gcc optimization flag break code

Question

This code works fine when no optimization flag are set:

#include <cstdio>
int main(){
  float *ptr = ({float var[10] = {1,2,3,4,5,6,7,8,9,10}; var;});
  float *ptr1 = ({float var[10]; for(int i_=0;i_<10;i_++)var[i_]=i_+1; var;});
  float *ptr2 = ({float var[10]; var[4]=5; var;});
  printf("\n value = %f %f %f",ptr[4],ptr1[4],ptr2[4]);
}

It returns 5 5 5 as expected. But when optimization flag are set, it returns 5 0 5.

ptr1 have problem something related to the loop. Why? Maybe its a bug?

I'm using the latest, 4.8.0, tested x64, x86 as well other builds. All same behaviour.

Yup, reeks of undefined behaviour. I didn't know this was legal, thoguh, so thanks. — chris
it is a bug, in your code. you cannot return a pointer of a stack object and use it outside the scope. — Bryan Chen
@Cubbi, Oh, thanks. I didn't realize -pedantic is off by default on Coliru. — chris

Cubbi Cubbi · Accepted Answer · 2013-05-15T03:32:32

You're using a GNU language extension, so let's look at the GNU documentation:

In a statement expression, any temporaries created within a statement are destroyed at that statement's end.

var is destroyed every time, and a pointer to its first element (which is what's returned by the expressions) is not safe to dereference.

gcc optimization flag break code

2 Answers