bsearch(3p) — Linux manual page


BSEARCH(3P)             POSIX Programmer's Manual            BSEARCH(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       bsearch — binary search a sorted table

SYNOPSIS         top

       #include <stdlib.h>

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

DESCRIPTION         top

       The functionality described on this reference page is aligned
       with the ISO C standard. Any conflict between the requirements
       described here and the ISO C standard is unintentional. This
       volume of POSIX.1‐2017 defers to the ISO C standard.

       The bsearch() function shall search 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.  If the nel argument
       has the value zero, the comparison function pointed to by compar
       shall not be called and no match shall be found.

       The comparison function pointed to by compar shall be called with
       two arguments that point to the key object and to an array
       element, in that order.

       The application shall ensure that the comparison function pointed
       to by compar does not alter the contents of the array. The
       implementation may reorder elements of the array between calls to
       the comparison function, but shall not alter the contents of any
       individual element.

       The implementation shall ensure that the first argument is always
       a pointer to the key.

       When the same objects (consisting of width bytes, irrespective of
       their current positions in the array) are passed more than once
       to the comparison function, the results shall be consistent with
       one another.  That is, the same object shall always compare the
       same way with the key.

       The application shall ensure that the function returns 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 application shall ensure that
       the array consists 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.

RETURN VALUE         top

       The bsearch() function shall return 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

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

       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

       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 *);
               while (scanf("%ms", &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.

RATIONALE         top

       The requirement that the second argument (hereafter referred to
       as p) to the comparison function is a pointer to an element of
       the array implies that for every call all of the following
       expressions are non-zero:

           ( (char *)p - (char *)base ) % width == 0
           (char *)p >= (char *)base
           (char *)p < (char *)base + nel * width



SEE ALSO         top

       hcreate(3p), lsearch(3p), qsort(3p), tdelete(3p)

       The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see .

IEEE/The Open Group               2017                       BSEARCH(3P)

Pages that refer to this page: stdlib.h(0p)hcreate(3p)