Where can I find the code for malloc my gcc compiler is using at the moment? I actually want to write my own malloc function which will be a little different from the original one. I know I can use hooks et all, but I want to see the real code.
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).