Question

I have a static pointer

   static int **2dArr;

and then I allocate memory for 2d array.
How can I free that memory and replace it with another array?

   //////////////////////////////////////////////////////////////////////////
  void func(int **arr) {
    int i,j,k,tmp;
    int **destMatrix = NULL;
destMatrix = (int**) malloc(N * sizeof(int *));

if ((destMatrix == NULL)) {
    fprintf(stderr, "out of memory\n");
    exit(2);
}

for (i = 0; i < N; i++) {
    destMatrix[i] = (int*) malloc(N * sizeof(int));

    if (destMatrix[i] == NULL) {
        fprintf(stderr, "out of memory\n");
        exit(2);
    }
}



for(i = 0; i < N; ++i) {
    free(arr[i]);
}
free(arr);
arr = NULL;
arr = destMatrix;

}
  //////////////////////////////////////////////////////////////////////////
  int main() {
   2dArr = (int**) malloc(N * sizeof(int *));
   if (2dArr== NULL) {
    fprintf(stderr, "out of memory\n");
    exit(2);
     }


for (i = 0; i < N; i++) {
    2dArr[i] = (int*) malloc(N * sizeof(int));

    if (2dArr[i] == NULL) {
            fprintf(stderr, "out of memory\n");
            exit(2);
    }
}
    func(2dArr);
    // try to print new array, access violation
 }

I free memory and try to replace the pointer. But then I have a access violation. How can I do it correctly?

Answer 1

Couple things:

• You need to pass the argument by reference in func:
  void func(int ***arr) {
• Then change the way you free things:
  free((*arr)[i]);
  and
  free(*arr);
• and finally change how you reassign arr:
  *arr = destMatrix;
• also, don’t name variables starting with a number — it doesn’t work.
• (also, you had ++i instead of i++ at the for loop at the end of func ) this doesn’t actually matter — see the comment below

I debugged these issues using Valgrind, check it out.

Working code is below:

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

#define N 10 
//////////////////////////////////////////////////////////////////////////
void func(int ***arr) {
  int **destMatrix = NULL;
  destMatrix = malloc(N * sizeof(int *));

  if ((destMatrix == NULL)) {
    fprintf(stderr, "out of memory\n");
    exit(2);
  }

  int i;
  for (i = 0; i < N; i++) {
    destMatrix[i] = malloc(N * sizeof(int));

    if (destMatrix[i] == NULL) {
      fprintf(stderr, "out of memory\n");
      exit(2);
    }
  }

  for(i = 0; i < N; i++) {
    free((*arr)[i]);
  }
  free(*arr);
  *arr = destMatrix;

}
//////////////////////////////////////////////////////////////////////////
int main() {
  int **tdArr = malloc(N * sizeof(int *));
  if (tdArr== NULL) {
    fprintf(stderr, "out of memory\n");
    exit(2);
  }

  int i;
  for (i = 0; i < N; i++) {
    tdArr[i] = malloc(N * sizeof(int));

    if (tdArr[i] == NULL) {
      fprintf(stderr, "out of memory\n");
      exit(2);
    }
  }

  func(&tdArr);
  // try to print new array, does not get access violation

  //assign numbers 0-99
  int j;
  for (i=0; i<N; i++){
    for (j=0; j<N; j++){
      tdArr[i][j] = i*10+j;
    }
  }
  //print numbers
  for (i=0; i<N; i++){
    for (j=0; j<N; j++){
      printf("%d\n", tdArr[i][j]);
    }
  }

  //free internal arrays
  for (i=0; i<N; i++){
    free(tdArr[i]);
  }
  //free outer array
  free(tdArr);

  return 0;
}