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
53 table can be created using hcreate(). The hdestroy_r() function per‐
54 forms the analogous task for a hash table described by *htab, which was
55 previously 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 specifies only the ENOMEM error.
107
109 For an explanation of the terms used in this section, see
110 attributes(7).
111 ┌──────────────────────────┬───────────────┬────────────────────────┐
113 │Interface │ Attribute │ Value │
114 ├──────────────────────────┼───────────────┼────────────────────────┤
115 │hcreate(), hsearch(), │ Thread safety │ MT-Unsafe race:hsearch │
116 │hdestroy() │ │ │
117 ├──────────────────────────┼───────────────┼────────────────────────┤
118 │hcreate_r(), hsearch_r(), │ Thread safety │ MT-Safe race:htab │
119 │hdestroy_r() │ │ │
120 └──────────────────────────┴───────────────┴────────────────────────┘
122 The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and
123 are described in POSIX.1-2001 and POSIX.1-2008.
124 The functions hcreate_r(), hsearch_r(), and hdestroy_r() are GNU exten‐
126 sions.
127
129 Hash table implementations are usually more efficient when the table
130 contains enough free space to minimize collisions. Typically, this
131 means that nel should be at least 25% larger than the maximum number of
132 elements that the caller expects to store in the table.
133 The hdestroy() and hdestroy_r() functions do not free the buffers
135 pointed to by the key and data elements of the hash table entries. (It
136 can't do this because it doesn't know whether these buffers were allo‐
137 cated dynamically.) If these buffers need to be freed (perhaps because
138 the program is repeatedly creating and destroying hash tables, rather
139 than creating a single table whose lifetime matches that of the pro‐
140 gram), then the program must maintain bookkeeping data structures that
141 allow it to free them.
142
144 SVr4 and POSIX.1-2001 specify that action is significant only for
145 unsuccessful searches, so that an ENTER should not do anything for a
146 successful search. In libc and glibc (before version 2.3), the imple‐
147 mentation violates the specification, updating the data for the given
148 key in this case.
149 Individual hash table entries can be added, but not deleted.
151
153 The following program inserts 24 items into a hash table, then prints
154 some of them.
155 #include <stdio.h>
157 #include <stdlib.h>
158 #include <search.h>
159 static char *data[] = { "alpha", "bravo", "charlie", "delta",
161 "echo", "foxtrot", "golf", "hotel", "india", "juliet",
162 "kilo", "lima", "mike", "november", "oscar", "papa",
163 "quebec", "romeo", "sierra", "tango", "uniform",
164 "victor", "whisky", "x-ray", "yankee", "zulu"
165 };
166 int
168 main(void)
169 {
170 ENTRY e, *ep;
171 int i;
172 hcreate(30);
174 for (i = 0; i < 24; i++) {
176 e.key = data[i];
177 /* data is just an integer, instead of a
178 pointer to something */
179 e.data = (void *) i;
180 ep = hsearch(e, ENTER);
181 /* there should be no failures */
182 if (ep == NULL) {
183 fprintf(stderr, "entry failed\n");
184 exit(EXIT_FAILURE);
185 }
186 }
187 for (i = 22; i < 26; i++) {
189 /* print two entries from the table, and
190 show that two are not in the table */
191 e.key = data[i];
192 ep = hsearch(e, FIND);
193 printf("%9.9s -> %9.9s:%d\n", e.key,
194 ep ? ep->key : "NULL", ep ? (int)(ep->data) : 0);
195 }
196 hdestroy();
197 exit(EXIT_SUCCESS);
198 }
199
201 bsearch(3), lsearch(3), malloc(3), tsearch(3)
202
204 This page is part of release 4.16 of the Linux man-pages project. A
205 description of the project, information about reporting bugs, and the
206 latest version of this page, can be found at
207 https://www.kernel.org/doc/man-pages/.
208GNU 2017-09-15 HSEARCH(3)