Integer array to string

Question

This function creates takes an int * buffer and creates a neatly formatted string (useful for printing the contents of an array).

• Is the code easy to follow?
• Is it efficient?
• Am I allocating and freeing memory properly?
• Bonus question: is there an idiomatic way to do this in C, such as through a set of library routines?

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
char *int_array_to_string(int *arr, size_t length)
{
 char **strings = calloc(length, sizeof(char *));
 int num_chars = 0;
 for (size_t i = 0; i < length; i++)
 {
 char *num_string = calloc(32, sizeof(char)); // 32 digits should fit any int
 sprintf(num_string, "%d", arr[i]);
 strings[i] = num_string;
 num_chars += strlen(num_string);
 }
 num_chars += 2 * (length - 1); // extra bytes for comma and space chars following the numbers (except for the last number)
 num_chars += 2; // bytes for curly braces at the beginning and end
 char *result = calloc(num_chars, sizeof(char));
 size_t i = 0;
 result[i++] = '{';
 for (size_t j = 0; j < length; j++)
 {
 char *str = strings[j];
 for (size_t k = 0; k < strlen(str); k++)
 result[i++] = str[k];
 free(str);
 result[i++] = ',';
 result[i++] = ' ';
 }
 free(strings);
 result[num_chars - 1] = '}';
 return result;
}

Answer 1

• result is not zero-terminated. You should allocate one byte more.

• To my taste, there are too many allocations. Compute the required size, and allocating once. You already allocate 32 bytes per array element, so

   char * result = malloc(length * 32 + whatever_extra_space_necessary);
  

• Now recall that sprintf returns an amount of bytes it actually printed, and remove a superfluous call to strlen:

   char * where = result;
   for (size_t i = 0; i < length; i++) {
   size_t printed = sprintf(where, "%d, ", arr[i]);
   where += printed;
   }
  

• Your code prints an unpleasant ", " after the last element of an array. If it is a conscious decision, it is all right; if it is not, consider printing the first element separately:

   print("%d", arr[0]);
   for (i = 1; ....)
   print(", %d", arr[i]);