C. Segmentation Fault when function modifies dynamically allocated 2d array

Question

What i need is a function that modifies given pointer to 2d matrix like this:

void intMatrixAll(int row, int col, int **matrix);

Now, a function should allocate memory and the matrix could be used. Rows and cols are given at run-time.

#include <stdio.h>
#include <stdlib.h>
#define PRINTINT(X) printf("%d\n", X);
void intMatrixAll(int row, int col, int **matrix);

int main(void) {
   int testArrRow = 4;
   int testArrCol = 6;
   int **testMatrix = NULL;
   intMatrixAll(testArrRow, testArrCol, testMatrix);
   testMatrix[2][2] = 112; //sementation fault here :(
   PRINTINT(testMatrix[2][2]);
   system("PAUSE");
   return 0;
}

void intMatrixAll(int row, int col, int **matrix) {
   printf("intMatrixAll\n");
   //allocate pointers:
   matrix = malloc(row * sizeof(int *));
   if(matrix == NULL) printf("Failed to allocate memmory.\n");
   for(int i=0; i<row; i++) {
      //allocate space for cols: 
      matrix[i] = malloc(col * sizeof(int));
      if(matrix[i] == NULL) {
         printf("Failed to allocate memmory for arr[%d].\n", i);
         exit(0);
      }
   }
}

Why am i getting error?

Answer 1

Test matrix is still NULL. You need to return the newly allocated pointer from intMatrixAll(). Either return the value from the function, or pass in the address of testMatrix so it can be set.

#include <stdio.h>
#include <stdlib.h>
#define PRINTINT(X) printf("%d\n", X);
void intMatrixAll(int row, int col, int **matrix);

int main(void) {
   int testArrRow = 4;
   int testArrCol = 6;
   int **testMatrix = NULL;
   intMatrixAll(testArrRow, testArrCol, &testMatrix);
   testMatrix[2][2] = 112; //sementation fault here :(
   PRINTINT(testMatrix[2][2]);
   system("PAUSE");
   return 0;
}

void intMatrixAll(int row, int col, int ***matrix) {
   printf("intMatrixAll\n");
   //allocate pointers:
   *matrix = malloc(row * sizeof(int *));
   if(*matrix == NULL) printf("Failed to allocate memmory.\n");
   for(int i=0; i<row; i++) {
      //allocate space for cols: 
      *matrix[i] = malloc(col * sizeof(int));
      if(*matrix[i] == NULL) {
         printf("Failed to allocate memmory for arr[%d].\n", i);
         exit(0);
      }
   }
}

Answer 2

Because modifying matrix inside intMatrixAll() does not modify testMatrix in main(). If you want to be able to modify main's variable, you need to pass a pointer to it. So you need to change intMatrixAll to:

void intMatrixAll(int row, int col, int ***matrix)

Inside intMatrixAll, you'll now need to change matrix to *matrix (and for when you are indexing it, you'll want (*matrix)[...].

Finally, you need to change your call to intMatrixAll to:

intMatrixAll(testArrRow, testArrCol, &testMatrix);

The reason why is that C only supports pass-by-value and pass-by-value does not support the called function changing the value of a variable in the caller.

In order to modify the value of a variable in the caller, you need to pass a pointer to the variable and then have the called function dereference it.

Answer 3

Thank YOU this helped me as well