1HSEARCH(3) Linux Programmer's Manual HSEARCH(3)
2
6 hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash ta‐
7 ble management
8
10 #include <search.h>
11 int hcreate(size_t nel);
13 ENTRY *hsearch(ENTRY item, ACTION action);
15 void hdestroy(void);
17 #define _GNU_SOURCE /* See feature_test_macros(7) */
19 #include <search.h>
20 int hcreate_r(size_t nel, struct hsearch_data *htab);
22 int hsearch_r(ENTRY item, ACTION action, ENTRY **retval,
24 struct hsearch_data *htab);
25 void hdestroy_r(struct hsearch_data *htab);
27
29 The three functions hcreate(), hsearch(), and hdestroy() allow the
30 caller to create and manage a hash search table containing entries con‐
31 sisting of a key (a string) and associated data. Using these func‐
32 tions, only one hash table can be used at a time.
33 The three functions hcreate_r(), hsearch_r(), hdestroy_r() are reen‐
35 trant versions that allow a program to use more than one hash search
36 table at the same time. The last argument, htab, points to a structure
37 that describes the table on which the function is to operate. The pro‐
38 grammer should treat this structure as opaque (i.e., do not attempt to
39 directly access or modify the fields in this structure).
40 First a hash table must be created using hcreate(). The argument nel
42 specifies the maximum number of entries in the table. (This maximum
43 cannot be changed later, so choose it wisely.) The implementation may
44 adjust this value upward to improve the performance of the resulting
45 hash table.
46 The hcreate_r() function performs the same task as hcreate(), but for
48 the table described by the structure *htab. The structure pointed to
49 by htab must be zeroed before the first call to hcreate_r().
50 The function hdestroy() frees the memory occupied by the hash table
52 that was created by hcreate(). After calling hdestroy() a new hash ta‐
53 ble can be created using hcreate(). The hdestroy_r() function performs
54 the analogous task for a hash table described by *htab, which was pre‐
55 viously created using hcreate_r().
56 The hsearch() function searches the hash table for an item with the
58 same key as item (where "the same" is determined using strcmp(3)), and
59 if successful returns a pointer to it.
60 The argument item is of type ENTRY, which is defined in <search.h> as
62 follows:
63 typedef struct entry {
65 char *key;
66 void *data;
67 } ENTRY;
68 The field key points to a null-terminated string which is the search
70 key. The field data points to data that is associated with that key.
71 The argument action determines what hsearch() does after an unsuccess‐
73 ful search. This argument must either have the value ENTER, meaning
74 insert a copy of item (and return a pointer to the new hash table entry
75 as the function result), or the value FIND, meaning that NULL should be
76 returned. (If action is FIND, then data is ignored.)
77 The hsearch_r() function is like hsearch() but operates on the hash ta‐
79 ble described by *htab. The hsearch_r() function differs from
80 hsearch() in that a pointer to the found item is returned in *retval,
81 rather than as the function result.
82
84 hcreate() and hcreate_r() return nonzero on success. They return 0 on
85 error, with errno set to indicate the cause of the error.
86 On success, hsearch() returns a pointer to an entry in the hash table.
88 hsearch() returns NULL on error, that is, if action is ENTER and the
89 hash table is full, or action is FIND and item cannot be found in the
90 hash table. hsearch_r() returns nonzero on success, and 0 on error.
91 In the event of an error, these two functions set errno to indicate the
92 cause of the error.
93
95 hcreate_r() and hdestroy_r() can fail for the following reasons:
96 EINVAL htab is NULL.
98 hsearch() and hsearch_r() can fail for the following reasons:
100 ENOMEM action was ENTER, key was not found in the table, and there was
102 no room in the table to add a new entry.
103 ESRCH action was FIND, and key was not found in the table.
105 POSIX.1-2001 specifies only the ENOMEM error.
107
109 Multithreading (see pthreads(7))
110 The hcreate(), hsearch(), and hdestroy() functions use a global space
111 for storing the table, so they are not thread-safe.
112 The hcreate_r(), hsearch_r(), and hdestroy_r() functions are thread-
114 safe.
115
117 The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and
118 are described in POSIX.1-2001. The functions hcreate_r(), hsearch_r(),
119 and hdestroy_r() are GNU extensions.
120
122 Hash table implementations are usually more efficient when the table
123 contains enough free space to minimize collisions. Typically, this
124 means that nel should be at least 25% larger than the maximum number of
125 elements that the caller expects to store in the table.
126 The hdestroy() and hdestroy_r() functions do not free the buffers
128 pointed to by the key and data elements of the hash table entries. (It
129 can't do this because it doesn't know whether these buffers were allo‐
130 cated dynamically.) If these buffers need to be freed (perhaps because
131 the program is repeatedly creating and destroying hash tables, rather
132 than creating a single table whose lifetime matches that of the pro‐
133 gram), then the program must maintain bookkeeping data structures that
134 allow it to free them.
135
137 SVr4 and POSIX.1-2001 specify that action is significant only for
138 unsuccessful searches, so that an ENTER should not do anything for a
139 successful search. In libc and glibc (before version 2.3), the imple‐
140 mentation violates the specification, updating the data for the given
141 key in this case.
142 Individual hash table entries can be added, but not deleted.
144
146 The following program inserts 24 items into a hash table, then prints
147 some of them.
148 #include <stdio.h>
150 #include <stdlib.h>
151 #include <search.h>
152 char *data[] = { "alpha", "bravo", "charlie", "delta",
154 "echo", "foxtrot", "golf", "hotel", "india", "juliet",
155 "kilo", "lima", "mike", "november", "oscar", "papa",
156 "quebec", "romeo", "sierra", "tango", "uniform",
157 "victor", "whisky", "x-ray", "yankee", "zulu"
158 };
159 int
161 main(void)
162 {
163 ENTRY e, *ep;
164 int i;
165 hcreate(30);
167 for (i = 0; i < 24; i++) {
169 e.key = data[i];
170 /* data is just an integer, instead of a
171 pointer to something */
172 e.data = (void *) i;
173 ep = hsearch(e, ENTER);
174 /* there should be no failures */
175 if (ep == NULL) {
176 fprintf(stderr, "entry failed\n");
177 exit(EXIT_FAILURE);
178 }
179 }
180 for (i = 22; i < 26; i++) {
182 /* print two entries from the table, and
183 show that two are not in the table */
184 e.key = data[i];
185 ep = hsearch(e, FIND);
186 printf("%9.9s -> %9.9s:%d\n", e.key,
187 ep ? ep->key : "NULL", ep ? (int)(ep->data) : 0);
188 }
189 hdestroy();
190 exit(EXIT_SUCCESS);
191 }
192
194 bsearch(3), lsearch(3), malloc(3), tsearch(3)
195
197 This page is part of release 3.53 of the Linux man-pages project. A
198 description of the project, and information about reporting bugs, can
199 be found at http://www.kernel.org/doc/man-pages/.
200GNU 2013-07-22 HSEARCH(3)