Crowded stack with huge local variable?

Question

As said, each process is assigned an 8 mb stack. This stack will be used to store local variables. Therefore, if I take an array with a maximum size than on the stack, it should overflow

int main() { int arr[88388608]; int arr1[88388608]; int arr2[88388608]; while(1); return 0; } 

But I can’t get the result!

Answer 1

Welcome to the world of compiler optimization!

Due to the as-if rule, the compiler is only required to create something that will have the same observable results as the source code. So, the compiler, if free:

• remove unused arrays
• remove empty loop
• store dynamic arrays from the main external part of the stack - since main is a special function that will be called only once by the environment

If you want to observe stack overflow (bad, not our good site :-)), you should:

• use some code to populate arrays
• compile all remote optimization and preferably in debug mode tell the compiler that I wrote as accurately as you can

The following code does SIGSEGV with CLang 3.4.1 when compiled as cc -g foo.c -o foo

 #include <stdio.h> #define SIZE 88388608 void fill(int *arr, size_t size, int val) { for (size_t i=0; i<size; i++) { arr[i] = val; } } int main() { int arr[SIZE]; int arr1[SIZE]; int arr2[SIZE]; fill(arr, SIZE, 0); fill(arr1, SIZE, 0); fill(arr2, SIZE, 0); printf("%d %d %d\n", arr[12], arr1[15], arr2[18]); return 0; } 

and even this code works fine when compiling as an optimization level -O2 ... Compilers are now too smart for me, and I am not good enough at the assembly code, which would be the only real way to understand what is actually done!