man-pages/man3p/bsearch.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "BSEARCH" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" bsearch 
.SH NAME
bsearch \- binary search a sorted table
.SH SYNOPSIS
.LP
\fB#include <stdlib.h>
.br
.sp
void *bsearch(const void *\fP\fIkey\fP\fB, const void *\fP\fIbase\fP\fB,
size_t\fP \fInel\fP\fB,
.br
\ \ \ \ \ \  size_t\fP \fIwidth\fP\fB, int (*\fP\fIcompar\fP\fB)(const
void *, const void *));
.br
\fP
.SH DESCRIPTION
.LP
The \fIbsearch\fP() function shall search an array of \fInel\fP objects,
the initial element of which is pointed to by
\fIbase\fP, for an element that matches the object pointed to by \fIkey\fP.
The size of each element in the array is specified by
\fIwidth\fP. If the \fInel\fP argument has the value zero, the comparison
function pointed to by \fIcompar\fP shall not be
called and no match shall be found.
.LP
The comparison function pointed to by \fIcompar\fP shall be called
with two arguments that point to the \fIkey\fP object and
to an array element, in that order.
.LP
The application shall ensure that the comparison function pointed
to by \fIcompar\fP 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.
.LP
The implementation shall ensure that the first argument is always
a pointer to the key.
.LP
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.
.LP
The application shall ensure that the function returns an integer
less than, equal to, or greater than 0 if the \fIkey\fP
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 \fIkey\fP object, in that order.
.SH RETURN VALUE
.LP
The \fIbsearch\fP() 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.
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
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.
.LP
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.
.sp
.RS
.nf

\fB#include <stdio.h>
#include <stdlib.h>
#include <string.h>
.sp

#define TABSIZE    1000

.sp

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);
}
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
The pointers to the key and the element at the base of the table should
be of type pointer-to-element.
.LP
The comparison function need not compare every byte, so arbitrary
data may be contained in the elements in addition to the
values being compared.
.LP
In practice, the array is usually sorted according to the comparison
function.
.SH RATIONALE
.LP
The requirement that the second argument (hereafter referred to as
\fIp\fP) 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:
.sp
.RS
.nf

\fB((char *)p - (char *(base) % width == 0
(char *)p >= (char *)base
(char *)p < (char *)base + nel * width
\fP
.fi
.RE
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIhcreate\fP() , \fIlsearch\fP() , \fIqsort\fP() , \fItsearch\fP()
, the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<stdlib.h>\fP
.SH 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 .