Static allocation of huge amounts of memory inside the main function

Question

I have a program that has to declare a huge integer array of the size 1000000 in C (using GNU GCC compiler). I tried to declare the array in two different ways.

The two possible codes are :

#include <stdio.h>
int arr[1000000];
int main()
{
  return 0; 
}

and

#include <stdio.h>
int main()
{
  int arr[1000000];
  return 0;
}

The latter version hangs during runtime. What could be the possible reason?

Thanks a lot!!!

Answer 1

The second version allocates on the stack, the size of which may be limited on your system for any given process. The first one allocates in the data segment of the process, the size of which isn't limited (at least for these orders of magnitude of allocation size)

From this SO answer you can learn how to check for the stack allocation limit for various platforms like Linux and Windows. If you're on Linux it's as simple as:

ulimit -a

Answer 2

Since you used the word static in the title, it is a wonder that it did not occur to actually declare the variable static.

int main()
{
  static int arr[1000000];
  return 0;
}

You could also use dynamic allocation.

Answer 3

A rule of thumb is to use dynamic allocation for huge objects, including huge arrays. Variables allocated in global space or in a function generally use program memory space. Many operating systems restrict the amount of memory for a program to use.

In general, operating systems can allocate more memory when using dynamic memory allocation. They can also turn it into virtual memory (such as paging it out to the hard drive).