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

You are getting a segmentation fault which means that your program is attempting to access a memory address that has not been assigned to its process. The array a is a local variable and thus allocated memory from the stack. As unwind pointed out a requires 120 Mbytes of storage. This is almost certainly larger than the stack space that the OS has allocated to your process. As soon as the for loop walks off the end of the stack you get a segmentation fault.

In Linux the stack size is controlled by the OS not the compiler so try the following:-

$ ulimit -a

In the response you should see a line something like this:-

stack size (kbytes)            (-s)  10240

This means that each process gets 10Mbyte of storage, nowhere near enough for your large array.

You can adjust the stack size with a ulimit -s <stack size> command but I suspect it will not allow you to select a 120Mbyte stack size!

The simplest solution is to make a a global variable instead of an local variable.