Sorry I know this is stupid but where is linux libc source code available? What I downloaded from GNU didn't seem to be what I wanted, specifically I could find nothing in the pthreads function family.
Is there an online (hypertexted cross-referenced) version somewhere?
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/