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