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diff --git a/docs/Glibc_vs_uClibc.html b/docs/Glibc_vs_uClibc.html new file mode 100644 index 000000000..c14eb8cf2 --- /dev/null +++ b/docs/Glibc_vs_uClibc.html @@ -0,0 +1,240 @@ +<!--#include file="header.html" --> + +<h2>uClibc vs. glibc</h2> + +<p> + uClibc and Glibc are not the same -- there are a number of differences which + may or may not cause you problems. This document attempts to list these + differences and, when completed, will contain a full list of all relevant + differences. + <br><br></p> + <ol> + <li>uClibc is smaller than glibc. We attempt to maintain a glibc compatible + interface, allowing applications that compile with glibc to easily compile with + uClibc. However, we do not include _everything_ that glibc includes, and + therefore some applications may not compile. If this happens to you, please + report the failure to the uclibc mailing list, with detailed error messages. + </li><br> + <li>uClibc is much more configurable then glibc. This means that a developer + may have compiled uClibc in such a way that significant amounts of + functionality have been omitted. + </li><br> + <li>uClibc does not even attempt to ensure binary compatibility across releases. + When a new version of uClibc is released, you may or may not need to recompile + all your binaries. + </li><br> + <li><ul><li> malloc(0) in glibc returns a valid pointer to something(!?!?) while in + uClibc calling malloc(0) returns a NULL. The behavior of malloc(0) is listed + as implementation-defined by SuSv3, so both libraries are equally correct. + This difference also applies to realloc(NULL, 0). I personally feel glibc's + behavior is not particularly safe. To enable glibc behavior, one has to + explicitly enable the MALLOC_GLIBC_COMPAT option. + </li><br><li> + glibc's malloc() implementation has behavior that is tunable via the + MALLOC_CHECK_ environment variable. This is primarily used to provide extra + malloc debugging features. These extended malloc debugging features are not + available within uClibc. There are many good malloc debugging libraries + available for Linux (dmalloc, electric fence, valgrind, etc) that work much + better than the glibc extended malloc debugging. So our omitting this + functionality from uClibc is not a great loss. + </li><br> + </ul></li> + <li>uClibc does not provide a database library (libdb). + </li><br> + <li>uClibc does not support NSS (/lib/libnss_*), which allows glibc to easily + support various methods of authentication and DNS resolution. uClibc only + supports flat password files and shadow password files for storing + authentication information. If you need something more complex than this, + you can compile and install pam. + </li><br> + <li>uClibc's libresolv is only a stub. Some, but not all of the functionality + provided by glibc's libresolv is provided internal to uClibc. Other functions + are not at all implemented. + </li><br> + <li>libnsl provides support for Network Information Service (NIS) which was + originally called "Yellow Pages" or "YP", which is an extension of RPC invented + by Sun to share Unix password files over the network. I personally think NIS + is an evil abomination and should not be used. These days, using ldap is much + more effective mechanism for doing the same thing. uClibc provides a stub + libnsl, but has no actual support for Network Information Service (NIS). + We therefore, also do not provide any of the headers files provided by glibc + under /usr/include/rpcsvc. + </li><br> + <li>uClibc's locale support is not 100% complete yet. We are working on it. + </li><br> + <li>uClibc's math library only supports long double as inlines, and even + then the long double support is quite limited. Also, very few of the + float math functions are implemented. Stick with double and you should + be just fine. + </li><br> + <li>uClibc's libcrypt does not support the reentrant crypt_r, setkey_r and + encrypt_r, since these are not required by SuSv3. + </li><br> + <li>uClibc directly uses kernel types to define most opaque data types. + </li><br> + <li>uClibc directly uses the linux kernel's arch specific 'stuct stat'. + </li><br> + <li>uClibc's librt library currently lacks all aio routines, all clock + routines, and all shm routines (only the timer routines and the mq + routines are implemented). + </li><br> +</ol> +<hr> +<h3>Manuel's Notes</h3> + + Some general comments...<br> + <p> + The intended target for all my uClibc code is ANSI/ISO C99 and SUSv3 + compliance. While some glibc extensions are present, many will eventually + be configurable. Also, even when present, the glibc-like extensions may + differ slightly or be more restrictive than the native glibc counterparts. + They are primarily meant to be porting _aides_ and not necessarily + drop-in replacements. + </p><br> +Now for some details...<br><br> + +<u>time functions</u><br> +<ol> +<li>Leap seconds are not supported.</li><br> +<li>/etc/timezone and the whole zoneinfo directory tree are not supported. + To set the timezone, set the TZ environment variable as specified in + http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap08.html + or you may also create an /etc/TZ file of a single line, ending with a + newline, containing the TZ setting. For example + echo CST6CDT > /etc/TZ +</li><br> +<li>Currently, locale specific eras and alternate digits are not supported. + They are on my TODO list. +</li> +</ol><br> +<u>wide char support</u><br> +<ol> +<li>The only multibyte encoding currently supported is UTF-8. The various + ISO-8859-* encodings are (optionally) supported. The internal + representation of wchar's is assumed to be 31 bit unicode values in + native endian representation. Also, the underlying char encoding is + assumed to match ASCII in the range 0-0x7f. +</li> +<li>In the next iteration of locale support, I plan to add support for + (at least some) other multibyte encodings. +</li> +</ol> +<u>locale support</u><br> +<ol> +<li>The target for support is SUSv3 locale functionality. While nl_langinfo + has been extended, similar to glibc, it only returns values for related + locale entries. +</li> +<li>Currently, all SUSv3 libc locale functionality should be implemented + except for wcsftime and collating item support in regex. +</li> +</ol> +<u>stdio</u><br> +<ol> +<li>Conversion of large magnitude floating-point values by printf suffers a loss + of precision due to the algorithm used. +</li><br> +<li>uClibc's printf is much stricter than glibcs, especially regarding positional + args. The entire format string is parsed first and an error is returned if + a problem is detected. In locales other than C, the format string is checked + to be a valid multibyte sequence as well. Also, currently at most 10 positional + args are allowed (although this is configurable). +</li><br> +<li>BUFSIZ is configurable, but no attempt is made at automatic tuning of internal + buffer sizes for stdio streams. In fact, the stdio code in general sacrifices + sophistication/performace for minimal size. +</li><br> +<li>uClibc allows glibc-like custom printf functions. However, while not + currently checked, the specifier must be <= 0x7f. +</li><br> +<li>uClibc allows glibc-like custom streams. However, no in-buffer seeking is + done. +</li><br> +<li>The functions fcloseall() and __fpending() can behave differently than their + glibc counterparts. +</li><br> +<li>uClibc's setvbuf is more restrictive about when it can be called than glibc's + is. The standards specify that setvbuf must occur before any other operations + take place on the stream. +</li><br> +<li>Right now, %m is not handled properly by printf when the format uses positional + args. +</li><br> +<li>The FILEs created by glibc's fmemopen(), open_memstream(), and fopencookie() + are not capable of wide orientation. The corresponding uClibc routines do + not have this limitation. +</li><br> +<li>For scanf, the C99 standard states "The fscanf function returns the value of + the macro EOF if an input failure occurs before any conversion." But glibc's + scanf does not respect conversions for which assignment was surpressed, even + though the standard states that the value is converted but not stored. +</li></ol><br> +<hr><h3>Glibc bugs</h3><br> +glibc bugs that Ulrich Drepper has refused to acknowledge or comment on + ( <a href="http://sources.redhat.com/ml/libc-alpha/2003-09/">http://sources.redhat.com/ml/libc-alpha/2003-09/</a> ) +<br> +<ol> +<li>The C99 standard says that for printf, a %s conversion makes no special + provisions for multibyte characters. SUSv3 is even more clear, stating + that bytes are written and a specified precision is in bytes. Yet glibc + treats the arg as a multibyte string when a precision is specified and + not otherwise. +</li><br> +<li>Both C99 and C89 state that the %c conversion for scanf reads the exact + number of bytes specified by the optional field width (or 1 if not specified). + uClibc complies with the standard. There is an argument that perhaps the + specified width should be treated as an upper bound, based on some historical + use. However, such behavior should be mentioned in the Conformance document. +</li><br> +<li>glibc's scanf is broken regarding some numeric patterns. Some invalid + strings are accepted as valid ("0x.p", "1e", digit grouped strings). + In spite of my posting examples clearly illustrating the bugs, they remain + unacknowledged by the glibc developers. +</li><br> +<li>glibc's scanf seems to require a 'p' exponent for hexadecimal float strings. + According to the standard, this is optional. +</li><br> +<li>C99 requires that once an EOF is encountered, the stream should be treated + as if at end-of-file even if more data becomes available. Further reading + can be attempted by clearing the EOF flag though, via clearerr() or a file + positioning function. For details concerning the original change, see + Defect Report #141. glibc is currently non-compliant, and the developers + did not comment when I asked for their official position on this issue. +</li><br> +<li>glibc's collation routines and/or localedef are broken regarding implicit + and explicit UNDEFINED rules. +</li><br></ol> +More to follow as I think of it... +<br><br><hr> +<h3>Profiling:</h3> +<p> +uClibc no longer supports 'gcc -fprofile-arcs -pg' style profiling, which +causes your application to generate a 'gmon.out' file that can then be analyzed +by 'gprof'. Not only does this require explicit extra support in uClibc, it +requires that you rebuild everything with profiling support. There is both a +size and performance penalty to profiling your applications this way, as well +as Heisenberg effects, where the act of measuring changes what is measured. +</p> +<p> +There exist a number of less invasive alternatives that do not require you to +specially instrument your application, and recompile and relink everything. +</p><p> +The OProfile system-wide profiler is an excellent alternative: + <a href="http://oprofile.sourceforge.net/">http://oprofile.sourceforge.net/</a> +</p><p> +Many people have had good results using the combination of Valgrind +to generate profiling information and KCachegrind for analysis: + <a href="http://developer.kde.org/~sewardj/">http://developer.kde.org/~sewardj/</a> + <a href="http://kcachegrind.sourceforge.net/">http://kcachegrind.sourceforge.net/</a> +</p><p> +Prospect is another alternative based on OProfile: + <a href="http://prospect.sourceforge.net/">http://prospect.sourceforge.net/</a> +</p><p> +And the Linux Trace Toolkit (LTT) is also a fine tool: + <a href="http://www.opersys.com/LTT/">http://www.opersys.com/LTT/</a> +</p><p> +FunctionCheck: + <a href="http://www710.univ-lyon1.fr/~yperret/fnccheck/">http://www710.univ-lyon1.fr/~yperret/fnccheck/</a> +</p> + +<!--#include file="footer.html" --> |