huschi.net

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 differ (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  ini-
       tial  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 func-
       tion, 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  com-
       parison  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 mem-
       bers 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 con-
       sisting 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 corre-
       sponding 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 com-
       pared.
       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, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications 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 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 .

IEEE/The Open Group                  2003                          BSEARCH(3P)