c - How does argument passing work?

Question

I want to know how passing arguments to functions in C works. Where are the values being stored and how and they retrieved? How does variadic argument passing work? Also since it's related: what about return values?

I have a basic understanding of CPU registers and assembler, but not enough that I thoroughly understand the ASM that GCC spits back at me. Some simple annotated examples would be much appreciated.

Answer 1

Considering this code:

int foo (int a, int b) {
  return a + b;
}

int main (void) {
  foo(3, 5);
  return 0;
}

Compiling it with gcc foo.c -S gives the assembly output:

foo:
    pushl   %ebp
    movl    %esp, %ebp
    movl    12(%ebp), %eax
    movl    8(%ebp), %edx
    leal    (%edx,%eax), %eax
    popl    %ebp
    ret

main:
    pushl   %ebp
    movl    %esp, %ebp
    subl    $8, %esp
    movl    $5, 4(%esp)
    movl    $3, (%esp)
    call    foo
    movl    $0, %eax
    leave
    ret

So basically the caller (in this case main) first allocates 8 bytes on the stack to accomodate the two arguments, then puts the two arguments on the stack at the corresponding offsets (4 and 0), and then the call instruction is issued which transfers the control to the foo routine. The foo routine reads its arguments from the corresponding offsets at the stack, restores it, and puts its return value in the eax register so it's available to the caller.