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bsearch – binary search of a sorted table



Standard C Library (libc, -lc)


#include <stdlib.h>

void *
bsearch(const void *key, const void *base, size_t nmemb, size_t size, int (*compar) (const void *, const void *));


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

The contents of the array should be in ascending sorted order according to the comparison function referenced by compar. The compar routine is expected to have two arguments which point to the key object and to an array member, in that order, and should return an integer less than, equal to, or greater than zero if the key object is found, respectively, to be less than, to match, or be greater than the array member. See the int_compare sample function in qsort(3) for a comparison function that is also compatible with bsearch().


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


A sample program that searches people by age in a sorted array:
#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct person {         const char      *name;         int             age; };

static int compare(const void *a, const void *b) {         const int *age;         const struct person *person;

        age = a;         person = b;

        return (*age - person->age); }

int main(void) {         struct person *friend;         int age;         /* Sorted array */         const struct person friends[] = {                 { "paul", 22 },                 { "anne", 25 },                 { "fred", 25 },                 { "mary", 27 },                 { "mark", 35 },                 { "bill", 50 }         };         const size_t len = sizeof(friends) / sizeof(friends[0]);

        age = 22;         friend = bsearch(&age, friends, len, sizeof(friends[0]), compare);         assert(strcmp(friend->name, "paul") == 0);         printf("name: %s\nage: %d\n", friend->name, friend->age);

        age = 25;         friend = bsearch(&age, friends, len, sizeof(friends[0]), compare);

        /*          * For multiple elements with the same key, it is implementation          * defined which will be returned          */         assert(strcmp(friend->name, "fred") == 0 ||          strcmp(friend->name, "anne") == 0);         printf("name: %s\nage: %d\n", friend->name, friend->age);

        age = 30;         friend = bsearch(&age, friends, len, sizeof(friends[0]), compare);         assert(friend == NULL);         printf("friend aged 30 not found\n"); }


db(3), lsearch(3), qsort(3)


The bsearch() function conforms to ISO/IEC 9899:1990 ("ISO C90").

BSEARCH (3) July 17, 2019

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