C program to find the frequency characters in a string using recursion

Write a C program to find the frequency characters in a string using recursion.

  #include <stdio.h>
  #include <string.h>

  /* calculates the frequency of characters in the given string */
  void calcFrequency(char *str, int *freq) {
        int index;
        if (*str != '\0') {
                index = *str - 'a';
                freq[index]++;
                calcFrequency(str + 1, freq);
        }
        return;
  }

  int main() {
        char str[256];
        int i, frequency[26];

        /* get the input string from the user */
        printf("Enter your input string(only lowercase):");
        fgets(str, 256, stdin);
        str[strlen(str) - 1] = '\0';

        /* fill frequency with 0s */
        memset(frequency, 0, sizeof(frequency));

        /* finds the frequency of characters in the given string */
        calcFrequency(str, frequency);
        printf("Frequency of characters:\n");

        /* prints the result */
        for (i = 0; i < 26; i++) {
                if (frequency[i]) {
                        printf("%c => %d\n", i + 'a', frequency[i]);
                }
        }

        return 0;

  }

  Output:

  jp@jp-VirtualBox:~/$ ./a.out
  Enter your input string(only lowercase):helloworld
  Frequency of characters:
  d => 1
  e => 1
  h => 1
  l => 3
  o => 2
  r => 1
  w => 1