C: Creating a static array with variable dependent on runtime functions?

Question

As static arrays are created during compile-time, may I set the length of the array with a variable obtained through e.g. another function? I believe this is dependent on whether the compiler can be smart enough to know what value this variable will have?

Example code where I obtain the size trough snprinf(). This code is compiling without errors or warnings using gcc with the -Wall flag.

#include <stdio.h>
    
int main(void)
{
    int mac[6] = {0xAA,0xBB,0xCC,0xDD,0xEE,0xFF};
    int size = snprintf(NULL, 0, "%02X", mac[0]);
    char str[size + 1];

    snprintf(str, size + 1, "%02X", mac[0]);

    printf("%s\n", str);

    return 0;
}

Compiling and running results in:

AA

Why is this possible?

Answer 1

str is not a static array it is what C calls a VLA, variable length array.