1BSEARCH(P) POSIX Programmer's Manual BSEARCH(P)
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6 bsearch - binary search a sorted table
7
9 #include <stdlib.h>
10 void *bsearch(const void *key, const void *base, size_t nel,
12 size_t width, int (*compar)(const void *, const void *));
13
16 The bsearch() function shall search an array of nel objects, the ini‐
17 tial element of which is pointed to by base, for an element that
18 matches the object pointed to by key. The size of each element in the
19 array is specified by width. If the nel argument has the value zero,
20 the comparison function pointed to by compar shall not be called and no
21 match shall be found.
22 The comparison function pointed to by compar shall be called with two
24 arguments that point to the key object and to an array element, in that
25 order.
26 The application shall ensure that the comparison function pointed to by
28 compar does not alter the contents of the array. The implementation
29 may reorder elements of the array between calls to the comparison func‐
30 tion, but shall not alter the contents of any individual element.
31 The implementation shall ensure that the first argument is always a
33 pointer to the key.
34 When the same objects (consisting of width bytes, irrespective of their
36 current positions in the array) are passed more than once to the com‐
37 parison function, the results shall be consistent with one another.
38 That is, the same object shall always compare the same way with the
39 key.
40 The application shall ensure that the function returns an integer less
42 than, equal to, or greater than 0 if the key object is considered,
43 respectively, to be less than, to match, or to be greater than the
44 array element. The application shall ensure that the array consists of
45 all the elements that compare less than, all the elements that compare
46 equal to, and all the elements that compare greater than the key
47 object, in that order.
48
50 The bsearch() function shall return a pointer to a matching member of
51 the array, or a null pointer if no match is found. If two or more mem‐
52 bers compare equal, which member is returned is unspecified.
53
55 No errors are defined.
56 The following sections are informative.
58
60 The example below searches a table containing pointers to nodes con‐
61 sisting of a string and its length. The table is ordered alphabetically
62 on the string in the node pointed to by each entry.
63 The code fragment below reads in strings and either finds the corre‐
65 sponding node and prints out the string and its length, or prints an
66 error message.
67 #include <stdio.h>
70 #include <stdlib.h>
71 #include <string.h>
72 #define TABSIZE 1000
75 struct node { /* These are stored in the table. */
79 char *string;
80 int length;
81 };
82 struct node table[TABSIZE]; /* Table to be searched. */
83 .
84 .
85 .
86 {
87 struct node *node_ptr, node;
88 /* Routine to compare 2 nodes. */
89 int node_compare(const void *, const void *);
90 char str_space[20]; /* Space to read string into. */
91 .
92 .
93 .
94 node.string = str_space;
95 while (scanf("%s", node.string) != EOF) {
96 node_ptr = (struct node *)bsearch((void *)(&node),
97 (void *)table, TABSIZE,
98 sizeof(struct node), node_compare);
99 if (node_ptr != NULL) {
100 (void)printf("string = %20s, length = %d\n",
101 node_ptr->string, node_ptr->length);
102 } else {
103 (void)printf("not found: %s\n", node.string);
104 }
105 }
106 }
107 /*
108 This routine compares two nodes based on an
109 alphabetical ordering of the string field.
110 */
111 int
112 node_compare(const void *node1, const void *node2)
113 {
114 return strcoll(((const struct node *)node1)->string,
115 ((const struct node *)node2)->string);
116 }
117
119 The pointers to the key and the element at the base of the table should
120 be of type pointer-to-element.
121 The comparison function need not compare every byte, so arbitrary data
123 may be contained in the elements in addition to the values being com‐
124 pared.
125 In practice, the array is usually sorted according to the comparison
127 function.
128
130 The requirement that the second argument (hereafter referred to as p)
131 to the comparison function is a pointer to an element of the array
132 implies that for every call all of the following expressions are non-
133 zero:
134 ((char *)p - (char *(base) % width == 0
137 (char *)p >= (char *)base
138 (char *)p < (char *)base + nel * width
139
141 None.
142
144 hcreate() , lsearch() , qsort() , tsearch() , the Base Definitions vol‐
145 ume of IEEE Std 1003.1-2001, <stdlib.h>
146
148 Portions of this text are reprinted and reproduced in electronic form
149 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
150 -- Portable Operating System Interface (POSIX), The Open Group Base
151 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
152 Electrical and Electronics Engineers, Inc and The Open Group. In the
153 event of any discrepancy between this version and the original IEEE and
154 The Open Group Standard, the original IEEE and The Open Group Standard
155 is the referee document. The original Standard can be obtained online
156 at http://www.opengroup.org/unix/online.html .
157IEEE/The Open Group 2003 BSEARCH(P)