If you think the lib is buggy, please read first the FAQ. It
might be that others had the same problem and there's an easy
solution. You should also check the section "Recommended Tools to
Install the GNU C Library" in the INSTALL file since
some bugs are bugs of the tools and not of glibc.
Once you've found a bug, make sure it's really a bug. A good way to do this is to see if the GNU C library behaves the same way some other C library does. If so, probably you are wrong and the libraries are right (but not necessarily). If not, one of the libraries is probably wrong.
Next, go to http://www-gnats.gnu.org:8080/cgi-bin/wwwgnats.pl,
and look through the bug database. Check here to verify the
problem has not already be reported. You should also look at the
file BUGS (distributed with libc) to check for known
bugs.
Once you're sure you've found a new bug, try to narrow it down to the smallest test case that reproduces the problem. In the case of a C library, you really only need to narrow it down to one library function call, if possible. This should not be too difficult.
The final step when you have a simple test case is to report the
bug. When reporting a bug, send your test case, the results you
got, the results you expected, what you think the problem might
be (if you've thought of anything), your system type, the
versions of the GNU C library, the GNU CC compiler, and the GNU
Binutils which you are using. Also include the files
config.status and config.make which are
created by running configure; they will be in
whatever directory was current when you ran
configure.
All bug reports for the GNU C library should be sent using the
glibcbug shell script which comes with the GNU libc
to bugs@gnu.org (the older address
bugs@gnu.ai.mit.edu is still
working), or submitted through the GNATS web interface at
http://www-gnats.gnu.org:8080/cgi-bin/wwwgnats.pl.
Suggestions and questions should be sent to the mailing list at bugs-glibc@prep.ai.mit.edu. If you don't read the gnewsgroup gnu.bug.glibc, you can subscribe to the list by asking bug-glibc-request@prep.ai.mit.edu.
Please DO NOT send bug report for the GNU C library to <bug-gcc@prep.ai.mit.edu>. That list is for bug reports for GNU CC. GNU CC and the GNU C library are separate entities maintained by separate people.