This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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.

       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 http://www.opengroup.org/unix/online.html .

       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                       BSEARCH(3POSIX)

       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.

       No errors are defined.

       Thefollowingsectionsareinformative.

       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 *);
               .
               .
               .
               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);
                   }
                   free(node.string);
               }
           }
           /*
               This routine compares two nodes based on an
               alphabetical ordering of the string field.
           */
           int
           node_compare(const void *node1, const void *node2)
           {
               return strcoll(((const struct node *)node1)->string,
                   ((const struct node *)node2)->string);
           }

       None.

       bsearch — binary search a sorted table

       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.

       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

       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 unspecified.

hcreate(), lsearch(), qsort(), tdelete()

       The Base Definitions volume of POSIX.1‐2017, <stdlib.h>

       #include <stdlib.h>

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