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



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

NAME
       bsearch -- binary search a sorted table

SYNOPSIS
       #include <stdlib.h>

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

DESCRIPTION
       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
       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.

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
       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);
           }

APPLICATION USAGE
       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
       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

FUTURE DIRECTIONS
       None.

SEE ALSO
       hcreate(), lsearch(), qsort(), tdelete()

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

COPYRIGHT
       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(3)
