Obtaining glibc code can be done via apt-get command. Here's basic process done in terminal, which takes no longer than 5 mins:

1. Create glibc-source directory with mkdir glibc-source and cd glibc-source
2. run apt-get source glibc
3. Once apt-get is done, you'll have 3 files and one directory, glibc-2.23/, glibc_2.23-0ubuntu3.dsc, glibc_2.23-0ubuntu3.debian.tar.xz, glibc_2.23.orig.tar.xz. The file you want is glibc-2.23/stdlib/atoi.c

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:

1. 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)

2. 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

1. Metager with glibc: http://code.metager.de/source/xref/gnu/glibc/

2. 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)

3. Glibc git is mirrored to github (which has some searching functions) https://github.com/bminor/glibc Buildroot 2018.05 notably uses this mirror.

4. 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/

The POSIX interface of malloc is defined here.

If you want to find out how the C library in GNU/Linux (glibc) implements malloc, go and get the source code from http://ftp.gnu.org/gnu/glibc/ or browse the git repository and look at the malloc/malloc.c file.

There is also the base documentation of the Memory Allocator by Doug Lea that describes the theory of a m(emory)alloc(ator) (read this carrefully, and then decide if you really need to implement your own malloc).