(C) How are the implementations of the stdlib functions stored and linked to in header files if the source code does not have to be provided directly?

new to using C

Header files for libraries like stdlib do not contain the actual implementation code for the functions they provide access to. I understand that the actual source text for libraries like this aren't needed to compile, but how does this work specifically? Are the implementation details for these libraries contained within the compiler?

When you use a function like printf(), including the header file essentially pastes in code for the declaration of the function, but normally the implementation code would need to be available as well. What form is it stored in? (and where?) Is this compiler specific? Would it be possible to write custom code and reference it in this way without modifying the behavior of the compiler?

I've been searching around and found some info that is relevant but nothing specific. This could be related to not formulating the question well. Thanks.

Solution

When you link a program, the compiler will implicitly add some extra libraries to your program:

$ ls
main.c
$ cc -c main.c
$ cc main.o
$ ls
main.c main.o a.out

You can discover the extra libraries a program uses with ldd. Here, there are three libraries linked into the program, and I didn't ask for any of them:

$ ldd a.out
linux-vdso.so => (0x00...)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00...)
/lib64/ld-linux-x86-64.so.2 (0x00...)

So, what happens if we link without these libraries? That's easy enough, just use the linker (ld) directly, instead of calling it through cc. When you use ld, it doesn't give you these extra libraries, so you get an error:

$ ld main.o
Undefined symbols:
  "_printf", referenced from:
    _main in main.o

The implementation for printf() is stored in the standard C library, which is usually just another library on your system... the only difference is that it gets automatically included into your program when you compile C.

You can use nm to find out what symbols are in a library, so I can use it to find printf() in libc:

$ nm -D /lib/x86_64-linux-gnu/libc-2.13.so | grep printf
...
000000000004e4b0 T printf
...

So, now that we know that libc has printf(), we can use -lc to tell the linker to include libc, and that will get rid of the errors about printf() being missing:

$ ld main.o -lc

There might be some other bits missing, and that's why we use cc to link our programs instead of ld: cc gives us all the default libraries.