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I'm trying to create an undefined 2D m x m array as global variable. But after I type the m value, I get a 

segmentation fail(core image dumped)

error. Can anyone help me with this? Thank you very much. Here's my code:

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

double **A;

int main(void) {

    int m, i, j;

    scanf("%d", &m);

    A = (double **)malloc(m * m * sizeof(double));

    for (i = 0; i < m; i++) {
        for (j = 0; j < m; j++) {
            A[i][j] = i + j;
        }
    }

    return 0;
}
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7
  • 3
    You're dereferencing A[i], which is uninitialized. Commented Aug 25, 2018 at 19:48
  • can someone find a good dupe? I'm too tired to google the proper keywords. Commented Aug 25, 2018 at 20:03
  • 1
    what the hell, not the perfect dupe, but ... stackoverflow.com/questions/36890624/malloc-a-2d-array-in-c Commented Aug 25, 2018 at 20:05
  • 1
    Despite what you may think, you are allocating a 1D array, so use double * instead of double **. Access via: A[(i * m) + j] instead of A[i][j] Commented Aug 25, 2018 at 20:21
  • Craig, so how can I do to create a 2D array? Commented Aug 25, 2018 at 20:32

1 Answer 1

Reset to default
1

If you want to allocate memory block of size m * m your way then you need to use single pointer arithmetic to access elements.

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

double *A;

int main(void) {

    int m;
    scanf("%d", &m);

    A = malloc(m * m * sizeof(double));

    for (int i = 0; i < m; i++) {
        for(int j = 0; j < m; j++) {
            *(A + i * m + j) = i + j;
        }
    }

    return 0;
}

Another way to do this is to use an array of pointers where each pointer points to a memory of size m as explained in here or check the below code

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

double **A;

int main(void) {

    int m;
    scanf("%d", &m) ;

    A = malloc(m * sizeof(double*));

    for(int i = 0; i < m; i++){
        A[i] = malloc(m * sizeof(double));
    }

    for (int i = 0; i < m; i++){
        for(int j = 0; j < m; j++){
            A[i][j] = i + j;
        }
    }

    return 0;
}
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4 Comments

also, OP should check that scanf returned a valid value, else m could be 2453463646
You can get the original syntax back by typing the result as pointer-to-array, making the allocated block an array-of-arrays, and keeping the nice multidim syntax OP wanted without needing separate allocations. Your solution is an important step to understanding how that works, though.
yeah, a real 2D access alternative would be a plus. But I understand that you don't want to invest on your answer since you got a really unjustified downvote...
2D access alternative added.

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