C++ stack array limit?

Question

I'm running some code which may be pointing out I don't understand the difference between the heap and stack that well. Below I have some example code, where I either declare an array on the stack or the heap of 1234567 elements. Both work.

int main(int argc, char** argv){

  int N = 1234567;

  int A[N];

  //int* A = new int[N];

}

But if we take N to be 12345678, I get a seg fault with int A[N], whereas the heap declaration still works fine. (I'm using g++ O3 -std=c++0x if that matters). What madness is this? Does the stack have a (rather small) array size limit?

Answer 1

This is because the stack is of a much smaller size than the heap. The heap can occupy all memory available to the program. By default VC++ compiles the stack with a size of 1 MB. The stack offers better performance but is for smaller quantities of data. In general it is not used for large data structures. This is why functions accepting lists/arrays/dictionaries/ect in c++ generally take a pointer or reference to that structure. Parameters passed by value are copied onto the stack and passing such structures would frequently cause programs to crash.

In your example you're using N int's, an int is 4 bytes. That makes the size of A[N] ~4.7 MB, much larger than the size of your stack.