uClibc and Glibc Differences

Ref:

Glibc_vs_uClibc_Differences.txt 

uClibc v.s. glibc 

Glibc, uClibc, Newlib 

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).