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BSEARCH(3P) POSIX Programmer's Manual BSEARCH(3P)
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.
bsearch — binary search a sorted table
#include <stdlib.h>
void *bsearch(const void *key, const void *base, size_t nel,
size_t width, int (*compar)(const void *, const void *));
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.1‐2008 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.
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.
No errors are defined.
The following sections are informative.
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);
}
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.
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
None.
hcreate(3p), lsearch(3p), qsort(3p), tdelete(3p)
The Base Definitions volume of POSIX.1‐2008, stdlib.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 BSEARCH(3P)
Pages that refer to this page: stdlib.h(0p), hcreate(3p)
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