Tribblix: manual page: bsearch.3c

BSEARCH(3C) Standard C Library Functions 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

The bsearch() function is a binary search routine generalized from
Knuth (6.2.1) Algorithm B. It returns a pointer into a table (an
array) indicating where a datum may be found or a null pointer if the
datum cannot be found. The table must be previously sorted in
increasing order according to a comparison function pointed to by
compar.

The key argument points to a datum instance to be sought in the
table. The base argument points to the element at the base of the
table. The nel argument is the number of elements in the table. The
size argument is the number of bytes in each element.

The comparison function pointed to by compar is called with two
arguments that point to the key object and to an array element, in
that order. The function must return an integer less than, equal to,
or greater than 0 if the key object is considered, respectively, to
be less than, equal to, or greater than the array element.

RETURN VALUES

The bsearch() function returns a pointer to a matching member of the
array, or a null pointer if no match is found. If two or more
members compare equal, which member is returned is unspecified.

USAGE

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

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

If the number of elements in the table is less than the size reserved
for the table, nel should be the lower number.

The bsearch() function safely allows concurrent access by multiple
threads to disjoint data, such as overlapping subtrees or tables.

EXAMPLES

Example 1: Examples for searching a table containing pointers to

nodes.

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 program 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>
struct node { /* these are stored in the table */
char *string;
int length;
};
static struct node table[] = { /* table to be searched */
{ "asparagus", 10 },
{ "beans", 6 },
{ "tomato", 7 },
{ "watermelon", 11 },
};

main()
{
struct node *node_ptr, node;
/* routine to compare 2 nodes */
static int node_compare(const void *, const void *);
char str_space[20]; /* space to read string into */

node.string = str_space;
while (scanf("%20s", node.string) != EOF) {
node_ptr = bsearch( &node,
table, sizeof(table)/sizeof(struct node),
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: %20s\n", node.string);
}
}
return(0);
}

/* routine to compare two nodes based on an */
/* alphabetical ordering of the string field */
static int
node_compare(const void *node1, const void *node2) {
return (strcmp(
((const struct node *)node1)->string,
((const struct node *)node2)->string));
}

ATTRIBUTES