Question

I just read about Statement Expressions Extension in GCC, and I found some unexpected behavior when using it.

Please observe this example:

#include <stdio.h>

int main(void)
{

    char* res1 = ({
                        char arr[] ={'h', 'e', '\0'}; // was char *arr[]
                        arr[0] = 'x';
                        char* ptr = arr;
                        ptr;
                 });


    char* res2 = ({
                        char arr[] ={'h', 'e', '\0'}; // was char *arr[]
                        arr[0] = 'X';
                        char* ptr = arr;
                        ptr;
                 });

    printf ("%s %p\n", res1, res1);
    printf ("%s %p\n", res2, res2);

    return 0;
}

Output:

X 0x7fff93098160
X 0x7fff93098160

I noticing that, the variables arr in first block and arr in second block taking the same memory address.

Why that happening??

Answer 1

Both occurrences of arr are array objects with automatic storage duration; they’re local to the enclosing block { ... } within the statement expression.

Each statement expression grabs the address of that local variable; that address is saved in res1 and res2and used *after* the end of the block, when the objectarr` no longer exists.

This is the same problem as a function returning the address of a local variable. The address becomes invalid when the variable ceases to exist, and the program’s behavior is undefined.

So don’t do that.