Is it legal to take the address of a function parameter?

Question

Is this code snippet well defined in ANSI C? On my system (Linux x86_64) it seems to run just fine and print an address, but will it always be the case? E.g. the parameter might be passed via a register, and taking the address of that doesn't seem right.

#include <stdio.h>

void foo(int a)
{
   printf("%p\n", &a);
}

int main(void)
{
   foo(42);
   return 0;
}

EDIT: Looks like GCC will put the value that is passed by register into the stack before taking the address of it.

<foo>:
  55                      push   rbp
  48 89 e5                mov    rbp,rsp
  48 83 ec 10             sub    rsp,0x10
  89 7d fc                mov    DWORD PTR [rbp-0x4],edi
  b8 1c 06 40 00          mov    eax,0x40061c
  48 8d 55 fc             lea    rdx,[rbp-0x4]
  48 89 d6                mov    rsi,rdx
  48 89 c7                mov    rdi,rax
  b8 00 00 00 00          mov    eax,0x0
  e8 d8 fe ff ff          call   4003c0 <printf@plt>
  c9                      leave  
  c3                      ret   

Answer 1

To address your confusion: Yes, the argument might be passed by a register, but when it becomes a local variable in the called function, it's just like any other local variable. If it's address is taken and used, the compiler will have to ensure it has a real address through which it can be accessed by creating a real stack variable. If not, the compiler may be able to optimize to keep it in a register without ever instantiating it in memory.