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Diffstat (limited to 'toolchain/uClibc/Glibc_vs_uClibc_Differences.txt')
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diff --git a/toolchain/uClibc/Glibc_vs_uClibc_Differences.txt b/toolchain/uClibc/Glibc_vs_uClibc_Differences.txt new file mode 100644 index 000000000..4ed24639e --- /dev/null +++ b/toolchain/uClibc/Glibc_vs_uClibc_Differences.txt @@ -0,0 +1,215 @@ + 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. + + +1) 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. + +2) 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. + +3) 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. + +4) 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. + +4.1) 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. + +5) uClibc does not provide a database library (libdb). + +6) 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. + +7) 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. + +8) 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. + +9) uClibc's locale support is not 100% complete yet. We are working on it. + +10) 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. + +11) uClibc's libcrypt does not support the reentrant crypt_r, setkey_r and +encrypt_r, since these are not required by SuSv3. + +12) uClibc directly uses kernel types to define most opaque data types. + +13) uClibc directly uses the linux kernel's arch specific 'stuct stat'. + +14) 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). + +<other things as we notice them> + + + +****************************** Manuel's Notes ****************************** + +Some general comments... + +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. + +Now for some details... + +time functions +-------------- +1) Leap seconds are not supported. +2) /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 +3) Currently, locale specific eras and alternate digits are not supported. + They are on my TODO list. + +wide char support +----------------- +1) 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. +2) In the next iteration of locale support, I plan to add support for + (at least some) other multibyte encodings. + +locale support +-------------- +1) 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. +2) Currently, all SUSv3 libc locale functionality should be implemented + except for wcsftime and collating item support in regex. + +stdio +----- +1) Conversion of large magnitude floating-point values by printf suffers a loss + of precision due to the algorithm used. +2) 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). +3) 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. +4) uClibc allows glibc-like custom printf functions. However, while not + currently checked, the specifier must be <= 0x7f. +5) uClibc allows glibc-like custom streams. However, no in-buffer seeking is + done. +6) The functions fcloseall() and __fpending() can behave differently than their + glibc counterparts. +7) 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. +8) Right now, %m is not handled properly by printf when the format uses positional + args. +9) 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. +10) 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. + +glibc bugs that Ulrich Drepper has refused to acknowledge or comment on + ( http://sources.redhat.com/ml/libc-alpha/2003-09/ ) +----------------------------------------------------------------------- +1) 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. +2) 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. +3) 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. +4) glibc's scanf seems to require a 'p' exponent for hexadecimal float strings. + According to the standard, this is optional. +5) 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. +6) glibc's collation routines and/or localedef are broken regarding implicit + and explicit UNDEFINED rules. + +More to follow as I think of it... + + + + +Profiling: +------------------------------------------------------------------- + +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. + +There exist a number of less invasive alternatives that do not require you to +specially instrument your application, and recompile and relink everything. + +The OProfile system-wide profiler is an excellent alternative: + http://oprofile.sourceforge.net/ + +Many people have had good results using the combination of Valgrind +to generate profiling information and KCachegrind for analysis: + http://developer.kde.org/~sewardj/ + http://kcachegrind.sourceforge.net/ + +Prospect is another alternative based on OProfile: + http://prospect.sourceforge.net/ + +And the Linux Trace Toolkit (LTT) is also a fine tool: + http://www.opersys.com/LTT/ + +FunctionCheck: + http://www710.univ-lyon1.fr/~yperret/fnccheck/ + |