BSEARCH(P) POSIX Programmer's Manual BSEARCH(P)NAMEbsearch - 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 ALSOhcreate() , lsearch() , qsort() , tsearch() , the Base Definitions vol‐
ume 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(P)
