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Manual page for BSEARCH(3)

bsearch - binary search a sorted table

SYNOPSIS

#include <search.h>

char *bsearch ((char *) key, (char *) base, nel, sizeof (*key), compar) unsigned nel; int (*compar)( );

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. 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 as accordingly the first argument is to be considered less than, equal to, or greater than the second.

EXAMPLE

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, in which case it prints out the string and its length, or it prints an error message.

#include <stdio.h>
#include <search.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((char *)(&node),
			   (char *)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);
}

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-character.

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

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

DIAGNOSTICS

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