Getting a stack overflow exception when declaring a large array

Your array is way too big to fit into the stack, consider using the heap:

int *sieve = malloc(2000000 * sizeof(*sieve));

If you really want to change the stack size, take a look at this document.

Tip: - Don't forget to free your dynamically allocated memory when it's no-longer needed.

c arrays memory stack allocation

There are 3 ways:

Allocate array on heap - use malloc(), as other posters suggested. Do not forget to free() it (although for main() it is not that important - OS will clean up memory for you on program termination).
Declare the array on unit level - it will be allocated in data segment and visible for everybody (adding static to declaration will limit the visibility to unit).
Declare your array as static - in this case it will be allocated in data segment, but visible only in main().

c arrays memory stack allocation

You would be better off allocating it on the heap, not the stack. something like

int main(int argc, char* argv[]){    int * sieve;    sieve = malloc(20000);    return 0;}

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