Header File 

stdlib.h 

Category 

Memory and String Manipulation Routines 

Prototype 

void *bsearch(const void *key, const void *base, size_t nelem, size_t width, int (_USERENTRY *fcmp)(const void *, const void *)); 

Description 

Binary search of an array. 

bsearch searches a table (array) of nelem elements in memory, and returns the address of the first entry in the table that matches the search key. The array must be in order. If no match is found, bsearch returns 0.

• nelem gives the number of elements in the table.
• width specifies the number of bytes in each table entry.

fcmp is called with two arguments: elem1 and elem2. Each argument points to an item to be compared. The comparison function compares each of the pointed-to items (*elem1 and *elem2), and returns an integer based on the results of the comparison.

• For bsearch, the fcmp return value is
• < 0 if *elem1 < *elem2
• == 0 if *elem1 == *elem2
• > 0 if *elem1 > *elem2

bsearch returns the address of the first entry in the table that matches the search key. If no match is found, bsearch returns 0. 

Example  

#include <stdlib.h>
#include <stdio.h>
typedef int (*fptr)(const void*, const void*);
#define NELEMS(arr) (sizeof(arr) / sizeof(arr[0]))
int numarray[] = {123, 145, 512, 627, 800, 933};
int numeric (const int *p1, const int *p2)
{
   return(*p1 - *p2);
}
#pragma argsused
int lookup(int key)
{
   int  *itemptr;
   /* The cast of (int(*)(const void *,const void*))
      is needed to avoid a type mismatch error at
      compile time */
   itemptr = (int *) bsearch (&key, numarray, NELEMS(numarray),
      sizeof(int), (fptr)numeric);
   return (itemptr != NULL);
}
int main(void)
{
   if (lookup(512))
      printf("512 is in the table.\n");
   else
      printf("512 isn't in the table.\n");
   return 0;
}

Portability