The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


bsearch - binary search a sorted table


#include <stdlib.h>

void *bsearch(const void *key, const void *base, size_t nel,
    size_t width, int (*compar)(const void *, const void *));


The bsearch() function searches an array of nel objects, the initial element of which is pointed to by base, for an element that matches the object pointed to by key. The size of each element in the array is specified by width.

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, to match, or to be greater than the array element. The array must consist of: all the elements that compare less than, all the elements that compare equal to, and all the elements that compare greater than the key object, in that order.


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.


No errors are defined.


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.

The code fragment below 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 */
    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.
node_compare(const void *node1, const void *node2)
    return strcoll(((const struct node *)node1)->string,
        ((const struct node *)node2)->string);


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.

In practice, the array is usually sorted according to the comparison function.




hsearch(), lsearch(), qsort(), tsearch(), <stdlib.h>.


Derived from Issue 1 of the SVID.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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