Why doesn't gcc o warn when size of array is uninitialized in this code?

Question

Okay, so this is a stripped down variant of a bug I had. The bug was that I initialized an array using a variable that wasn't initialized. Earlier I used a function to declare the number of elements using a function, but after a cleanup I forgot about it and moved all declarations to the top of the function.

I used the flags -std=c99 -Wall -Wextra -pedantic -O, and usually gcc warns about values being used before they are uninitialized, but in this specific case it didn't. So, my question is:

Is this a bug in gcc or is it possible for f(&n) to post-initialize the array size in some weird way?

#include <stdio.h>

void f(int * x) {
  *x = 8;
}


int main(void) {

  int n;
  float a[n]; // Compiler should warn that n may contain garbage

  a[7] = 3.1415;
  printf("%f\n", a[7]);

  f(&n);  // Removing this causes the compiler warn as expected

  return 0;
}

EDIT: It may be this gcc bug?

looks like a bug to me

user3629249
– user3629249

2015-10-20 18:11:57 +00:00
Commented Oct 20, 2015 at 18:11 — user3629249
– user3629249, Commented Oct 20, 2015 at 18:11

Davislor · Accepted Answer · 2015-10-20 18:23:02Z

1

GCC is accepting float a[n] as a variable-length array. It should, however, warn you that n contains garbage when it’s used. Perhaps VLA initialization is getting rearranged in a way that makes that fact non-obvious to the code generator? If n were initialized before use, moving the call to f() above the declaration of a would clearly be wrong, but this program produces undefined behavior.

answered Oct 20, 2015 at 18:23

Davislor

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2 Comments

skogshjort Over a year ago

How do you mean? I it's unclear; removing f(&n) from main causes the compiler to catch and warn that n is used uninitialized.

Davislor Over a year ago

Yes, that definitely sounds like a compiler bug. Suppose you’d initialized int n = 1; float a[n]; f(&n);. In that case, the compiler clearly needs to update n after it allocates the VLA. Some refactoring during the optimization phase is confusing it.

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