libc source location - for download or online viewing?
If you're on a Debian-derived system, you can use apt-get source libc6. This will unpack a eglibc-2.12.1 directory (version number might differ, of course) in your current working directory, and the pthreads support are in the nptl/ directory below that. linuxthreads/ is for the older threading style, in case you're an archaeologist.
Most linuxes use a libc version named glibc.
The LXR (online cross-reference system) for glibc is e.g. here http://koala.cs.pub.ro/lxr/glibc/ for 2.9 version (link is broken). I must say that something may be not lxr'ed because some sources are generated in the build process, for example - as i can remember - wrappers around a system calls.
Pthreads are in nptl/ folder. Right link to libc sources is http://ftp.gnu.org/gnu/glibc/glibc-2.14.tar.bz2 (or change 2.14 to your version)
Update: After closing of koala's lxr, there are:
Metager with glibc: http://code.metager.de/source/xref/gnu/glibc/(Served with Sun's OpenGrok, which was originally used to generate online x-ref for Solaris/OpenSolaris)Google code search (I know that it was closed; but I also know it's other version which is up):
http://code.google.com/codesearch and try to search something glibc-specific
UPD (march 2013) They killed codesearch again:
404. That’s an error.The requested URL /codesearch was not found on this server. That’s all we know.
UPD 2017
Metager with glibc: http://code.metager.de/source/xref/gnu/glibc/There is online git by glibc authors: https://sourceware.org/git/?p=glibc.git (tree is browserable at https://sourceware.org/git/?p=glibc.git;a=tree)
Glibc git is mirrored to github (which has some searching functions) https://github.com/bminor/glibc Buildroot 2018.05 notably uses this mirror.
There is search like google's codesearch in all debian packages: https://codesearch.debian.net/. It can search in glibc sources by "package:glibc request" request and also have file browser: http://sources.debian.net/src/glibc/
Try the FreeBSD and Linux Kernel Cross-Reference.
Have fun :)
Info on the glibc repository: http://sourceware.org/glibc/wiki/GlibcGit
Clone it to get your own copy and search it however you like:
git clone git://sourceware.org/git/glibc.git
I load it up in an IDE project (using whatever preferred IDE) and the code navigation works quite well to let me find what I'm interested in.
Browse the source online http://sourceware.org/git/?p=glibc.git