uClibc vs. glibc

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.
   
   
   • 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).

Manuel's Notes

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.

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.

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.

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.

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

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.

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/