CODEWITHSUNDEEP
cmatrixsegmentation-fault
fenigo69
fenigo69

Reputation: 53

Segmentation fault in matrix in C

I've tried to create a matrix in C and have some input value, but I don't know why it throws me a "segmentation error". This is my code:

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

int main() {
    int i;
    int j;
    int **a;
    a = malloc(5 * sizeof(int));
    for (i = 0; i < 5; i++) {
      a[i] = malloc(4 * sizeof(int));
    }
    for (i = 0; i < 5; i++) {
      for (j = 0; j < 4; j++) {
        scanf("%d", (a[i][j]));
      }
    }
    return 0;
}

Upvotes: 1

Views: 1786

Answers (2)

Lundin
Lundin

Reputation: 213276

Given the answer by @Bathsheba, this is what your code should look like:

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

int main (void)
{
  int (*a)[4] = malloc( sizeof(int[5][4]) );

  for(int i=0;i<5;i++)
  {
    for(int j=0;j<4;j++)
    {
      scanf("%d", &a[i][j]);
    }
  }

  free(a);
  return 0;
}

Upvotes: 2

Bathsheba
Bathsheba

Reputation: 234635

  1. Your allocation for a should be

a=malloc(5*sizeof(int*));

Note well the pointer type in the sizof. On some systems (Windows desktops in the early 2000s) an int just happened to be the same size as an int*, but you must not assume that.

  1. scanf takes a pointer as the parameter: the clearest way to write this is scanf("%d", &a[i][j]);

  2. Don't forget to free the memory once you're all done.

Finally, I die a little inside every time I see a matrix modelled like this. Granted, it allows you to use the [][] notation, but normally it's better to model it as a contiguous memory block and use the idiom i * columns + j to access the element at (i, j).

Upvotes: 0

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