bsearch(3C)bsearch(3C)NAMEbsearch() - binary search a sorted table
SYNOPSISDESCRIPTION
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 accord‐
ing 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 compari‐
son function, which is called with two arguments that point to the ele‐
ments 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 sec‐
ond 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 com‐
pared.
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 con‐
sisting of a string and its length. The table is ordered alphabeti‐
cally 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 mes‐
sage.
#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 */
.
.
.
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(const void *node1, const void *node2)
struct node *node1, *node2;
{
return strcoll(((const struct node *)node1)->string,
((const struct node *)node2)->string);
}
WARNINGS
If the table being searched contains two or more entries that match the
selection criteria, a random entry is returned by as determined by the
search algorithm.
SEE ALSOhsearch(3C), lsearch(3C), qsort(3C), tsearch(3C), thread_safety(5).
STANDARDS CONFORMANCEbsearch(3C)