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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017 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.
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.1‐2017, stdlib.h(0p)
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(3P)
Pages that refer to this page: stdlib.h(0p), hcreate(3p)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|