//Declaration: void *bsearch(const void *key, const void *buf, size_t num, size_t size, int (*compare)(const void *, const void *)); 
//Return:      returns a pointer to the first member that matches *key. 
               A null pointer is returned if not found. 
//Parameter:   The array must be sorted in ascending order. 
               The number of elements in the array is num.
               The size (in bytes) of each element is described by size. 
#include <stdlib.h>
  #include <ctype.h>
  #include <stdio.h>

  char *alpha = "abcdefghijklmnopqrstuvwxyz";

  int comp(const void *ch, const void *s);

  int main(void)
  {
    char ch;
    char *p;

    printf("Enter a character: ");
    ch = getchar();
    ch = tolower(ch);
    p = (char *) bsearch(&ch, alpha, 26, 1, comp);
    if(p) printf(" %c is in alphabet\n", *p);
    else printf("is not in alphabet\n");

    return 0;
  }

  /* Compare two characters. */
  int comp(const void *ch, const void *s)
  {
    return *(char *)ch - *(char *)s;
  }