How is the stack laid out in C?

What does the stack look like?

Where does the compiler put function arguments?

int main(int argc, char* argv[]) {
  printf("&argc: %p\n", &argc);
  printf("&argv: %p\n", &argv);
  return 0;
}

This prints:

&argc: 0x7fff5dfd47e8
&argv: 0x7fff5dfd47e0

(Here, we’re using the %p format specifier for pointer types.)

The addresses decrease as we go through the arguments: &arg1 < &arg2 < ... < &argn. This is an implementation detail: C does not define how the arguments are laid out in memory.

Next, what happens when a function calls another? Let’s build up the stack:

void test(int i) {
  printf("Nested %d times; &i: %p\n", i, &i);
  if (i == 5) {
    return;
  } else {
    test(i+1);
  }
}

int main(int argc, char* argv[]) {
  test(0);
  return 0;
}

This prints:

Nested 0 times; &i: 0x7fff536f17cc
Nested 1 times; &i: 0x7fff536f17ac
Nested 2 times; &i: 0x7fff536f178c
Nested 3 times; &i: 0x7fff536f176c
Nested 4 times; &i: 0x7fff536f174c
Nested 5 times; &i: 0x7fff536f172c

The stack grows down: each call is shifted 32 bytes down the stack. This means the stack frame for test is 32 bytes.

Another component of the stack frame is the return value. When we write return 5, this places the value 5 at a location on the stack. Where? C does not gives us easy access to this, but there is a non-standard GCC function __builtin_return_address which does.

int test(int i1, int i2) {
  void* return_addr = __builtin_return_address(0);
  printf("&return: %p\n", return_addr);
  printf("&i1:     %p\n", (void*) &i1);
  printf("&i2:     %p\n", (void*) &i2);
  return 2;
}

&return: 0x10dd82f59
&i1:     0x7fff51e7d7bc
&i2:     0x7fff51e7d7b8

Heh, the return address is at the bottom of memory - not on the stack at all? What happens when we put values there? Answer: it blows up! Probably very-undefined-behavior?

What is in this stack frame? It depends on the calling convention. More about that in the future ...

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