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

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.