bsearch.3c (2010 09)

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bsearch(3C) bsearch (3C)
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 (the key) is to be considered less
than, equal to, or greater than the second argument (the array element).
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>
#include <stdlib.h>
#include <string.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;
/* routine to compare 2 nodes */
int node_compare(const void *, const void *);
char str_space[20]; /* space to read string into */
.
.
.
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