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



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

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  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 spec-
       ified  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 ta-
       ble 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 *);
                  char str_space[20];   /* Space to read string into. */
                  .
                  .
                  .
                  node.string = str_space;
                  while (scanf("%s", 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.
              */
              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(), tsearch(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  Issue  6,  Copyright (C) 2001-2003 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 Stan-
       dard,  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 .



IEEE/The Open Group                  2003                          BSEARCH(3P)