gcc: how is “implementation-defined” malloc(0) defined?

Question

C standard (eg C99) tells that malloc(0) returns "a null pointer or a unique pointer that can be successfully passed to free()". Which of the two, is implementation-defined . (This has been asked on StackOverflow many times, eg 1 , 2 , 3 , 4 .)

My question is: how does GCC define it?

Answer 1

GCC doesn't define it at all. GCC uses the malloc() implementation provided by the standard library. On OS X this will be libSystem, on Linux this will usually be glibc or eglibc, etc. These libraries are open source, so you can browse the source code.