best way to realloc a 2d array in c?

Question

I have to implement a function for a project in one of my classes and I am having trouble with resizing the dimensions of a 2-D dynamic array, represented as a board of characters. If I resize a board of a size such as 10 X 10 to a smaller board such as 5 X 5, the resize seems to work properly and truncates the arrays accordingly. However, if I try to increase the size of the board I get strange results or segmentation faults, I'm assuming because realloc somehow failed. Any feedback at all would be helpful; I am trying my best to understand pointers and can't seem to wrap my head around how to accomplish this without memory errors. In the larger program that I am writing I have a function to free the allocated memory of the board, but here I am mainly concerned about the way I have implemented realloc.

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

char** constructBoard(int num_rows, int num_cols, char empty_space){
//this function dynamically allocates a matrix of chars
  char** board = (char**)malloc(num_rows * sizeof(char*));
  for(int row = 0; row < num_rows; ++row){
    board[row] = (char*)malloc(num_cols * sizeof(char));
    for(int col = 0; col < num_cols; ++col){
      board[row][col] = empty_space;
    }
  }
  return board;
}

void displayBoard(char** board, const int num_rows, const int num_cols){
//this function prints out the board
  int rowNum = num_rows - 1;
  for(int row = 0; row < num_rows; ++row){
    if(num_rows - row <= 10) {
      printf(" ");
      printf("%d ", rowNum);
    }
    else{
      printf("%d ", rowNum);
    }
    rowNum = rowNum - 1;
    for(int col = 0; col < num_cols; ++col){
      printf(" %c", board[row][col]);
      printf(" ");
    }
    printf("\n");
  }
  printf("  ");
  for(int i = 0; i < num_cols; ++i){
    if(i == 0){
      printf("  %d", i);
      printf(" ");
    }
    else if(i < 10) {
      printf(" %d", i);
      printf(" ");
    }
    else{
      printf("%d", i);
      printf(" ");
    }
  }
  printf("\n");
}

void resizeBoard(char*** board, int new_num_row, int new_num_col, int* num_rows, int* num_cols){
//this function is supposed to resize the dimensions of the board without causing memory leaks
  (*board) = realloc(*board, (new_num_row * sizeof(char*)));
  for(int y = *num_rows; y < new_num_row; ++y){
    (**board)[y] = '*';
  }
  for(int x = 0; x < new_num_row; ++x){
    (*board)[x] = realloc((*board)[x], new_num_col * sizeof(char));
  }

  *num_rows = new_num_row;
  *num_cols = new_num_col;
}


int main() {
  char empty_space = '*';
  int num_rows = 5;
  int num_cols = 5;
  char** board;
  int new_num_row = 7;
  int new_num_col = 7;
  board = constructBoard(num_rows, num_cols, empty_space);
  displayBoard(board, num_rows, num_cols);

  resizeBoard(&board, new_num_row, new_num_col, &num_rows, &num_cols);

  return 0;
}

You don't have a 2d array here. Why do you assume that `realloc()` has failed, when you don't check the value returned by `realloc()`? When you find yourself using 3 stars, consider rethinking the program design. — ad absurdum, Dec 13 '17 at 05:24
[You might want to read this](https://stackoverflow.com/questions/42094465/correctly-allocating-multi-dimensional-arrays) — ad absurdum, Dec 13 '17 at 05:27

score 1 · Answer 1 · answered Dec 13 '17 at 05:38

C don't have multidimensional arrays (only arrays of arrays, or arrays of pointers). So consider some better approach, where you'll have some abstract data type (e.g. for your matrix-like objects, which keep their dimension).

Of course, you'll better explicitly initialize every cell when redimensioning your board.

And don't forget to test failure of malloc, calloc, realloc

Compile your code with all warnings and debug info (gcc -Wall -Wextra -g with GCC) and use the debugger gdb and valgrind.

best way to realloc a 2d array in c?

1 Answers1