     IMMEDIATE:      B,D,E,F=data bit, C=flag code, P=pointer address bit
             ................................
     inum    BBBBBBBBBBBBBBBBBBBBBBBBBBBBBB10
     ichr    BBBBBBBBBBBBBBBBBBBBBBBB11110100
     iflag                   CCCCCCC101110100
     isym                    CCCCCCC001110100
             IMCAR:  only in car of evaluated code, cdr has cell's GC bit
     ispcsym                 000CCCC00CCCC100
     iloc    0DDDDDDDDDDDEFFFFFFFFFFF11111100
     pointer PPPPPPPPPPPPPPPPPPPPPPPPPPPPP000
     gloc    PPPPPPPPPPPPPPPPPPPPPPPPPPPPP001

        HEAP CELL:   G=gc_mark; 1 during mark, 0 other times.
             1s and 0s here indicate type.     G missing means sys (not GC'd)
             SIMPLE
     cons    ..........SCM car..............0  ...........SCM cdr.............G
     closure ..........SCM code...........011  ...........SCM env...........CCG
             HEADERs:
     ssymbol .........long length....G0000101  ..........char *chars...........
     msymbol .........long length....G0000111  ..........char *chars...........
     string  .........long length....G0001101  ..........char *chars...........
     vector  .........long length....G0001111  ...........SCM **elts...........
     bvect   .........long length....G0010101  ..........long *words...........
      spare                          G0010111
     ivect   .........long length....G0011101  ..........long *words...........
     uvect   .........long length....G0011111  ......unsigned long *words......
      spare                          G0100101
     svect   .........long length....G0100111  ........ short *words...........
     fvect   .........long length....G0101101  .........float *words...........
     dvect   .........long length....G0101111  ........double *words...........
     cvect   .........long length....G0110101  ........double *words...........

     contin  .........long length....G0111101  .............*regs..............
     specfun ................xxxxxxxxG1111111  ...........SCM name.............
     cclo    ..short length..xxxxxx10G1111111  ...........SCM **elts...........
                             PTOBs
        port int portnum.CwroxxxxxxxxG0110111  ..........FILE *stream..........
      socket int portnum.C001xxxxxxxxG0110111  ..........FILE *stream..........
      inport int portnum.C011xxxxxxxxG0110111  ..........FILE *stream..........
     outport int portnum.0101xxxxxxxxG0110111  ..........FILE *stream..........
      ioport int portnum.C111xxxxxxxxG0110111  ..........FILE *stream..........
     fport   int portnum.C   00000000G0110111  ..........FILE *stream..........
     pipe    int portnum.C   00000001G0110111  ..........FILE *stream..........
     strport 00000000000.0   00000010G0110111  ..........FILE *stream..........
     sfport  int portnum.C   00000011G0110111  ..........FILE *stream..........
             SUBRs
      spare                          010001x1
      spare                          010011x1
     subr_0  ..........int hpoff.....01010101  ...........SCM (*f)()...........
     subr_1  ..........int hpoff.....01010111  ...........SCM (*f)()...........
     cxr     ..........int hpoff.....01011101  .........double (*f)()..........
     subr_3  ..........int hpoff.....01011111  ...........SCM (*f)()...........
     subr_2  ..........int hpoff.....01100101  ...........SCM (*f)()...........
     asubr   ..........int hpoff.....01100111  ...........SCM (*f)()...........
     subr_1o ..........int hpoff.....01101101  ...........SCM (*f)()...........
     subr_2o ..........int hpoff.....01101111  ...........SCM (*f)()...........
     lsubr_2 ..........int hpoff.....01110101  ...........SCM (*f)()...........
     lsubr   ..........int hpoff.....01110111  ...........SCM (*f)()...........
     rpsubr  ..........int hpoff.....01111101  ...........SCM (*f)()...........
                             SMOBs
     free_cell
             000000000000000000000000G1111111  ...........*free_cell........000
     flo     000000000000000000000001G1111111  ...........float num............
     dblr    000000000000000100000001G1111111  ..........double *real..........
     dblc    000000000000001100000001G1111111  .........complex *cmpx..........
     bignum  ...int length...0000001 G1111111  .........short *digits..........
     bigpos  ...int length...00000010G1111111  .........short *digits..........
     bigneg  ...int length...00000011G1111111  .........short *digits..........
                             xxxxxxxx = code assigned by newsmob();
     promise 000000000000000fxxxxxxxxG1111111  ...........SCM val..............
     arbiter 000000000000000lxxxxxxxxG1111111  ...........SCM name.............
     macro   000000000000000mxxxxxxxxG1111111  ...........SCM name.............
     array   ...short rank..cxxxxxxxxG1111111  ............*array..............

Operations
==========

Garbage Collection
------------------

The garbage collector is in the latter half of sys.c.  The primary goal of garbage collection (or GC) is to recycle those cells no longer in use.  Immediates always appear as parts of other objects, so they are not subject to explicit garbage collection.

All cells reside in the heap (composed of heap segments). Note that this is different from what Computer Science usually defines as a heap.

Marking Cells
.............

The first step in garbage collection is to mark all heap objects in use.  Each heap cell has a bit reserved for this purpose.  For pairs (cons cells) the lowest order bit (0) of the CDR is used.  For other types, bit 8 of the CAR is used.  The GC bits are never set except during garbage collection.  Special C macros are defined in scm.h to allow easy manipulation when GC bits are possibly set.  CAR, TYP3, and TYP7 can be used on GC marked cells as they are.

 -- Macro: GCCDR x
     Returns the CDR of a cons cell, even if that cell has been GC marked.

 -- Macro: GCTYP16 x
     Returns the 16 bit type code of a cell.

We need to (recursively) mark only a few objects in order to assure that all accessible objects are marked.  Those objects are sys_protects[] (for example, dynwinds), the current C-stack and the hash table for symbols, symhash.

 -- Function: void gc_mark (SCM OBJ)
     The function gc_mark() is used for marking SCM cells.  If OBJ is marked, gc_mark() returns.  If OBJ is unmarked, gc_mark sets the mark bit in OBJ, then calls gc_mark() on any SCM components of OBJ.  The last call to gc_mark() is tail-called (looped).

 -- Function: void mark_locations (STACKITEM X[], sizet LEN))
     The function mark_locations is used for marking segments of C-stack or saved segments of C-stack (marked continuations).  The argument LEN is the size of the stack in units of size (STACKITEM).

     Each longword in the stack is tried to see if it is a valid cell pointer into the heap.  If it is, the object itself and any objects it points to are marked using gc_mark.  If the stack is word rather than longword aligned (#define WORD_ALIGN), both alignments are tried. This arrangement will occasionally mark an object which is no longer used.  This has not been a problem in practice and the advantage of using the c-stack far outweighs it.

Sweeping the Heap
.................

After all found objects have been marked, the heap is swept.

The storage for strings, vectors, continuations, doubles, complexes, and bignums is managed by malloc.  There is only one pointer to each malloc object from its type-header cell in the heap.  This allows malloc objects to be freed when the associated heap object is garbage collected.

 -- Function: static void gc_sweep ()
     The function gc_sweep scans through all heap segments.  The mark bit is cleared from marked cells.  Unmarked cells are spliced into FREELIST, where they can again be returned by invocations of NEWCELL.

     If a type-header cell pointing to malloc space is unmarked, the malloc object is freed.  If the type header of smob is collected, the smob's free procedure is called to free its storage.

Memory Management for Environments
----------------------------------

  * Ecache was designed and implemented by Radey Shouman.

  * This documentation of ecache was written by Tom Lord.

The memory management component of SCM contains special features which optimize the allocation and garbage collection of environments.

The optimizations are based on certain facts and assumptions:

The SCM evaluator creates many environments with short lifetimes and these account of a large portion of the total number of objects allocated.

The general purpose allocator allocates objects from a freelist, and collects using a mark/sweep algorithm.  Research into garbage collection suggests that such an allocator is sub-optimal for object populations containing a large portion of short-lived members and that allocation strategies involving a copying collector are more appropriate.

It is a property of SCM, reflected throughout the source code, that a simple copying collector can not be used as the general purpose memory manager: much code assumes that the run-time stack can be treated as a garbage collection root set using conservative garbage collection techniques, which are incompatible with objects that change location.

Nevertheless, it is possible to use a mostly-separate copying-collector, just for environments.  Roughly speaking, cons pairs making up environments are initially allocated from a small heap that is collected by a precise copying collector.  These objects must be handled specially for the collector to work.  The (presumably) small number of these objects that survive one collection of the copying heap are copied to the general purpose heap, where they will later be collected by the mark/sweep collector.  The remaining pairs are more rapidly collected than they would otherwise be and all of this collection is accomplished without having to mark or sweep any other segment of the heap.

Allocating cons pairs for environments from this special heap is a heuristic that approximates the (unachievable) goal:

     allocate all short-lived objects from the copying-heap, at no extra cost in allocation time.

Implementation Details
......................

A separate heap (ecache_v) is maintained for the copying collector.  Pairs are allocated from this heap in a stack-like fashion. Objects in this heap may be protected from garbage collection by:

 1. Pushing a reference to the object on a stack specially maintained for that purpose.  This stack (scm_estk) is used in place of the C run-time stack by the SCM evaluator to hold local variables which refer to the copying heap.

 2. Saving a reference to every object in the mark/sweep heap which directly references the copying heap in a root set that is specially maintained for that purpose (scm_egc_roots).  If no object in the mark/sweep heap directly references an object from the copying heap, that object can be preserved by storing a direct reference to it in the copying-collector root set.

 3. Keeping no other references to these objects, except references between the objects themselves, during copying collection.

When the copying heap or root-set becomes full, the copying collector is invoked.  All protected objects are copied to the mark-sweep heap.  All references to those objects are updated.  The copying collector root-set and heap are emptied.

References to pairs allocated specificly for environments are inaccessible to the Scheme procedures evaluated by SCM.  These pairs are manipulated by only a small number of code fragments in the interpreter.  To support copying collection, those code fragments (mostly in eval.c) have been modified to protect environments from garbage collection using the three rules listed above.

During a mark-sweep collection, the copying collector heap is marked and swept almost like any ordinary segment of the general purpose heap.  The only difference is that pairs from the copying heap that become free during a sweep phase are not added to the freelist.

The environment cache is disabled by adding #define NO_ENV_CACHE to eval.c; all environment cells are then allocated from the regular heap.

Relation to Other Work
......................

This work seems to build upon a considerable amount of previous work into garbage collection techniques about which a considerable amount of literature is available.

Signals
-------

 -- Function: init_signals
     (in scm.c) initializes handlers for SIGINT and SIGALRM if they are supported by the C implementation.  All of the signal handlers immediately reestablish themselves by a call to signal().

 -- Function: int_signal sig
 -- Function: alrm_signal sig
     The low level handlers for SIGINT and SIGALRM.

If an interrupt handler is defined when the interrupt is received, the code is interpreted.  If the code returns, execution resumes from where the interrupt happened.  Call-with-current-continuation allows the stack to be saved and restored.

SCM does not use any signal masking system calls.  These are not a portable feature.  However, code can run uninterrupted by use of the C macros DEFER_INTS and ALLOW_INTS.

 -- Macro: DEFER_INTS
     sets the global variable ints_disabled to 1.  If an interrupt occurs during a time when ints_disabled is 1, then deferred_proc is set to non-zero, one of the global variables SIGINT_deferred or SIGALRM_deferred is set to 1, and the handler returns.

 -- Macro: ALLOW_INTS
     Checks the deferred variables and if set the appropriate handler is called.

     Calls to DEFER_INTS can not be nested.  An ALLOW_INTS must happen before another DEFER_INTS can be done.  In order to check that this constraint is satisfied #define CAREFUL_INTS in scmfig.h.

C Macros
--------

 -- Macro: ASRTER cond arg pos subr
     signals an error if the expression (COND) is 0.  ARG is the offending object, SUBR is the string naming the subr, and POS indicates the position or type of error.  POS can be one of

       * ARGn (> 5 or unknown ARG number)

       * ARG1

       * ARG2

       * ARG3

       * ARG4

       * ARG5

       * WNA (wrong number of args)

       * OVFLOW

       * OUTOFRANGE

       * NALLOC

       * EXIT

       * HUP_SIGNAL

       * INT_SIGNAL

       * FPE_SIGNAL

       * BUS_SIGNAL

       * SEGV_SIGNAL

       * ALRM_SIGNAL

       * a C string (char *)

     Error checking is not done by ASRTER if the flag RECKLESS is defined.  An error condition can still be signaled in this case with a call to wta(arg, pos, subr).

 -- Macro: ASRTGO cond label
     goto LABEL if the expression (COND) is 0.  Like ASRTER, ASRTGO does is not active if the flag RECKLESS is defined.

Changing Scm
------------

When writing C-code for SCM, a precaution is recommended.  If your routine allocates a non-cons cell which will not be incorporated into a SCM object which is returned, you need to make sure that a SCM variable in your routine points to that cell as long as part of it might be referenced by your code.

In order to make sure this SCM variable does not get optimized out you can put this assignment after its last possible use:

     SCM_dummy1 = foo;

or put this assignment somewhere in your routine:

     SCM_dummy1 = (SCM) &foo;

SCM_dummy variables are not currently defined.  Passing the address of the local SCM variable to any procedure also protects it.  The procedure scm_protect_temp is provided for this purpose.

 -- Function: void scm_protect_temp (SCM *PTR)
     Forces the SCM object PTR to be saved on the C-stack, where it will be traced for GC.

Also, if you maintain a static pointer to some (non-immediate) SCM object, you must either make your pointer be the value cell of a symbol (see errobj for an example) or (permanently) add your pointer to sys_protects using:

 -- Function: SCM scm_gc_protect (SCM OBJ)
     Permanently adds OBJ to a table of objects protected from garbage collection.  scm_gc_protect returns OBJ.

To add a C routine to scm:

 1. choose the appropriate subr type from the type list.

 2. write the code and put into scm.c.

 3. add a make_subr or make_gsubr call to init_scm.  Or put an entry into the appropriate iproc structure.

To add a package of new procedures to scm (see crs.c for example):

 1. create a new C file (foo.c).

 2. at the front of foo.c put declarations for strings for your procedure names.

          static char s_twiddle_bits[]="twiddle-bits!";
          static char s_bitsp[]="bits?";

 3. choose the appropriate subr types from the type list in code.doc.

 4. write the code for the procedures and put into foo.c

 5. create one iproc structure for each subr type used in foo.c

          static iproc subr3s[]= {
                  {s_twiddle-bits,twiddle-bits},
                  {s_bitsp,bitsp},
                  {0,0} };

 6. create an init_<name of file> routine at the end of the file which calls init_iprocs with the correct type for each of the iprocs created in step 5.

          void init_foo()
          {
            init_iprocs(subr1s, tc7_subr_1);
            init_iprocs(subr3s, tc7_subr_3);
          }

     If your package needs to have a finalization routine called to free up storage, close files, etc, then also have a line in init_foo like:

          add_final(final_foo);

     final_foo should be a (void) procedure of no arguments.  The finals will be called in opposite order from their definition.

     The line:

          add_feature("foo");

     will append a symbol 'foo to the (list) value of *features*.

 7. put any scheme code which needs to be run as part of your package into Ifoo.scm.

 8. put an if into InitSCMVERSION.scm which loads Ifoo.scm if your package is included:

          (if (defined? twiddle-bits!)
              (load (in-vicinity (implementation-vicinity)
                                 "Ifoo"
                                 (scheme-file-suffix))))

     or use (provided? 'foo) instead of (defined? twiddle-bits!) if you have added the feature.

 9. put documentation of the new procedures into foo.doc

 10. add lines to your Makefile to compile and link SCM with your object file.  Add a init_foo\(\)\; to the INITS=... line at the beginning of the makefile.

These steps should allow your package to be linked into SCM with a minimum of difficulty.  Your package should also work with dynamic linking if your SCM has this capability.

Special forms (new syntax) can be added to scm.

 1. define a new MAKISYM in scm.h and increment NUM_ISYMS.

 2. add a string with the new name in the corresponding place in isymnames in repl.c.

 3. add case clause to ceval() near i_quasiquote (in eval.c).

New syntax can now be added without recompiling SCM by the use of the procedure->syntax, procedure->macro, procedure->memoizing-macro, and defmacro.  For details, see Syntax.

Defining Subrs
--------------

If CCLO is #defined when compiling, the compiled closure feature will be enabled.  It is automatically enabled if dynamic linking is enabled.

The SCM interpreter directly recognizes subrs taking small numbers of arguments.  In order to create subrs taking larger numbers of arguments use:

 -- Function: make_gsubr name req opt rest fcn
     returns a cclo (compiled closure) object of name char * NAME which takes int REQ required arguments, int OPT optional arguments, and a list of rest arguments if int REST is 1 (0 for not).

     SCM (*fcn)() is a pointer to a C function to do the work.

     The C function will always be called with REQ + OPT + REST arguments, optional arguments not supplied will be passed UNDEFINED.  An error will be signaled if the subr is called with too many or too few arguments.  Currently a total of 10 arguments may be specified, but increasing this limit should not be difficult.

          /* A silly example, taking 2 required args,
             1 optional, and a list of rest args */

          #include <scm.h>

          SCM gsubr_21l(req1,req2,opt,rst)
               SCM req1,req2,opt,rst;
          {
            lputs("gsubr-2-1-l:\n req1: ", cur_outp);
            display(req1,cur_outp);
            lputs("\n req2: ", cur_outp);
            display(req2,cur_outp);
            lputs("\n opt: ", cur_outp);
            display(opt,cur_outp);
            lputs("\n rest: ", cur_outp);
            display(rst,cur_outp);
            newline(cur_outp);
            return UNSPECIFIED;
          }

          void init_gsubr211()
          {
            make_gsubr("gsubr-2-1-l", 2, 1, 1, gsubr_21l);
          }

Defining Smobs
--------------

Here is an example of how to add a new type named foo to SCM. The following lines need to be added to your code:

long tc16_foo;
     The type code which will be used to identify the new type.

static smobfuns foosmob = {markfoo,freefoo,printfoo,equalpfoo};
     smobfuns is a structure composed of 4 functions:

          typedef struct {
            SCM   (*mark)P((SCM));
            sizet (*free)P((CELLPTR));
            int   (*print)P((SCM exp, SCM port, int writing));
            SCM   (*equalp)P((SCM, SCM));
          } smobfuns;

     smob.mark
          is a function of one argument of type SCM (the cell to mark) and returns type SCM which will then be marked.  If no further objects need to be marked then return an immediate object such as BOOL_F.  The smob cell itself will already have been marked. Note This is different from SCM versions prior to 5c5.  Only additional data specific to a smob type need be marked by smob.mark.

          2 functions are provided:

          markcdr(ptr)
               returns CDR(ptr).

          mark0(ptr)
               is a no-op used for smobs containing no additional SCM data.  0 may also be used in this case.

     smob.free
          is a function of one argument of type CELLPTR (the cell to collected) and returns type sizet which is the number of malloced bytes which were freed.  Smob.free should free any malloced storage associated with this object.  The function free0(ptr) is provided which does not free any storage and returns 0.

     smob.print
          is 0 or a function of 3 arguments.  The first, of type SCM, is the smob object.  The second, of type SCM, is the stream on which to write the result.  The third, of type int, is 1 if the object should be writen, 0 if it should be displayed, and 2 if it should be writen for an error report.  This function should return non-zero if it printed, and zero otherwise (in which case a hexadecimal number will be printed).

     smob.equalp
          is 0 or a function of 2 SCM arguments.  Both of these arguments will be of type tc16foo.  This function should return BOOL_T if the smobs are equal, BOOL_F if they are not.  If smob.equalp is 0, equal? will return BOOL_F if they are not eq?.

tc16_foo = newsmob(&foosmob);
     Allocates the new type with the functions from foosmob.  This line goes in an init_ routine.

Promises and macros in eval.c and arbiters in repl.c provide examples of SMOBs.  There are a maximum of 256 SMOBs. Smobs that must allocate blocks of memory should use, for example, must_malloc rather than malloc see Allocating memory.

Defining Ptobs
--------------

ptobs are similar to smobs but define new types of port to which SCM procedures can read or write.  The following functions are defined in the ptobfuns:

     typedef struct {
       SCM   (*mark)P((SCM ptr));
       int   (*free)P((FILE *p));
       int   (*print)P((SCM exp, SCM port, int writing));
       SCM   (*equalp)P((SCM, SCM));
       int   (*fputc)P((int c, FILE *p));
       int   (*fputs)P((char *s, FILE *p));
       sizet (*fwrite)P((char *s, sizet siz, sizet num, FILE *p));
       int   (*fflush)P((FILE *stream));
       int   (*fgetc)P((FILE *p));
       int   (*fclose)P((FILE *p));
     } ptobfuns;

The .free component to the structure takes a FILE * or other C construct as its argument, unlike .free in a smob, which takes the whole smob cell.  Often, .free and .fclose can be the same function.  See fptob and pipob in sys.c for examples of how to define ptobs. Ptobs that must allocate blocks of memory should use, for example, must_malloc rather than malloc see Allocating memory.

Allocating memory
-----------------

SCM maintains a count of bytes allocated using malloc, and calls the garbage collector when that number exceeds a dynamically managed limit. In order for this to work properly, malloc and free should not be called directly to manage memory freeable by garbage collection. The following functions are provided for that purpose:

 -- Function: SCM must_malloc_cell (long LEN, SCM C, char *WHAT)
 -- Function: char * must_malloc (long LEN, char *WHAT)
     LEN is the number of bytes that should be allocated, WHAT is a string to be used in error or gc messages.  must_malloc returns a pointer to newly allocated memory.  must_malloc_cell returns a newly allocated cell whose car is C and whose cdr is a pointer to newly allocated memory.

 -- Function: void must_realloc_cell (SCM Z, long OLEN, long LEN, char *WHAT)
 -- Function: char * must_realloc (char *WHERE, long OLEN, long LEN, char *WHAT)
     must_realloc_cell takes as argument Z a cell whose cdr should be a pointer to a block of memory of length OLEN allocated with must_malloc_cell and modifies the cdr to point to a block of memory of length LEN.  must_realloc takes as argument WHERE the address of a block of memory of length OLEN allocated by must_malloc and returns the address of a block of length LEN.

     The contents of the reallocated block will be unchanged up to the minimum of the old and new sizes.

     WHAT is a pointer to a string used for error and gc messages.

must_malloc, must_malloc_cell, must_realloc, and must_realloc_cell must be called with interrupts deferred see Signals.  must_realloc and must_realloc_cell must not be called during initialization (non-zero errjmp_bad) -- the initial allocations must be large enough.

 -- Function: void must_free (char *PTR, sizet LEN)
     must_free is used to free a block of memory allocated by the above functions and pointed to by PTR.  LEN is the length of the block in bytes, but this value is used only for debugging purposes. If it is difficult or expensive to calculate then zero may be used instead.

Embedding SCM
-------------

The file scmmain.c contains the definition of main(). When SCM is compiled as a library scmmain.c is not included in the library; a copy of scmmain.c can be modified to use SCM as an embedded library module.

 -- Function: int main (int ARGC, char **ARGV)
     This is the top level C routine.  The value of the ARGC argument is the number of command line arguments.  The ARGV argument is a vector of C strings; its elements are the individual command line argument strings.  A null pointer always follows the last element: ARGV[ARGC] is this null pointer.

 -- Variable: char *execpath
     This string is the pathname of the executable file being run.  This variable can be examined and set from Scheme (see Internal State). EXECPATH must be set to executable's path in order to use DUMP (see Dump) or DLD.

Rename main() and arrange your code to call it with an ARGV which sets up SCM as you want it.

If you need more control than is possible through ARGV, here are descriptions of the functions which main() calls.

 -- Function: void init_sbrk (void)
     Call this before SCM calls malloc().  Value returned from sbrk() is used to gauge how much storage SCM uses.

 -- Function: char * scm_find_execpath (int ARGC, char **ARGV, char *SCRIPT_ARG)
     ARGC and ARGV are as described in main().  SCRIPT_ARG is the pathname of the SCSH-style script (see Scripting) being invoked; 0 otherwise.  scm_find_execpath returns the pathname of the executable being run; if scm_find_execpath cannot determine the pathname, then it returns 0.

scm_find_implpath is defined in scmmain.c.  Preceeding this are definitions ofGENERIC_NAME and INIT_GETENV.  These, along with IMPLINIT and DIRSEP control scm_find_implpath()'s operation.

If your application has an easier way to locate initialization code for SCM, then you can replace scm_find_implpath.

 -- Function: char * scm_find_implpath (char *EXECPATH)
     Returns the full pathname of the Scheme initialization file or 0 if it cannot find it.

     The string value of the preprocessor variable INIT_GETENV names an environment variable (default "SCM_INIT_PATH").  If this environment variable is defined, its value will be returned from scm_find_implpath.  Otherwise find_impl_file() is called with the arguments EXECPATH, GENERIC_NAME (default "scm"), INIT_FILE_NAME (default "InitSCMVERSION_scm"), and the directory separator string DIRSEP.  If find_impl_file() returns 0 and IMPLINIT is defined, then a copy of the string IMPLINIT is returned.

 -- Function: int init_buf0 (FILE *INPORT)
     Tries to determine whether INPORT (usually stdin) is an interactive input port which should be used in an unbuffered mode.  If so, INPORT is set to unbuffered and non-zero is returned. Otherwise, 0 is returned.

     init_buf0 should be called before any input is read from INPORT.  Its value can be used as the last argument to scm_init_from_argv().

 -- Function: void scm_init_from_argv (int ARGC, char **ARGV, char *SCRIPT_ARG, int IVERBOSE, int BUF0STDIN)
     Initializes SCM storage and creates a list of the argument strings PROGRAM-ARGUMENTS from ARGV.  ARGC and ARGV must already be processed to accomodate Scheme Scripts (if desired).  The scheme variable *SCRIPT* is set to the string SCRIPT_ARG, or #f if SCRIPT_ARG is 0. IVERBOSE is the initial prolixity level.  If BUF0STDIN is non-zero, stdin is treated as an unbuffered port.

Call init_signals and restore_signals only if you want SCM to handle interrupts and signals.

 -- Function: void init_signals (void)
     Initializes handlers for SIGINT and SIGALRM if they are supported by the C implementation.  All of the signal handlers immediately reestablish themselves by a call to signal().

 -- Function: void restore_signals (void)
     Restores the handlers in effect when init_signals was called.

 -- Function: SCM scm_top_level (char *INITPATH, SCM (*toplvl_fun)())
     This is SCM's top-level.  Errors longjmp here.  TOPLVL_FUN is a callback function of zero arguments that is called by scm_top_level to do useful work -- if zero, then repl, which implements a read-eval-print loop, is called.

     If TOPLVL_FUN returns, then scm_top_level will return as well.  If the return value of TOPLVL_FUN is an immediate integer then it will be used as the return value of scm_top_level.  In the main function supplied with SCM, this return value is the exit status of the process.

     If the first character of string INITPATH is ;, ( or whitespace, then scm_ldstr() is called with INITPATH to initialize SCM; otherwise INITPATH names a file of Scheme code to be loaded to initialize SCM.

     When a Scheme error is signaled; control will pass into scm_top_level by longjmp, error messages will be printed to current-error-port, and then TOPLVL_FUN will be called again.  TOPLVL_FUN must maintain enough state to prevent errors from being resignalled.  If toplvl_fun can not recover from an error situation it may simply return.

 -- Function: void final_scm (int FREEALL)
     Calls all finalization routines registered with add_final().  If FREEALL is non-zero, then all memory which SCM allocated with malloc() will be freed.

You can call indivdual Scheme procedures from C code in the TOPLVL_FUN argument passed to scm_top_level(), or from module subrs (registered by an init_ function, see Changing Scm).

Use apply to call Scheme procedures from your C code.  For example:

     /* If this apply fails, SCM will catch the error */
     apply(CDR(intern("srv:startup",sizeof("srv:startup")-1)),
           mksproc(srvproc),
           listofnull);

     func = CDR(intern(rpcname,strlen(rpcname)));
     retval = apply(func, cons(mksproc(srvproc), args), EOL);

Functions for loading Scheme files and evaluating Scheme code given as C strings are described in the next section, (see Callbacks).

Here is a minimal embedding program libtest.c:

     /* gcc -o libtest libtest.c libscm.a -ldl -lm -lc */
     #include "scm.h"
     /* include patchlvl.h for SCM's INIT_FILE_NAME. */
     #include "patchlvl.h"

     void libtest_init_user_scm()
     {
       fputs("This is libtest_init_user_scm\n", stderr); fflush(stderr);
       sysintern("*the-string*", makfrom0str("hello world\n"));
     }

     SCM user_main()
     {
       static int done = 0;
       if (done++) return MAKINUM(EXIT_FAILURE);
       scm_ldstr("(display *the-string*)");
       return MAKINUM(EXIT_SUCCESS);
     }

     int main(argc, argv)
          int argc;
          char **argv;
     {
       SCM retval;
       char *implpath, *execpath;

       init_user_scm = libtest_init_user_scm;
       execpath = dld_find_executable(argv[0]);
       fprintf(stderr, "dld_find_executable(%s): %s\n", argv[0], execpath);
       implpath = find_impl_file(execpath, "scm", INIT_FILE_NAME, dirsep);
       fprintf(stderr, "implpath: %s\n", implpath);
       scm_init_from_argv(argc, argv, 0L, 0, 0);

       retval = scm_top_level(implpath, user_main);

       final_scm(!0);
       return (int)INUM(retval);
     }

     -|
     dld_find_executable(./libtest): /home/jaffer/scm/libtest
     implpath: /home/jaffer/scm/InitSCMVERSION.scm
     This is libtest_init_user_scm
     hello world

Callbacks
---------

SCM now has routines to make calling back to Scheme procedures easier. The source code for these routines are found in rope.c.

 -- Function: int scm_ldfile (char *FILE)
     Loads the Scheme source file FILE.  Returns 0 if successful, non-0 if not.  This function is used to load SCM's initialization file InitSCMVERSION.scm.

 -- Function: int scm_ldprog (char *FILE)
     Loads the Scheme source file (in-vicinity (program-vicinity) FILE).  Returns 0 if successful, non-0 if not.

     This function is useful for compiled code init_ functions to load non-compiled Scheme (source) files.  program-vicinity is the directory from which the calling code was loaded (see Vicinity (SLIB)).

 -- Function: SCM scm_evstr (char *STR)
     Returns the result of reading an expression from STR and evaluating it.

 -- Function: void scm_ldstr (char *STR)
     Reads and evaluates all the expressions from STR.

If you wish to catch errors during execution of Scheme code, then you can use a wrapper like this for your Scheme procedures:

     (define (srv:protect proc)
       (lambda args
         (define result #f)                  ; put default value here
         (call-with-current-continuation
          (lambda (cont)
            (dynamic-wind (lambda () #t)
                          (lambda ()
                            (set! result (apply proc args))
                            (set! cont #f))
                          (lambda ()
                            (if cont (cont #f))))))
         result))

Calls to procedures so wrapped will return even if an error occurs.

Type Conversions
----------------

These type conversion functions are very useful for connecting SCM and C code.  Most are defined in rope.c.

 -- Function: SCM long2num (long N)
 -- Function: SCM ulong2num (unsigned long N)
     Return an object of type SCM corresponding to the long or unsigned long argument N.  If N cannot be converted, BOOL_F is returned.  Which numbers can be converted depends on whether SCM was compiled with the BIGDIG or FLOATS flags.

     To convert integer numbers of smaller types (short or char), use the macro MAKINUM(n).

 -- Function: long num2long (SCM NUM, char *POS, char *S_CALLER)
 -- Function: unsigned long num2ulong (SCM NUM, char *POS, char *S_CALLER)
 -- Function: short num2short (SCM NUM, char *POS, char *S_CALLER)
 -- Function: unsigned short num2ushort (SCM NUM, char *POS, char *S_CALLER)
 -- Function: unsigned char num2uchar (SCM NUM, char *POS, char *S_CALLER)
 -- Function: double num2dbl (SCM NUM, char *POS, char *S_CALLER)
     These functions are used to check and convert SCM arguments to the named C type.  The first argument NUM is checked to see it it is within the range of the destination type.  If so, the converted number is returned.  If not, the ASRTER macro calls wta with NUM and strings POS and S_CALLER.  For a listing of useful predefined POS macros, see C Macros.

     Note Inexact numbers are accepted only by num2dbl, num2long, and num2ulong (for when SCM is compiled without bignums).  To convert inexact numbers to exact numbers, see inexact->exact (Revised(5) Scheme).

 -- Function: unsigned long scm_addr (SCM ARGS, char *S_NAME)
     Returns a pointer (cast to an unsigned long) to the storage corresponding to the location accessed by aref(CAR(args),CDR(args)).  The string S_NAME is used in any messages from error calls by scm_addr.

     scm_addr is useful for performing C operations on strings or other uniform arrays (see Uniform Array).

 -- Function: unsigned long scm_base_addr(SCM RA, char *S_NAME)
     Returns a pointer (cast to an unsigned long) to the beginning of storage of array RA.  Note that if RA is a shared-array, the strorage accessed this way may be much larger than RA.

     Note While you use a pointer returned from scm_addr or scm_base_addr you must keep a pointer to the associated SCM object in a stack allocated variable or GC-protected location in order to assure that SCM does not reuse that storage before you are done with it.  See scm_gc_protect.

 -- Function: SCM makfrom0str (char *SRC)
 -- Function: SCM makfromstr (char *SRC, sizet LEN)
     Return a newly allocated string SCM object copy of the null-terminated string SRC or the string SRC of length LEN, respectively.

 -- Function: SCM makfromstrs (int ARGC, char **ARGV)
     Returns a newly allocated SCM list of strings corresponding to the ARGC length array of null-terminated strings ARGV.  If ARGV is less than 0, ARGV is assumed to be NULL terminated.  makfromstrs is used by scm_init_from_argv to convert the arguments SCM was called with to a SCM list which is the value of SCM procedure calls to program-arguments (see program-arguments).

 -- Function: char ** makargvfrmstrs (SCM ARGS, char *S_NAME)
     Returns a NULL terminated list of null-terminated strings copied from the SCM list of strings ARGS.  The string S_NAME is used in messages from error calls by makargvfrmstrs.

     makargvfrmstrs is useful for constructing argument lists suitable for passing to main functions.

 -- Function: void must_free_argv (char **ARGV)
     Frees the storage allocated to create ARGV by a call to makargvfrmstrs.

Continuations
-------------

The source files continue.h and continue.c are designed to function as an independent resource for programs wishing to use continuations, but without all the rest of the SCM machinery.  The concept of continuations is explained in call-with-current-continuation (Revised(5) Scheme).

The C constructs jmp_buf, setjmp, and longjmp implement escape continuations.  On VAX and Cray platforms, the setjmp provided does not save all the registers.  The source files setjump.mar, setjump.s, and ugsetjump.s provide implementations which do meet this criteria.

SCM uses the names jump_buf, setjump, and longjump in lieu of jmp_buf, setjmp, and longjmp to prevent name and declaration conflicts.

 -- Data type: CONTINUATION jmpbuf length stkbse other parent
     is a typedefed structure holding all the information needed to represent a continuation.  The OTHER slot can be used to hold any data the user wishes to put there by defining the macro CONTINUATION_OTHER.

 -- Macro: SHORT_ALIGN
     If SHORT_ALIGN is #defined (in scmfig.h), then the it is assumed that pointers in the stack can be aligned on short int boundaries.

 -- Data type: STACKITEM
     is a pointer to objects of the size specified by SHORT_ALIGN being #defined or not.

 -- Macro: CHEAP_CONTINUATIONS
     If CHEAP_CONTINUATIONS is #defined (in scmfig.h) each CONTINUATION has size sizeof CONTINUATION. Otherwise, all but root CONTINUATIONs have additional storage (immediately following) to contain a copy of part of the stack.

     Note On systems with nonlinear stack disciplines (multiple stacks or non-contiguous stack frames) copying the stack will not work properly.  These systems need to #define CHEAP_CONTINUATIONS in scmfig.h.

 -- Macro: STACK_GROWS_UP
     Expresses which way the stack grows by its being #defined or not.

 -- Variable: long thrown_value
     Gets set to the VALUE passed to throw_to_continuation.

 -- Function: long stack_size (STACKITEM *START)
     Returns the number of units of size STACKITEM which fit between START and the current top of stack.  No check is done in this routine to ensure that START is actually in the current stack segment.

 -- Function: CONTINUATION * make_root_continuation (STACKITEM *STACK_BASE)
     Allocates (malloc) storage for a CONTINUATION of the current extent of stack.  This newly allocated CONTINUATION is returned if successful, 0 if not.  After make_root_continuation returns, the calling routine still needs to setjump(NEW_CONTINUATION->jmpbuf) in order to complete the capture of this continuation.

 -- Function: CONTINUATION * make_continuation (CONTINUATION *PARENT_CONT)
     Allocates storage for the current CONTINUATION, copying (or encapsulating) the stack state from PARENT_CONT->stkbse to the current top of stack.  The newly allocated CONTINUATION is returned if successful, 0q if not.  After make_continuation returns, the calling routine still needs to setjump(NEW_CONTINUATION->jmpbuf) in order to complete the capture of this continuation.

 -- Function: void free_continuation (CONTINUATION *CONT)
     Frees the storage pointed to by CONT.  Remember to free storage pointed to by CONT->other.

 -- Function: void throw_to_continuation (CONTINUATION *CONT, long VALUE, CONTINUATION *ROOT_CONT)
     Sets thrown_value to VALUE and returns from the continuation CONT.

     If CHEAP_CONTINUATIONS is #defined, then throw_to_continuation does longjump(CONT->jmpbuf, val).

     If CHEAP_CONTINUATIONS is not #defined, the CONTINUATION CONT contains a copy of a portion of the C stack (whose bound must be CONT(ROOT_CONT)->stkbse).  Then:

       * the stack is grown larger than the saved stack, if neccessary.

       * the saved stack is copied back into it's original position.

       * longjump(CONT->jmpbuf, val);

Evaluation
----------

SCM uses its type representations to speed evaluation.  All of the subr types (see Subr Cells) are tc7 types.  Since the tc7 field is in the low order bit position of the CAR it can be retrieved and dispatched on quickly by dereferencing the SCM pointer pointing to it and masking the result.

All the SCM Special Forms get translated to immediate symbols (isym) the first time they are encountered by the interpreter (ceval).  The representation of these immediate symbols is engineered to occupy the same bits as tc7.  All the isyms occur only in the CAR of lists.

If the CAR of a expression to evaluate is not immediate, then it may be a symbol.  If so, the first time it is encountered it will be converted to an immediate type ILOC or GLOC (see Immediates).  The codes for ILOC and GLOC lower 7 bits distinguish them from all the other types we have discussed.

Once it has determined that the expression to evaluate is not immediate, ceval need only retrieve and dispatch on the low order 7 bits of the CAR of that cell, regardless of whether that cell is a closure, header, or subr, or a cons containing ILOC or GLOC.

In order to be able to convert a SCM symbol pointer to an immediate ILOC or GLOC, the evaluator must be holding the pointer to the list in which that symbol pointer occurs.  Turning this requirement to an advantage, ceval does not recursively call itself to evaluate symbols in lists; It instead calls the macro EVALCAR.  EVALCAR does symbol lookup and memoization for symbols, retrieval of values for ILOCs and GLOCs, returns other immediates, and otherwise recursively calls itself with the CAR of the list.

ceval inlines evaluation (using EVALCAR) of almost all procedure call arguments.  When ceval needs to evaluate a list of more than length 3, the procedure eval_args is called.  So ceval can be said to have one level lookahead.  The avoidance of recursive invocations of ceval for the most common cases (special forms and procedure calls) results in faster execution.  The speed of the interpreter is currently limited on most machines by interpreter size, probably having to do with its cache footprint.  In order to keep the size down, certain EVALCAR calls which don't need to be fast (because they rarely occur or because they are part of expensive operations) are instead calls to the C function evalcar.

 -- Variable: symhash
     Top level symbol values are stored in the symhash table. symhash is an array of lists of ISYMs and pairs of symbols and values.

 -- Immediate: ILOC
     Whenever a symbol's value is found in the local environment the pointer to the symbol in the code is replaced with an immediate object (ILOC) which specifies how many environment frames down and how far in to go for the value.  When this immediate object is subsequently encountered, the value can be retrieved quickly.

ILOCs work up to a maximum depth of 4096 frames or 4096 identifiers in a frame.  Radey Shouman added FARLOC to handle cases exceeding these limits.  A FARLOC consists of a pair whose CAR is the immediate type IM_FARLOC_CAR or IM_FARLOC_CDR, and whose CDR is a pair of INUMs specifying the frame and distance with a larger range than ILOCs span.

Adding #define TEST_FARLOC to eval.c causes FARLOCs to be generated for all local identifiers; this is useful only for testing memoization.

 -- Immediate: GLOC
     Pointers to symbols not defined in local environments are changed to one plus the value cell address in symhash.  This incremented pointer is called a GLOC.  The low order bit is normally reserved for GCmark; But, since references to variables in the code always occur in the CAR position and the GCmark is in the CDR, there is no conflict.

If the compile FLAG CAUTIOUS is #defined then the number of arguments is always checked for application of closures.  If the compile FLAG RECKLESS is #defined then they are not checked.  Otherwise, number of argument checks for closures are made only when the function position (whose value is the closure) of a combination is not an ILOC or GLOC.  When the function position of a combination is a symbol it will be checked only the first time it is evaluated because it will then be replaced with an ILOC or GLOC.

 -- Macro: EVAL expression env
 -- Macro: SIDEVAL expression env
     EVAL Returns the result of evaluating EXPRESSION in ENV.  SIDEVAL evaluates EXPRESSION in ENV when the value of the expression is not used.

     Both of these macros alter the list structure of EXPRESSION as it is memoized and hence should be used only when it is known that EXPRESSION will not be referenced again.  The C function eval is safe from this problem.

 -- Function: SCM eval (SCM EXPRESSION)
     Returns the result of evaluating EXPRESSION in the top-level environment.  eval copies expression so that memoization does not modify expression.

Program Self-Knowledge
======================

File-System Habitat
-------------------

Where should software reside?  Although individually a minor annoyance, cumulatively this question represents many thousands of frustrated user hours spent trying to find support files or guessing where packages need to be installed.  Even simple programs require proper habitat; games need to find their score files.

Aren't there standards for this?  Some Operating Systems have devised regimes of software habitats -- only to have them violated by large software packages and imports from other OS varieties.

In some programs, the expected locations of support files are fixed at time of compilation.  This means that the program may not run on configurations unanticipated by the authors.  Compiling locations into a program also can make it immovable -- necessitating recompilation to install it.

     Programs of the world unite!  You have nothing to lose but loss itself.

The function find_impl_file in scm.c is an attempt to create a utility (for inclusion in programs) which will hide the details of platform-dependent file habitat conventions.  It takes as input the pathname of the executable file which is running.  If there are systems for which this information is either not available or unrelated to the locations of support files, then a higher level interface will be needed.

 -- Function: char * find_impl_file (char *EXEC_PATH, char *GENERIC_NAME, char *INITNAME, char *SEP)

     Given the pathname of this executable (EXEC_PATH), test for the existence of INITNAME in the implementation-vicinity of this program.  Return a newly allocated string of the path if successful, 0 if not.  The SEP argument is a null-terminated string of the character used to separate directory components.

  * One convention is to install the support files for an executable program in the same directory as the program.  This possibility is tried first, which satisfies not only programs using this convention, but also uninstalled builds when testing new releases, etc.

  * Another convention is to install the executables in a directory named bin, BIN, exe, or EXE and support files in a directroy named lib, which is a peer the executable directory. This arrangement allows multiple executables can be stored in a single directory.  For example, the executable might be in /usr/local/bin/ and initialization file in /usr/local/lib/.

     If the executable directory name matches, the peer directroy lib is tested for INITNAME.

  * Sometimes lib directories become too crowded.  So we look in any subdirectories of lib or src having the name (sans type suffix such as .EXE) of the program we are running.  For example, the executable might be /usr/local/bin/foo and initialization file in /usr/local/lib/foo/.

  * But the executable name may not be the usual program name; So also look in any GENERIC_NAME subdirectories of lib or src peers.

  * Finally, if the name of the executable file being run has a (system dependent) suffix which is not needed to invoke the program, then look in a subdirectory (of the one containing the executable file) named for the executable (without the suffix); And look in a GENERIC_NAME subdirectory.  For example, the executable might be C:\foo\bar.exe and the initialization file in C:\foo\bar\.

Executable Pathname
-------------------

For purposes of finding InitSCMVERSION.scm, dumping an executable, and dynamic linking, a SCM session needs the pathname of its executable image.

When a program is executed by MS-DOS, the full pathname of that executable is available in argv[0].  This value can be passed directly to find_impl_file (see File-System Habitat).

In order to find the habitat for a unix program, we first need to know the full pathname for the associated executable file.

 -- Function: char * dld_find_executable (const char *COMMAND)
     dld_find_executable returns the absolute path name of the file that would be executed if COMMAND were given as a command.  It looks up the environment variable PATH, searches in each of the directory listed for COMMAND, and returns the absolute path name for the first occurrence.  Thus, it is advisable to invoke dld_init as:

          main (int argc, char **argv)
          {
              ...
              if (dld_init (dld_find_executable (argv[0]))) {
                  ...
              }
              ...
          }

          Note@: If the current process is executed using the execve call without passing the correct path name as argument 0, dld_find_executable (argv[0])  will also fail to locate the executable file.

     dld_find_executable returns zero if command is not found in any of the directories listed in PATH.

Script Support
--------------

Source code for these C functions is in the file script.c. Scripting for a description of script argument processing.

script_find_executable is only defined on unix systems.

 -- Function: char * script_find_executable (const char *NAME)
     script_find_executable returns the path name of the executable which is invoked by the script file NAME; NAME if it is a binary executable (not a script); or 0 if NAME does not exist or is not executable.

 -- Function: char ** script_process_argv (int ARGC; char **ARGV)
     Given an main style argument vector ARGV and the number of arguments, ARGC, script_process_argv returns a newly allocated argument vector in which the second line of the script being invoked is substituted for the corresponding meta-argument.

     If the script does not have a meta-argument, or if the file named by the argument following a meta-argument cannot be opened for reading, then 0 is returned.

     script_process_argv correctly processes argument vectors of nested script invocations.

 -- Function: int script_count_argv (char **ARGV)
     Returns the number of argument strings in ARGV.

Improvements To Make
====================

  * Allow users to set limits for malloc() storage.

  * Prefix and make more uniform all C function, variable, and constant names.  Provide a file full of #define's to provide backward compatability.

  * lgcd() needs to generate at most one bignum, but currently generates more.

  * divide() could use shifts instead of multiply and divide when scaling.

  * Currently, dumping an executable does not preserve ports.  When loading a dumped executable, disk files could be reopened to the same file and position as they had when the executable was dumped.

  * Copying all of the stack is wasteful of storage.  Any time a call-with-current-continuation is called the stack could be re-rooted with a frame which calls the contin just created.  This in combination with checking stack depth could also be used to allow stacks deeper than 64K on the IBM PC.

  * In the quest for speed, there has been some discussion about a "Forth" style Scheme interpreter.

          Provided there is still type code space available in SCM, if we devote some of the IMCAR codes to "inlined" operations, we should get a significant performance boost.  What is eliminated is the having to look up a GLOC or ILOC and then dispatch on the subr type.  The IMCAR operation would be dispatched to directly.  Another way to view this is that we make available special form versions of CAR, CDR, etc.  Since the actual operation code is localized in the interpreter, it is much easier than uncompilation and then recompilation to handle (trace car); For instance a switch gets set which tells the interpreter to instead always look up the values of the associated symbols.

  * Scott Schwartz <schwartz@galapagos.cse.psu.edu> suggests: One way to tidy up the dynamic loading stuff would be to grab the code from perl5.

VMS Dynamic Linking
-------------------

George Carrette (gjc@mitech.com) outlines how to dynamically link on VMS.  There is already some code in dynl.c to do this, but someone with a VMS system needs to finish and debug it.

 1. Say you have this main.c program:

          main()
          {init_lisp();
           lisp_repl();}

 2. and you have your lisp in files repl.c, gc.c, eval.c and there are some toplevel non-static variables in use called the_heap, the_environment, and some read-only toplevel structures, such as the_subr_table.

          $ LINK/SHARE=LISPRTL.EXE/DEBUG REPL.OBJ,GC.OBJ,EVAL.OBJ,LISPRTL.OPT/OPT

 3. where LISPRTL.OPT must contain at least this:

          SYS$LIBRARY:VAXCRTL/SHARE
          UNIVERSAL=init_lisp
          UNIVERSAL=lisp_repl
          PSECT_ATTR=the_subr_table,SHR,NOWRT,LCL
          PSECT_ATTR=the_heap,NOSHR,LCL
          PSECT_ATTR=the_environment,NOSHR,LCL

     Notice The psect (Program Section) attributes.

     LCL
          means to keep the name local to the shared library.  You almost always want to do that for a good clean library.

     SHR,NOWRT
          means shared-read-only.  Which is the default for code, and is also good for efficiency of some data structures.

     NOSHR,LCL
          is what you want for everything else.

     Note: If you do not have a handy list of all these toplevel variables, do not dispair.  Just do your link with the /MAP=LISPRTL.MAP/FULL and then search the map file,

          $SEARCH/OUT=LISPRTL.LOSERS LISPRTL.MAP  ",  SHR,NOEXE,  RD,  WRT"

     And use an emacs keyboard macro to muck the result into the proper form. Of course only the programmer can tell if things can be made read-only. I have a DCL command procedure to do this if you want it.

 4. Now MAIN.EXE would be linked thusly:

          $ DEFINE LISPRTL USER$DISK:[JAFFER]LISPRTL.EXE

          $LINK MAIN.OBJ,SYS$INPUT:/OPT
           SYS$LIBRARY:VAXCRTL/SHARE
           LISPRTL/SHARE

     Note the definition of the LISPRTL logical name.  Without such a definition you will need to copy LISPRTL.EXE over to SYS$SHARE (aka SYS$LIBRARY) in order to invoke the main program once it is linked.

 5. Now say you have a file of optional subrs, MYSUBRS.C.  And there is a routine INIT_MYSUBRS that must be called before using it.

          $ CC MYSUBRS.C
          $ LINK/SHARE=MYSUBRS.EXE MYSUBRS.OBJ,SYS$INPUT:/OPT
            SYS$LIBRARY:VAXCRTL/SHARE
            LISPRTL/SHARE
            UNIVERSAL=INIT_MYSUBRS

     Ok.  Another hint is that you can avoid having to add the PSECT declaration of NOSHR,LCL by declaring variables status in the C language source.  That works great for most things.

 6. Then the dynamic loader would have to do this:

          {void (*init_fcn)();
           long retval;
           retval = lib$find_image_symbol("MYSUBRS","INIT_MYSUBRS",&init_fcn,
                                          "SYS$DISK:[].EXE");
           if (retval != SS$_NORMAL) error(...);
           (*init_fcn)();}

     But of course all string arguments must be (struct dsc$descriptor *) and the last argument is optional if MYSUBRS is defined as a logical name or if MYSUBRS.EXE has been copied over to SYS$SHARE.  The other consideration is that you will want to turn off C-c or other interrupt handling while you are inside most lib$ calls.

     As far as the generation of all the UNIVERSAL=... declarations.  Well, you could do well to have that automatically generated from the public LISPRTL.H file, of course.

     VMS has a good manual called the Guide to Writing Modular Procedures or something like that, which covers this whole area rather well, and also talks about advanced techniques, such as a way to declare a program section with a pointer to a procedure that will be automatically invoked whenever any shared image is dynamically activated.  Also, how to set up a handler for normal or abnormal program exit so that you can clean up side effects (such as opening a database). But for use with LISPRTL you probably don't need that hair.

     One fancier option that is useful under VMS for LISPLIB.EXE is to define all your exported procedures through an call vector instead of having them just be pointers into random places in the image, which is what you get by using UNIVERSAL.

     If you set up the call vector thing correctly it will allow you to modify and relink LISPLIB.EXE without having to relink programs that have been linked against it.

Procedure and Macro Index
*************************

This is an alphabetical list of all the procedures and macros in SCM.

Variable Index
**************

This is an alphabetical list of all the global variables in SCM.

Type Index
**********

This is an alphabetical list of data types and feature names in SCM.

This is an alphabetical list of concepts introduced in this manual.