The OS will initialize the memory of the stack to 0 before the program starts. But before main is called, the compiler is free to insert other code that runs before main. gcc inserts a function named __libc_start_main that runs before main. This code will modify the content of the stack. So when main is run, the stack where the uninitialized local variable is has a decent chance of not being 0 anymore.
This is easily testable.
#include <stdio.h>
int main(void) {
char* pointers[20];
int i;
for (i = 0; i < 20; ++i) {
printf("%p\n", pointers[i]);
}
return 0;
}
And yes when I run it, most of the pointers are not null.
