NAME

bsearch() − binary search a sorted table

SYNOPSIS

#include <stdlib.h>

void *bsearch(

const void *key,
const void *base,
size_t nel,
size_t size,
int (*compar)(const void *, const void *)

);

DESCRIPTION

bsearch() is a binary search routine generalized from Knuth (6.2.1) Algorithm B.  It returns a pointer into a table indicating where a datum may be found.  The table must be previously sorted in increasing order according to a provided comparison function.  key points to a datum instance to be sought in the table.  base points to the element at the base of the table.  nel is the number of elements in the table.  size is the size of each element in the table.  compar is the name of the comparison function, which is called with two arguments that point to the elements being compared.  The function must return an integer less than, equal to, or greater than zero indicating that the first argument is to be considered less than, equal to, or greater than the second. 

NOTES

The pointers to the key and the element at the base of the table should be of type pointer-to-element, and cast to type pointer-to-void. 

The comparison function need not compare every byte, so arbitrary data can be contained in the elements in addition to the values being compared. 

Although declared as type pointer-to-void, the value returned should be cast into type pointer-to-element. 

RETURN VALUE

A NULL pointer is returned if the key cannot be found in the table. 

EXAMPLES

The example below searches a table containing pointers to nodes consisting of a string and its length.  The table is ordered alphabetically on the string in the node pointed to by each entry. 

This code fragment reads in strings and either finds the corresponding node and prints out the string and its length, or prints an error message. 

#include <stdio.h>
 #define TABSIZE  1000
 struct node {                        /* these are stored in the table */
    char *string;
    int length;
};
struct node table[TABSIZE];          /* table to be searched */
    .
    .
    .
{
    struct node *node_ptr, node;
    int node_compare( );  /* routine to compare 2 nodes */
    char str_space[20];   /* space to read string into */
    .
    .
    .
    node.string = str_space;
    while (scanf("%s", node.string) != EOF) {
        node_ptr = (struct node *)bsearch((void *)(&node),
               (void *)table, TABSIZE,
               sizeof(struct node), node_compare);
        if (node_ptr != NULL) {
            (void)printf("string = %20s, length = %d\n",
                node_ptr->string, node_ptr->length);
        } else {
            (void)printf("not found: %s\n", node.string);
        }
    }
}
/* This routine compares two nodes based on an
      alphabetical ordering of the string field.  */
int
node_compare(node1, node2)
struct node *node1, *node2;
{
    return strcmp(node1->string, node2->string);
}

WARNINGS

If the table being searched contains two or more entries that match the selection criteria, a random entry is returned by bsearch() as determined by the search algorithm. 

SEE ALSO

hsearch(3C), lsearch(3C), qsort(3C), tsearch(3C). 

STANDARDS CONFORMANCE

bsearch(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1, ANSI C

Hewlett-Packard Company  —  HP-UX Release 10.20:  July 1996