Most likely the compiler does some sort of simple optimizations resulting in the string still being in the same place on the stack. In other words, the compiler allows the stack to grow to store 'str'. But it doesn't shrink the stack in the scope of main, because it is not required to do so.
If you really want to see the result of saving the address of variables on the stack, call a function.
#include <stdio.h>
char * str_ptr = NULL;
void onstack(void)
{
char str[4] = {0};
sprintf(str,"AGH");
str_ptr = str;
}
int main(int argc, char *argv[])
{
onstack();
int x = 0x61626364;
printf("str_ptr: %s\n", str_ptr);
printf("x:%i\n",x);
getchar();
return 0;
}
With gcc -O0 -std=c99 strcorrupt.c
I get random output on the first printf. It will vary from machine to machine and architecture to architecture.