This manual documents the SCM Scheme implementation.  The most recent information about SCM can be found on SCM's WWW home page:
http://www-swiss.ai.mit.edu/~jaffer/SCM.html

Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1996 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.

Copying
*******

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

Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995
Free Software Foundation, Inc.
675 Mass Ave, Cambridge, MA 02139, USA

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.

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^4 Report on the Algorithmic Language Scheme [R4RS] and the [IEEE] P1178 specification.

  * Support for [SICP], [R2RS], [R3RS], and (proposed) [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 (jaffer@ai.mit.edu)
     Most of SCM.

Radey Shouman
     Arrays. gsubrs, compiled closures, and records.

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.

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

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

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

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

[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.

[GUILE]
     Tom Lord. The Guile Architecture for Ubiquitous Computing. Usenix Symposium on Tcl/Tk, 1995.

[SLIB]
     Todd R. Eigenschink, Dave Love, and Aubrey Jaffer. SLIB, The Portable Scheme Library. Version 2a3, June 1995.

     SLIB.

[JACAL]
     Aubrey Jaffer. JACAL Symbolic Mathematics System. Version 1a5, April 1994.

     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.

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

      scm  [-a kbytes] [-ibvqmu] [-p number]
     [-c expression] [-e expression] [-f filename] [-l filename] [-r feature] [-- | - | -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 Init.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, Init.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.

Init.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 kb
     specifies that scm should allocate an initial heapsize of KB 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: -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: -r feature
     requires FEATURE. This will load a file from [SLIB] if that FEATURE is not already supported. If FEATURE is 2, 3, 4, or 5 scm will require the features neccessary to support [R2RS], [R3RS], [R4RS], or proposed [R5RS], respectively.

 -- 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: -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 [R4RS] macro capability. To use a specific [R4RS] 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 [R4RS] macro capability. [R4RS] 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 further options are to be treated as program aguments.

 -- Command Option: -
 -- Command Option: --
     specifies that there are no more options on the command line.

 -- Command Option: -d filename
     loads SLIB database-utilities and opens FILENAME as a database.

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

     If options appear on the command line after -o FILENAME, 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: --help
     prints usage information and URL; then exit.

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

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

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

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

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

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: *R4RS-macro*
     controls whether loading and interaction support [R4RS] 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.

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 [R4RS] macros and enters interactive (with macros) session.

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

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.

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

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.  However, the run-scheme (xscheme.el) which comes included with Gnu Emacs 18 will work only with MIT Cscheme.  If you are using Emacs 18, get the emacs packages:

       * ftp-swiss.ai.mit.edu:/pub/scheme-editor-packages/cmuscheme.el

       * ftp-swiss.ai.mit.edu:/pub/scheme-editor-packages/comint.el

     These files are already standard in Emacs 19.

     If your Emacs can not run a process in a buffer, see ``under 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 routine I use most for debugging is:

 -- Procedure: 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 '<proc-name> and ) around an expression in order to see its value as a program operates.

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.

     ftp-swiss.ai.mit.edu:pub/scm/slib-psd1-3.tar.gz
     prep.ai.mit.edu:pub/gnu/jacal/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

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

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 scm_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 *scm_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 mull-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
===================

When a program is executed by MS-DOS, the full pathname of that executable is available in argv[0].  This value can be passed to dld_find_executable (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.

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

The SCM distribution has Makefile which contains rules for making scmlit, a ``bare-bones'' version of SCM sufficient for running build.scm.  build.scm 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.scm, there are several ways to proceed:

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

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

  * Create your own script or Makefile.

  * Buy a SCM executable from jaffer@ai.mit.edu.  See the end of the ANNOUNCE file in the distribution for details.

  * Use scmconfig (From: bos@scrg.cs.tcd.ie):

     Build and install scripts using GNU autoconf are available from scmconfig4e6.tar.gz in the distribution directories.  See README.unix in scmconfig4e6.tar.gz for further instructions.

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

The file build.scm builds and runs a relational database of how to compile and link SCM executables.  It has information for most 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.scm.

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.scm followed by command line arguments.

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

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:

     name              processor         operating-system  compiler
     symbol            processor-family  operating-system  symbol
     symbol            atom              symbol            symbol
     ================= ================= ================= =================
     *unknown*         *unknown*         unix              *unknown*
     acorn-unixlib     acorn             *unknown*         *unknown*
     aix               powerpc           aix               *unknown*
     amiga-aztec       m68000            amiga             aztec
     amiga-dice-c      m68000            amiga             dice-c
     amiga-sas/c-5.10  m68000            amiga             sas/c
     atari-st-gcc      m68000            atari.st          gcc
     atari-st-turbo-c  m68000            atari.st          turbo-c
     borland-c-3.1     8086              ms-dos            borland-c
     djgpp             i386              ms-dos            gcc
     gcc               *unknown*         unix              gcc
     highc.31          i386              ms-dos            highc
     hp-ux             hp-risc           hp-ux             *unknown*
     linux             i386              linux             gcc
     linux-elf         i386              linux             gcc
     microsoft-c       8086              ms-dos            microsoft-c
     microsoft-c-nt    i386              ms-dos            microsoft-c
     microsoft-quick-c 8086              ms-dos            microsoft-quick-c
     ms-dos            8086              ms-dos            *unknown*
     os/2-cset         i386              os/2              c-set++
     os/2-emx          i386              os/2              gcc
     sun               sparc             sun-os            *unknown*
     svr4              *unknown*         unix              *unknown*
     turbo-c-2         8086              ms-dos            turbo-c
     unicos            cray              unicos            *unknown*
     unix              *unknown*         unix              *unknown*
     vms               vax               vms               *unknown*
     vms-gcc           vax               vms               gcc
     watcom-9.0        i386              ms-dos            watcom

 -- 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 Init.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

       * 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:

     lit
     none
          Lightweight -- no features

     cautious
          Normally, the number of arguments arguments to interpreted closures (from LAMBDA) are checked if the function part of a form is not a symbol or only the first time the form is executed if the function part is a symbol.  defining reckless disables any checking.  If you want to have SCM always check the number of arguments to interpreted closures define feature cautious.

     careful-interrupt-masking
          Define this for extra checking of interrupt masking.  This is for debugging C code in sys.c and repl.c.

     debug
          Turns on features cautious careful-interrupt-masking stack-limit and uses -g flags for debugging SCM source code.

     reckless
          If your scheme code runs without any errors you can disable almost all error checking by compiling all files with reckless.

     stack-limit
          Use to enable checking for stack overflow.  Define value of the C preprocessor variable STACK_LIMIT to be the size to which SCM should allow the stack to grow.  STACK_LIMIT should be less than the maximum size the hardware can support, as not every routine checks the stack.

     bignums
          Large precision integers.

     arrays
          Use if you want arrays, uniform-arrays and uniform-vectors.

     array-for-each
          array-map! and array-for-each (arrays must also be defined).

     inexact
          Use if you want floating point numbers.

     engineering-notation
          Use if you want floats to display in engineering notation (exponents always multiples of 3) instead of scientific notation.

     single-precision-only
          Use if you want all inexact real numbers to be single precision.  This only has an effect if SINGLES is also defined (which is the default). This does not affect complex numbers.

     sicp
          Use if you want to run code from:

          H. Abelson, G. J. Sussman, and J. Sussman, Structure and Interpretation of Computer Programs, The MIT Press, Cambridge, Massachusetts, USA

          (eq? '() '#f) is the major difference.

     rev2-procedures
          These procedures were specified in the Revised^2 Report on Scheme but not in R4RS.

     record
          The Record package provides a facility for user to define their own record data types.  See SLIB for documentation.

     compiled-closure
          Use if you want to use compiled closures.

     generalized-c-arguments
          make_gsubr for arbitrary (< 11) arguments to C functions.

     tick-interrupts
          Use if you want the ticks and ticks-interrupt functions.

     i/o-extensions
          Commonly available I/O extensions: exec, line I/O, file positioning, file delete and rename, and directory functions.

     turtlegr
          Turtle graphics calls for both Borland-C and X11 from sjm@ee.tut.fi.

     curses
          For the curses screen management package.

     edit-line
          interface to the editline or GNU readline library.

     regex
          String regular expression matching.

     socket
          BSD socket interface.

     posix
          Posix functions available on all Unix-like systems.  fork and process functions, user and group IDs, file permissions, and link.

     unix
          Those unix features which have not made it into the Posix specs: nice, acct, lstat, readlink, symlink, mknod and sync.

     windows
          Microsoft Windows executable.

     dynamic-linking
          Be able to load compiled files while running.

     dump
          Convert a running scheme program into an executable file.

     heap-can-shrink
          Use if you want segments of unused heap to not be freed up after garbage collection.  This may reduce time in GC for *very* large working sets.

     cheap-continuations
          If you only need straight stack continuations, executables compile with this feature will run faster and use less storage than not having it. Machines with unusual stacks need this.  Also, if you incorporate new C code into scm which uses VMS system services or library routines (which need to unwind the stack in an ordrly manner) you may need to use this feature.

     memoize-local-bindings
          Saves the interpeter from having to look up local bindings for every identifier reference

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:

  * ftp-swiss.ai.mit.edu:/pub/scm/slib2a6.tar.gz

  * prep.ai.mit.edu:/pub/gnu/jacal/slib2a6.tar.gz

  * ftp.maths.tcd.ie:pub/bosullvn/jacal/slib2a6.tar.gz

  * ftp.cs.indiana.edu:/pub/scheme-repository/imp/slib2a6.tar.gz

Unpack SLIB (tar xzf slib2a6.tar.gz or unzip -ao slib2a6.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 Init.scm is installed).  require.scm should have the contents:

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

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)))
     (load (in-vicinity (library-vicinity) "require"))

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.

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 Finishing 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:

  * prep.ai.mit.edu: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.

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
     _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
     _QC             Microsoft QuickC
     __STDC__        ANSI C compliant
     __TURBOC__      Turbo C and Borland C
     __WATCOMC__     Watcom C on MS-DOS
     __ZTC__         Zortech C

     _AIX            AIX operating system
     AMIGA           SAS/C 5.10 or Dice C on AMIGA
     atarist         ATARI-ST under Gnu CC
     GNUDOS          DJGPP (obsolete in version 1.08)
     __GO32__        DJGPP (future?)
     hpux            HP-UX
     linux           Linux
     MCH_AMIGA       Aztec_c 5.2a on AMIGA
     MSDOS           Microsoft C 5.10 and 6.00A
     __MSDOS__       Turbo C, Borland C, and DJGPP
     nosve           Control Data NOS/VE
     SVR2            System V Revision 2.
     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
     _Windows        Borland C 3.1 compiling for Windows
     _WIN32          MS VisualC++ 4.2 under Windows-NT
     vms             (and VMS) VAX-11 C under VMS.

     hp9000s800      HP RISC processor
     __i386__        DJGPP
     i386            DJGPP
     MULTIMAX        Encore computer
     pyr             Pyramid 9810 processor
     sparc           SPARC processor
     sequent         Sequent computer
     tahoe           CCI Tahoe processor

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

FILE: PROBLEM
     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 in ...
*.c: Parameter '...' is never used in ...
*.c: Condition is always false in ...
*.c: Unreachable code in function ...
     Ignore.

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
sys.c: constant in conditional expression
     ignore

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 size of HEAP_SEG_SIZE in setjump.h.

Input hangs
     #define NOSETBUF

ERROR: heap: need larger initial
     Need to increase the 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 ... in scmfig.h and recompile scm
add ... in scmfig.h and recompile scm
     Do it and recompile files.

ERROR: Init.scm not found
     Assign correct IMPLINIT in makefile or scmfig.h or define environment variable SCM_INIT_PATH to be the full pathname of Init.scm (see Installing SCM).

WARNING: require.scm not found
     define environment variable SCHEME_LIBRARY_PATH to be the full pathname of the scheme library [SLIB] or change library-vicinity in Init.scm to point to library or remove. See Installation (SLIB).

     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 mSec (0 in gc) 767 cells work, 233 bytes 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 mSec (60 in gc) 36976 cells work, 1548 bytes 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 Init.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
VAX: botched longjmp
     #define CHEAP_CONTIUATIONS in scmfig.h.

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.

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 jaffer@ai.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.

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

This section describes features which are either present in all builds of SCM or which must be enabled when SCM is compiled.

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

Scm conforms to the IEEE Standard 1178-1990.  IEEE Standard for the Scheme Programming Language. (see Bibliography), and Revised(4) Report on the Algorithmic Language Scheme. Revised(4) 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 [R4RS] Supported by SCM
------------------------------------

two clause if: (if <test> <consequent>)
     See Conditionals (Revised(4) Scheme).

let*
named let
     See Binding constructs (Revised(4) Scheme).

do
     See Iteration (Revised(4) Scheme).

All varieties of define
     See Definitions (Revised(4) Scheme).

list-tail
     See Pairs and lists (Revised(4) Scheme).

string-copy
string-fill!
     See Strings (Revised(4) Scheme).

make-vector of two arguments
vector-fill!
     See Vectors (Revised(4) Scheme).

apply of more than 2 arguments
     See Control features (Revised(4) Scheme).

- 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(4) Scheme).

delay
force
     See Control features (Revised(4) Scheme).
with-input-from-file
with-output-to-file
     See Ports (Revised(4) Scheme).
char-ready?
     See Input (Revised(4) Scheme).
transcript-on
transcript-off
     See System interface (Revised(4) Scheme).

Optionals of [R4RS] not Supported by SCM
----------------------------------------

numerator
denominator
rationalize
     See Numerical operations (Revised(4) Scheme).

[R4RS] appendix Macros
     See Macros (Revised(4) 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 (SLIB).

defmacro
     See Defmacro (SLIB).

dynamic-wind
     See Dynamic-Wind (SLIB).

eval
     See System (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).

System Interface
================

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

 -- 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.

 -- Function: vms-debug
     If SCM is compiled under VMS these commands will invoke the editor or debugger respectively.

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

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

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

 -- 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.

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.

 0. Wrong type in arg 0

 1. Wrong type in arg 1

 2. Wrong type in arg 2

 3. Wrong type in arg 3

 4. Wrong type in arg 4

 5. Wrong type in arg 5

 6. Wrong number of args

 7. numerical overflow

 8. Argument out of range

 9. Could not allocate (out-of-storage)

 10. EXIT (end-of-program)

 11. hang up (hang-up)

 12. user interrupt (user-interrupt)

 13. arithmetic error (arithmetic-error)

 14. bus error

 15. segment violation

 16. alarm (alarm-interrupt)

 -- Variable: errobj
     If 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.

 -- Function: error arg1 arg2 arg3 ...
     Alias for 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 Init.scm; Feel free to redefine it to suit your purposes.

CAUTIOUS enhancements
---------------------

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 Init.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 Init.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 Init.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
          the CPU time is printed after each top level form evaluated.

     >= 3
          messages about heap growth are printed.

     >= 4
          garbage collection (see Garbage Collection) messages are printed.

     >= 5
          a warning will be printed for each top-level symbol which is defined more than one time.

 -- 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. 4e6) of SCM.

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

Miscellaneous Procedures
========================

 -- 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). *load-pathname* is used to compute the value of program-vicinity (SLIB).

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

 -- 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.  eval-string does not change *load-pathname* or line-number.

 -- 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
     See copy-tree (SLIB).  This extends the SLIB version by also copying vectors.

 -- Function: acons obj1 obj2 obj3
     Returns (cons (cons obj1 obj2) obj3).  The expression (set! a-list (acons key datum a-list)) adds a new association to a-list.

 -- Function: terms
     This command displays the GNU General Public License.

 -- Function: list-file filename
     Displays the text contents of FILENAME.

 -- Procedure: 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.

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 (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.

 -- Callback procedure: user-interrupt ...
 -- Callback procedure: alarm-interrupt ...
     Establishes a response for SIGINT (control-C interrupt) and SIGALRM 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
=======================

 -- 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(4) Scheme).

 -- Function: 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.

 -- 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.

 -- 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.  A non-file input-port must be unbuffered in order for char-ready? to work correctly on it.  The initial value of (current-input-port) is unbuffered if the platform supports it.

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

 -- Function: open-io-file filename
 -- Function: close-io-port port
     These functions are analogous to the standard scheme file functions. The ports are open to FILENAME in read/write mode.  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: 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.

 -- 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.

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 6.  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(4) Scheme)) it indicates that the port has reached end-of-file.  For example:

          (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#\>

Syntax Extensions
=================

 -- Read 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)

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

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

 -- Read syntax: #! any thing
     On the first line of a file will be ignored when loaded by SCM.  This makes SCM files usable as POSIX shell scripts if the first line is:

          #!/usr/local/bin/scm

     When such a file is invoked it executes /usr/local/bin/scm with the name of this file as the first argument.  The following shell script will print factorial of its argument:

          #!/usr/local/bin/scm
          ;;; -*-scheme-*- tells emacs this is a scheme file.
          (define (fact n) (if (< n 2) 1 (* n (fact (+ -1 n)))))
          (display (fact (string->number (caddr (program-arguments)))))
          (newline)
          (quit)

     This technique has some 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 programs being moved.

     The following approach solves both problems -- at the expense of slightly slower startup.  type; should appear on every line to be executed by the shell.  These lines do 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 know absolute locations in order to invoke a program.

          #!/bin/sh
          type;exec scm $0 $*
          ;;; -*-scheme-*- tells emacs this is a scheme file.
          (define (fact n) (if (< n 2) 1 (* n (fact (+ -1 n)))))
          (display (fact (string->number (caddr (program-arguments)))))
          (newline)
          (quit)

 -- 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(4) 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.

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

Low Level Syntactic Hooks
=========================

 -- 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).

     Note: When adding new # syntaxes, have your code save the previous value of read: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 use read:sharp).

 -- 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
     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:

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

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

     An environment is a list of environment frames.  There are 2 types of environment frames:

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

               ((variable1 ...) value1 ...)

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

               (variable1 . value1) (variable2 .  value2) ...

 -- Special Form: @apply procedure argument-list
     Returns the result of applying procedure to argument-list.  (apply procedure argument-list) will produce the same result.

 -- Special Form: @call-with-current-continuation procedure)
     Returns the result of applying PROCEDURE to the current continuation.  A continuation is a SCM object of type contin (see Continuations).  The procedure (call-with-current-continuation PROCEDURE) is defined to have the same effect as (@call-with-current-continuation procedure).

Packages
********

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.

Compiling And Linking
=====================

 -- Function: compile-file name1 name2 ...
     If the HOBBIT compiler is installed in the (implementation-vicinity), compiles the files NAME1 NAME2 ... to an object file name NAME1<object-suffix>, where <object-suffix> is the object file suffix for your computer (for instance, .o).  NAME1 must be in the current directory; NAME2 ... can be in other directories.

 -- Function: link-named-scm name module1 ...
     Creates a new SCM executable with name NAME.  NAME will include the object modules MODULE1 ... which can be produced with compile-file.

          cd ~/scm/
          scm -e'(link-named-scm"cute""cube")'
          (delete-file "scmflags.h")
          (call-with-output-file
            "scmflags.h"
            (lambda (fp)
              (for-each
                (lambda (string) (write-line string fp))
                '("#define IMPLINIT \"/home/jaffer/scm/Init.scm\""
                  "#define COMPILED_INITS init_cube();"
                  "#define BIGNUMS"
                  "#define FLOATS"
                  "#define ARRAYS"))))
          (system "gcc -Wall -O2 -c continue.c findexec.c time.c
           repl.c scl.c eval.c sys.c subr.c unif.c rope.c scm.c")
          ...
          scm.c: In function `scm_init_extensions':
          scm.c:95: warning: implicit declaration of function `init_cube'
          scm.c: In function `scm_cat_path':
          scm.c:589: warning: implicit declaration of function `realloc'
          scm.c:594: warning: implicit declaration of function `malloc'
          scm.c: In function `scm_try_path':
          scm.c:612: warning: implicit declaration of function `free'
          (system "cc -o cute continue.o findexec.o time.o repl.o scl.o
           eval.o sys.o subr.o unif.o rope.o scm.o cube.o  -lm -lc")

          Compilation finished at Sun Jul 21 00:59:17

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

If SCM has been compiled with dynl.c then the additional properties of load and require (from [SLIB]) specified here are supported.  The require forms are preferred.  The variable *catalog* can be extended to define other require-able packages.  See Link.scm for details.

 -- Function: load filename lib1 ...
     In addition to the [R4RS] 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 are for 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)

 -- Function: require 'db
 -- Function: require 'wb
     Either form will dynamically load the WB database system from the wb:vicinity (../wb/) specified in Link.scm.  See scm/ANNOUNCE for ftp sites where WB is available.

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.

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

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.  This procedure will 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 System Interface) 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 thunk boot-tail is called by SCM to process command line arguments.  dump sets boot-tail to the THUNK it is called with.

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.

These procedures augment the standard capabilities in Numerical operations (Revised(4) Scheme).

 -- 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: $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
-------------------

Arrays read and write as a # followed by the rank (number of dimensions) followed by 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 3 3) =>
     #2((ho ho ho) (ho ho ho) (ho ho ho))

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

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

When constructing an array, BOUND is either an inclusive range of indices expressed as a two element list, or an upper bound expressed as a single integer.  So

     (make-array 'foo 3 3) == (make-array 'foo '(0 2) '(0 2))

 -- Function: array? obj
     Returns #t if the OBJ is an array, and #f if not.

 -- Function: make-array initial-value bound1 bound2 ...
     Creates and returns an array that has as many dimensions as there are BOUNDs and fills it with INITIAL-VALUE.

 -- Function: array-ref array index1 index2 ...
     Returns the INDEX1, INDEX2, ...'th element of ARRAY.

 -- Function: array-in-bounds? array index1 index2 ...
     Returns #t if its arguments would be acceptable to ARRAY-REF.

 -- Function: array-set! array new-value index1 index2 ...
     Sets the INDEX1, INDEX2, ...'th element of ARRAY to NEW-VALUE.  The value returned by array-set! is unspecified.

 -- Function: make-shared-array array mapper bound1 bound2 ...
     make-shared-array can be used to create shared subarrays of other arrays.  The MAPPER is a function that translates coordinates in the new array into coordinates in the old array.  A MAPPER must be linear, and its range must stay within the bounds of the old array, but it can be otherwise arbitrary.  A simple example:

          (define fred (make-array #f 8 8))
          (define freds-diagonal
            (make-shared-array fred (lambda (i) (list i i)) 8))
          (array-set! freds-diagonal 'foo 3)
          (array-ref fred 3 3) => foo
          (define freds-center
            (make-shared-array fred (lambda (i j) (list (+ 3 i) (+ 3 j))) 2 2))
          (array-ref freds-center 0 0) => foo

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

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

 -- Function: array-shape array
     Returns a list of inclusive bounds of integers.

          (array-shape (make-array 'foo '(-1 3) 5)) => ((-1 3) (0 4))

 -- Function: array-dimensions array
     Array-dimensions is similar to array-shape but replaces elements with a 0 minimum with one greater than the maximum. So:

          (array-dimensions (make-array 'foo '(-1 3) 5)) => ((-1 3) 5)

 -- Function: array-rank obj
     Returns the number of dimensions of OBJ.  If OBJ is not an array, 0 is returned.

 -- Function: array->list array
     Returns a list consisting of all the elements, in order, of ARRAY.

 -- Function: array-copy! source destination
     Copies every element from vector or array SOURCE to the corresponding element of DESTINATION.  DESTINATION must have the same rank as SOURCE, and be at least as large in each dimension.  The order of copying is unspecified.

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

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

 -- 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-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 and MAKE-UNIFORM-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.

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

 -- 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.

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

 -- Function: array-for-each proc array0 ...
     PROC is applied to each tuple of elements of ARRAY0 ... in row-major order.  The value returned is unspecified.

 -- Function: array-index-map! array proc
     applies PROC to the indices of each element of ARRAY in turn, storing the result in the corresponding element.  The value returned and the order of application are unspecified.

     One can implement ARRAY-INDEXES as

          (define (array-indexes array)
              (let ((ra (apply make-array #f (array-shape array))))
                (array-index-map! ra (lambda x x))
                ra))

     Another example:

          (define (apl:index-generator n)
              (let ((v (make-uniform-vector n 1)))
                (array-index-map! v (lambda (i) i))
                v))

 -- Function: scalar->array scalar array prototype
     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.

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.

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

     prototype       type                             display prefix

     #t              boolean (bit-vector)                    #b
     #\a             char (string)                           #a
     integer >0      unsigned integer                        #u
     integer <0      signed integer                          #e
     1.0             float (single precision)                #s
     1/3             double (double precision float)         #i
     +i              complex (double precision)              #c
     ()              conventional vector                     #

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

     (make-uniform-array #\a 3) => "$q2"

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

     (make-uniform-array #t 3) => #*000
     ==
     #b(#f #f #f) => #*000
     ==
     #1b(#f #f #f) => #*000

Other uniform vectors are written in a form similar to that of vectors, except that a single character from the above table is put between # and (.  For example, '#e(3 5 9) returns a uniform vector of signed integers.

 -- Function: uniform-vector-ref uve index
     Returns the element at the INDEX element in UVE.

 -- Function: uniform-vector-set! uve index new-value
     Sets the element at the INDEX element in UVE to NEW-VALUE.  The value returned by uniform-vector-set! is unspecified.

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

 -- Function: make-uniform-array prototype bound1 bound2 ...
     Creates and returns a uniform array of type corresponding to PROTOTYPE that has as many dimensions as there are BOUNDs and fills it with PROTOTYPE.

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

 -- Function: list->uniform-array rank prot lst
 -- Function: list->uniform-vector 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.

 -- Function: uniform-vector-fill! uve fill
     Stores FILL in every element of UVE.  The value returned is unspecified.

 -- Function: uniform-vector-length uve
     Returns the number of elements in UVE.

 -- Function: dimensions->uniform-array dims prototype fill
 -- Function: dimensions->uniform-array dims prototype
 -- Function: make-uniform-vector length prototype fill
 -- Function: make-uniform-vector length prototype
     Creates and returns a uniform array or vector of type corresponding to PROTOTYPE with dimensions DIMS or length LENGTH.  If the FILL argument is supplied, the returned array is filled with this value.

 -- Function: uniform-array-read! ura
 -- Function: uniform-array-read! ura port
 -- Function: uniform-vector-read! uve
 -- Function: uniform-vector-read! uve 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
 -- Function: uniform-vector-write uve
 -- Function: uniform-vector-write uve 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).

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

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

     #b(#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 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.

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

 -- 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: 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: 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: 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 deliverd 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 one of:

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

     -1
          which means to wait for any child process whose process group ID is equal to the (abs 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 is normally the process ID of the child process whose status is reported.  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 priveledged 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 pro-cess 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.

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 prep.ai.mit.edu:/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

  * prep.ai.mit.edu:/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 Data, Network, Protocol, and Service 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.  This is an inverse function to inet:address.

 -- 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)
         (cond ((char-ready? listener-socket)
                (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 ((char-ready? con)
                                 (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))

     (do ((cs #f (and (char-ready? con) (read-char con)))
          (ct #f (and (char-ready?) (read-char))))
         ((or (eof-object? cs) (eof-object? ct))
          (close-port con))
       (cond (cs (display cs)))
       (cond (ct (file-set-position con 0)
                 (display ct con)
                 (file-set-position con 0))))

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 other the body of the procedure in the CAR and a pointer to the environment in the CDR.

      -- 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.

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_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_cclo
     Subr and environment for compiled closure

     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.

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(4) 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.

 -- smob: tc16_promise
     made by DELAY.  See Control features (Revised(4) Scheme).

 -- smob: tc16_arbiter
     synchronization object.  See Process Synchronization.

 -- smob: tc16_macro
     macro expanding function.  See Low Level Syntactic Hooks.

 -- 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    0DDDDDDDDDDDDDDDEFFFFFFF11111100
     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.............G
             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
      spare                          G0100111
     fvect   .........long length....G0101101  .........float *words...........
     dvect   .........long length....G0101111  ........double *words...........
     cvect   .........long length....G0110101  ........double *words...........

     contin  .........long length....G0111101  .............*regs..............
     cclo    .........long length....G0111111  ...........SCM **elts...........
             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_2n..........int hpoff.....01110111  ...........SCM (*f)()...........
     rpsubr  ..........int hpoff.....01111101  ...........SCM (*f)()...........
                             PTOBs:
        port            0bwroxxxxxxxxG1110111  ..........FILE *stream..........
      socket ttttttt    00001xxxxxxxxG1110111  ..........FILE *stream..........
      inport uuuuuuuuuuU00011xxxxxxxxG1110111  ..........FILE *stream..........
     outport 0000000000000101xxxxxxxxG1110111  ..........FILE *stream..........
      ioport uuuuuuuuuuU00111xxxxxxxxG1110111  ..........FILE *stream..........
     fport              00   00000000G1110111  ..........FILE *stream..........
     pipe               00   00000001G1110111  ..........FILE *stream..........
     strport            00   00000010G1110111  ..........FILE *stream..........
     sfport             00   00000011G1110111  ..........FILE *stream..........
                             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.

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 one of the global variables sig_deferred or alrm_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: ASSERT 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 ASSERT 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 ASSERT, 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.

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 make your pointer be one of the sys_protects (see dynwinds for an example). The former method is prefered since it does not require any changes to the SCM distribution.

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 Init.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 Extensions.

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.  2 functions are provided:

          markcdr(ptr)
               which marks the current cell and returns CDR(ptr).

          mark0(ptr)
               which marks the current cell and returns BOOL_F.

     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.  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.

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.

Calling Scheme From C
---------------------

To use SCM as a whole from another program call init_scm or run_scm as is done in main() in scm.c.

In order to call indivdual Scheme procedures from C code more is required; SCM's storage system needs to be initialized.  The simplest way to do this for a statically linked single-thread program is to:

 1. make a SCM procedure which calls your code's startup routine.

 2. use the #define RTL flag when compiling scm.c to elide SCM's main().

 3. In your main(), call run_scm with arguments (argc and argv) to invoke your code's startup routine.

 4. link your code with SCM at compile time.

For a dynamically linked single-thread program:

 1. make an init_ procedure for your code which will set up any Scheme definitions you need and then call your startup routine (see Changing Scm).

 2. Start SCM with command line arguments to dynamically link your code. After your module is linked, the init_ procedure will be called, and hence your startup routine.

Now use apply (and perhaps intern) 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);

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 Init.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: unsigned short num2ushort (SCM NUM, char *POS, char *S_CALLER)
 -- Function: unsigned char num2uchar (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 ASSERT 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 num2long and num2ulong (for when SCM is compiled without bignums).  To convert inexact numbers to exact numbers, see inexact->exact (Revised(4) 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).

     Note: While you use a pointer returned from scm_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.

 -- 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 run_scm 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(4) Scheme).

 -- 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 setjmp(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 setjmp(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 longjmp(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.

       * longjmp(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.

There was some discussion a year ago about a "Forth" style Scheme interpreter.  This is the only improvement I know of which would beat SCM in speed.

     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.

 -- 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.

 -- 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.

Improvements To Make
====================

  * 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.

  * If an open fails because there are no unused file handles, GC should be done so that file handles which are no longer used can be collected.

  * 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.

  * Compaction could be done to malloced objects by freeing and reallocing all the malloc objects encountered in a scan of the heap. Whether compactions would actually occur is system depenedent.

  * 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.

  * lookupcar in eval.c should not memoize (to ILOCs) when it retrieves environments deeper or longer than 4095.  The values can still be retrieved (albeit slowly), but an ILOC should not be made.  The MEMOIZE_LOCALS flag could then be flushed.

  * The must- or make- routines need some sort of C macros or conditionalization so that they check:

       * that the LENGTH field fits into a size_t (as is checked now) for platforms with (sizeof(size_t) < sizeof(SCM)).

       * that the LENGTH field fits into 24 (or 56) bits on machines where size_t is 32 bits or more.

     This is trickier than it first looks because the must_malloc() routine is also used for allocating heap segments, which do not have the LENGTH field restriction.  Putting the 24 bit test into must_malloc() should be tested for speed impact.

Finishing Dynamic Linking
=========================

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
...

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.

Windows NT
..........

George Carrette (gjc@mitech.com) outlines how to dynamically link on Windows NT:

  * The Software Developers Kit has a sample called SIMPLDLL. Here is the gist of it, following along the lines of the VMS description above (contents of a makefile for the SDK NMAKE)

          LISPLIB.exp:
          LISPLIB.lib: LISPLIB.def
              $(implib) -machine:$(CPU) -def:LISPLIB.def -out:LISPLIB.lib

          LISPLIB.DLL : $(LISPLIB_OBJS) LISPLIB.EXP
              $(link) $(linkdebug)              \
              -dll                 \
              -out:LISPLIB.DLL     \
              LISPLIB.EXP $(LISPLIB_OBJS) $(conlibsdll)

  * The LISPDEF.DEF file has this:

          LIBRARY lisplib
          EXPORT
           init_lisp
           init_repl

  * And MAIN.EXE using:

          CLINK = $(link) $(ldebug) $(conflags) -out:$*.exe $** $(conlibsdll)

          MAIN.EXE : MAIN.OBJ LISPLIB.LIB
           $(CLINK)

  * And MYSUBRS.DLL is produced using:

          mysubrs.exp:
          mysubrs.lib: mysubrs.def
              $(implib) -machine:$(CPU) -def:MYSUBRS.def -out:MYSUBRS.lib

          mysubrs.dll : mysubrs.obj mysubrs.exp mysubrs.lib
              $(link) $(linkdebug) \
              -dll                 \
              -out:mysubrs.dll     \
              MYSUBRS.OBJ MYSUBRS.EXP LISPLIB.LIB $(conlibsdll)

  * Where MYSUBRS.DEF has

          LIBRARY mysubrs
          EXPORT
           INIT_MYSUBRS

  * And the dynamic loader looks something like this, calling the two procedures LoadLibrary and GetProcAddress.

          LISP share_image_load(LISP fname)
          {long iflag;
           LISP retval,(*fcn)(void);
           HANDLE hLib;
           DWORD err;
           char *libname,fcnname[64];
           iflag = nointerrupt(1);
           libname = c_string(fname);
           _snprintf(fcnname,sizeof(fcnname),"INIT_%s",libname);
           if (!(hLib = LoadLibrary(libname)))
             {err = GetLastError();
              retval = list2(fname,LSPNUM(err));
              serror1("library failed to load",retval);}
           if (!(fcn = (LISP (*)(void)) GetProcAddress(hLib,fcnname)))
             {err = GetLastError();
              retval = list2(fname,LSPNUM(err));
              serror1("could not find library init procedure",retval);}
           retval = (*fcn)();
           nointerrupt(iflag);
           return(retval);}

  * Note: in VMS the linker and dynamic loader is case sensitive, but all the language compilers, including C, will by default upper-case external symbols for use by the linker, although the debugger gets its own symbols and case sensitivity is language mode dependant.  In Windows NT things are case sensitive generally except for file and device names, which are case canonicalizing like in the Symbolics filesystem.

  * Also: All this WINDOWS NT stuff will work in MS-DOS MS-Windows 3.1 too, by a method of compiling and linking under Windows NT, and then copying various files over to MS-DOS/WINDOWS.

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 all the data types in SCM.