Memory allocation for a matrix in C

Question

Why is the following code resulting in Segmentation fault? (I'm trying to create two matrices of the same size, one with static and the other with dynamic allocation)

#include <stdio.h>
#include <stdlib.h>

//Segmentation fault!
int main(){
    #define X 5000
    #define Y 6000

    int i;
    int a[X][Y];

    int** b = (int**) malloc(sizeof(int*) * X);
    for(i=0; i<X; i++){
        b[i] = malloc (sizeof(int) * Y);
    }
}

Weirdly enough, if I comment out one of the matrix definitions, the code runs fine. Like this:

#include <stdio.h>
#include <stdlib.h>

//No Segmentation fault!
int main(){
    #define X 5000
    #define Y 6000

    int i;
    //int a[X][Y];

    int** b = (int**) malloc(sizeof(int*) * X);
    for(i=0; i<X; i++){
        b[i] = malloc (sizeof(int) * Y);
    }
}

or

#include <stdio.h>
#include <stdlib.h>

//No Segmentation fault!
int main(){
    #define X 5000
    #define Y 6000

    int i;
    int a[X][Y];

    //int** b = (int**) malloc(sizeof(int*) * X);
    //for(i=0; i<X; i++){
    //  b[i] = malloc (sizeof(int) * Y);
    //}
}

I'm running gcc on Linux on a 32-bit machine.

Edit: Checking if malloc() succeeds:

#include <stdio.h>
#include <stdlib.h>

//No Segmentation fault!
int main(){
    #define X 5000
    #define Y 6000

    int i;
    int a[X][Y];

    int* tmp;
    int** b = (int**) malloc(sizeof(int*) * X);
    if(!b){
        printf("Error on first malloc.\n");
    }
    else{
        for(i=0; i<X; i++){          
            tmp = malloc (sizeof(int) * Y);
            if(tmp)
               b[i] = tmp;
            else{
               printf("Error on second malloc, i=%d.\n", i);
               return;
            }
        }
    }    
}

Nothing is printed out when I run it (expect of course for "Segmentation fault")

Answer 1

Your a variable requires, on a 32-bit system, 5000 * 6000 * 4 = 120 MB of stack space. It's possible that this violates some limit, which causes the segmentation fault.

Also, it's of course possible that malloc() fails at some point, which might casue you to dereference a NULL pointer.