bsearch, bsearch_s

Defined in header `<stdlib.h>`
void* bsearch( const void key, const void ptr, size_t count, size_t size, int (comp)(const void, const void*) );	(1)
void* bsearch_s( const void key, const void ptr, rsize_t count, rsize_t size, int (comp)(const void , const void , void ), void *context );	(2)	(since C11)

1) Finds an element equal to element pointed to by key in an array pointed to by ptr. The array contains count elements of size bytes and must be partitioned with respect to key, that is, all the elements that compare less than must appear before all the elements that compare equal to, and those must appear before all the elements that compare greater than the key object. A fully sorted array satisfies these requirements. The elements are compared using function pointed to by comp. The behavior is undefined if the array is not already partitioned with respect to *key in ascending order according to the same criterion that comp uses.

2) Same as (1), except that the additional context argument context is passed to comp and that the following errors are detected at runtime and call the currently installed constraint handler function:

count or size is greater than RSIZE_MAX
key, ptr or comp is a null pointer (unless count is zero)

As with all bounds-checked functions, bsearch_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including stdlib.h.

If the array contains several elements that comp would indicate as equal to the element searched for, then it is unspecified which element the function will return as the result.

Parameters

key	-	pointer to the element to search for
ptr	-	pointer to the array to examine
count	-	number of element in the array
size	-	size of each element in the array in bytes
comp	-	comparison function which returns a negative integer value if the first argument is less than the second, a positive integer value if the first argument is greater than the second and zero if the arguments are equal. `key` is passed as the first argument, an element from the array as the second. The signature of the comparison function should be equivalent to the following: int cmp(const void a, const void b); The function must not modify the objects passed to it and must return consistent results when called for the same objects, regardless of their positions in the array.
context	-	additional information (e.g., collating sequence), passed to `comp` as the third argument

Return value

1) Pointer to an element in the array that compares equal to *key, or null pointer if such element has not been found.

2) Same as (1), except that the null pointer is also returned on runtime constraints violations.

Notes

Despite the name, neither C nor POSIX standards require this function to be implemented using binary search or make any complexity guarantees.

Unlike other bounds-checked functions, bsearch_s does not treat arrays of zero size as a runtime constraint violation and instead indicates element not found (the other function that accepts arrays of zero size is qsort_s).

Until bsearch_s, users of bsearch often used global variables to pass additional context to the comparison function.

Example

Run this code

#include <stdlib.h>
#include <stdio.h>

struct data {
    int nr;
    char const *value;
} dat[] = {
    {1, "Foo"}, {2, "Bar"}, {3, "Hello"}, {4, "World"}
};

int data_cmp(void const *lhs, void const *rhs) 
{
    struct data const *const l = lhs;
    struct data const *const r = rhs;

    if (l->nr < r->nr) return -1;
    else if (l->nr > r->nr) return 1;
    else return 0;

    // return (l->nr > r->nr) - (l->nr < r->nr); // possible shortcut
    // return l->nr - r->nr; // erroneous shortcut (fails if INT_MIN is present)
}

int main(void) 
{
    struct data key = { .nr = 3 };
    struct data const *res = bsearch(&key, dat, sizeof dat / sizeof dat[0],
                                     sizeof dat[0], data_cmp);
    if (res) {
        printf("No%d:%s\n", res->nr, res->value);
    } else {
        printf("No%d not found\n", key.nr);
    }
}

Output:

No 3: Hello

References

C11 standard (ISO/IEC 9899:2011):

7.22.5.1 The bsearch function (p: 355)

K.3.6.3.1 The bsearch_s function (p: 608-609)

C99 standard (ISO/IEC 9899:1999):

7.20.5.1 The bsearch function (p: 318-319)

C89/C90 standard (ISO/IEC 9899:1990):

4.10.5.1 The bsearch function

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