Passing a dynamically allocated array of pointers by reference C

Question

void parseFile(unsigned *line, char **matrix)
{

  FILE *fp;
  char buff[cols];
  char ***ptr;
  ptr = &matrix;

  fp = fopen("cazino_heist_maffs.csv", "r");
  if (fp == NULL)
  {
    // return 1;
  }

  int i = 0;

  while (fgets(buff, 255, (FILE *)fp) != NULL)
  {
    printf("%d: %s\n", *line, buff);

    for (i = 0; buff[i] != '?'; i++)
    {
      matrix[*line][i] = buff[i];
    }

    (*line)++;
    char **tmp;
    tmp = realloc(matrix, sizeof(matrix) * ((*line) + 1));
    if (tmp == NULL)
    {
      printf("fail");
      // return 1;
    }

    matrix = tmp;
    for (int i = *line; i < (*line) + 1; i++)
    {
      matrix[i] = malloc(100 * sizeof(char *));
    }
  }
  fclose(fp);
}

So I have this method to read from a file and fill a pointer to arrays. The function works inside the method however when I go back to main the matrix isn't filled.

  printf("ALL: \n");
  for (unsigned i = 0; i < *line; i++)
  {
    for (unsigned j = 0; matrix[i][j] != '='; j++)
    {
      printf("%c", matrix[i][j]);
    }
    printf("\n");
  }

If I put this inside the function it works fine but if I put it in the main it doesn't work.

   char **matrix;
   matrix = malloc2dArray(rows, cols);

   //
   parseFile(&line, matrix);

Also this is how I call the method. I assume I pass my matrix by value instead of reference however I am not sure how to do it with the double pointer. Any help?

Answer 1

To pass an variable to a function "by reference", you pass its address. You have a char ** , which means its address has type char *** . So make that the parameter type for the function.

void parseFile(unsigned *line, char ***a_matrix)
{
    char **matrix = *a_matrix;
    // do stuff
    *a_matrix = matrix;
}

And pass in the address:

parseFile(&line, &matrix);