Thread: A General Shuffle Array Function

I wrote a general array shuffle function which has a signature:

Code:

void shuffle(void * ptr, unsigned int elem_size, size_t size)

and I want to know if its proper to cast the void * ptr to a character pointer inside the shuffle function when I shuffle the elements?

Here's a complete example of the shuffle function with two different arrays.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdbool.h>
#include <string.h>

#define ARR_SIZE 10

void set_seed() {
  srand(time(NULL));
}

void shuffle(void * ptr, unsigned int elem_size, size_t size) {
  static bool set_seed_called = false;
  size_t counter = size - 1;
  char temp[elem_size];

  if (!set_seed_called) {
    set_seed();
    set_seed_called = true;
  }

  for (int i = 0; i < size; ++i) {
    int pos = rand() % (counter + 1);
    memcpy(temp, ((char*)ptr) + (elem_size * counter), elem_size);
    memcpy(((char*)ptr) + (elem_size * counter), ((char*)ptr) + (elem_size * pos), elem_size);
    memcpy(((char*)ptr) + (elem_size * pos), temp, elem_size);
    --counter;
  }
}

int main(int argc, char ** argv) {
  int i_arr[ARR_SIZE] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

  for (int i = 0; i < ARR_SIZE; ++i) {
    fprintf(stdout, "%d, ", i_arr[i]);
  }
  fputs("\n", stdout);
  
  shuffle(i_arr, sizeof(*i_arr), ARR_SIZE);
  
  for (int i = 0; i < ARR_SIZE; ++i) {
    fprintf(stdout, "%d, ", i_arr[i]);
  }
  fputs("\n", stdout);
  
  char* s_arr[ARR_SIZE] = {
    "aaaaa",
    "bbbbb",
    "ccccc",
    "ddddd",
    "eeeee",
    "fffff",
    "ggggg",
    "hhhhh",
    "iiiii",
    "jjjjj"
  };
  
  for (int i = 0; i < ARR_SIZE; ++i) {
    fprintf(stdout, "%s, ", s_arr[i]);
  }
  fputs("\n", stdout);
  
  shuffle(s_arr, sizeof(*s_arr), ARR_SIZE);
  
  for (int i = 0; i < ARR_SIZE; ++i) {
    fprintf(stdout, "%s, ", s_arr[i]);
  }
  fputs("\n", stdout);
 
  return 0;
}

Note: The shuffle functions works but I'd like to know if the casting is proper.