The simplest way is to extract the package source code somewhere: go to an appropriate directory, then run

apt source glibc

In gdb, add the corresponding directory to the source path by using the directory command.

You may also want the detached debug symbols; I don’t know whether Devuan provides debug symbol packages in general (dbgsym packages), but I see it ships libc6-dbg which has the same purpose:

sudo apt install libc6-dbg

Here’s an example gdb session:

$ gdb ls
...
(gdb) directory ~/Debian/glibc
Source directories searched: /home/steve/Debian/glibc:$cdir:$cwd
(gdb) break malloc
Breakpoint 1 at 0x46c8
(gdb) run
Starting program: /bin/ls 

Breakpoint 1, malloc (n=1441) at dl-minimal.c:50
50    if (alloc_end == 0)

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

You should check if your distribution is using the vanilla GLIBC or the EGLIBC fork (Debian and Ubuntu have switched to EGLIBC EDIT: they switched back around 2014).

Anyway, the repository browser for GLIBC is at http://sourceware.org/git/?p=glibc.git

http://code.woboq.org/userspace/glibc/, posted by @guruz below, is a good alternative.

The source is a bit complicated by the presence of multiple versions of the same files.

The project is online at GNU C Library.

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/

How can I navigate glibc and find the actual implementation for lib functions?

Type the function in the search bar at https://code.woboq.org/userspace/glibc or at https://github.com/bminor/glibc . Navigate the results manually until you find the definition.

If you want to index the project locally, use cscope, ctags, GLOBAL tags or clangd to index the project and then use that tools specific interface to search for the definition.

As an example, try looking up the definition of hidden_def in glibc

Type hidden_def glibc into google. My first hit is woboq.org https://code.woboq.org/userspace/glibc/include/libc-symbols.h.html#550 .

I use firefox. I type ctrl+f and type hidden_def. Type Enter until I find # define hidden_def at https://code.woboq.org/userspace/glibc/include/libc-symbols.h.html#550 .

Then select __hidden_ver1 and type ctrl+c ctrl+f and ctrl+v and search for it. In the web browser. I type enter until I find https://code.woboq.org/userspace/glibc/include/libc-symbols.h.html#540 . __hidden_ver2 is just below on line 542.

For most cases all you need is a browser, google, coboq.org and github.org.

It's a macro taking you to hidden_def1

There are no such macros as you mentioned, at least at the version hosted at woboq.org.

How does anyone follow it?

While IDE is a powerful help, each project is unique and requires different settings, that take time to figure out. Mostly browsing the source code is grep (or faster alternatives, like ag, very useful for big projects like glibc) and going through the result list.

Not only system dependent things, like setjmp

Developers are (shoudl be :) sane people - in most cases a function named setjmp will be in a file named setjmp.c. or setjmp.S. Or in the same directory as setjmp.h. Or inside directory named stdlib or setjmp.

Type setjmp in github search bar. https://github.com/bminor/glibc/search?q=setjmp You see there are multiple definitions for each architecture powerpc s390 etc. But files are all named setjmp. Go back. Type "Go to file" on https://github.com/bminor/glibc . Search for a file named x86/setjmp. There are 3 implementations, the most standard one seems to be https://github.com/bminor/glibc/blob/master/sysdeps/x86_64/setjmp.S .

even portable functions like fprintf

As above, saerch for file named fprintf . You quickly find https://github.com/bminor/glibc/blob/master/stdio-common/fprintf.c .