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-
-<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>
-
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