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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



bsearch - binary search a sorted table

bsearch_s - binary search a sorted table with additional safety checks


#include <stdlib.h>

void *bsearch(const void *key, const void *base, size_t nel, size_t size,
     int (*compar)(const void *, const void *));
#define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>

void *bsearch_s(const void *key, const void *base, rsize_t nel, 
	rsize_t size, int (*compar)(const void *k, const void *y, 
	void *context), void *context);


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.

The bsearch_s() function is part of the bounds checking interfaces specified in the C11 standard, Annex K. It is similar to the bsearch() function, except that it adds a third argument to the comparison function which is the context argument passed to the bsearch_s() function. The sole use of context by bsearch_s() is to pass it to the comparison function. See runtime_constraint_handler(3C) and INCITS/ISO/IEC 9899:2011.

Return Values

The bsearch() and bsearch_s() functions return a pointer to a matching member of the array, or a null pointer if no match is found. Additionally, the bsearch_s() function returns a null pointer if there is a runtime-constraint violation. If two or more members compare equal, which member is returned is unspecified.



size argument is not a valid value


Null pointer is passed


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.


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 },

    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);

/* 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));


See attributes(7) for descriptions of the following attributes:

Interface Stability
See below

The bsearch() function can be used safely in multithreaded applications.

The bsearch_s() function cannot be used safely in a multithreaded application due to the runtime constraint handler. For more information, see the runtime_constraint_handler(3C) man page.

See Also

hsearch(3C), lsearch(3C), qsort(3C), qsort_s(3C), tsearch(3C), attributes(7), standards(7), runtime_constraint_handler(3C)