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 void hdestroy(void);
14 ENTRY *hsearch(ENTRY item, ACTION action);
16 #define _GNU_SOURCE /* See feature_test_macros(7) */
18 #include <search.h>
19 int hcreate_r(size_t nel, struct hsearch_data *htab);
21 void hdestroy_r(struct hsearch_data *htab);
22 int hsearch_r(ENTRY item, ACTION action, ENTRY **retval,
24 struct hsearch_data *htab);
25
27 The three functions hcreate(), hsearch(), and hdestroy() allow the
28 caller to create and manage a hash search table containing entries con‐
29 sisting of a key (a string) and associated data. Using these func‐
30 tions, only one hash table can be used at a time.
31 The three functions hcreate_r(), hsearch_r(), hdestroy_r() are reen‐
33 trant versions that allow a program to use more than one hash search
34 table at the same time. The last argument, htab, points to a structure
35 that describes the table on which the function is to operate. The pro‐
36 grammer should treat this structure as opaque (i.e., do not attempt to
37 directly access or modify the fields in this structure).
38 First a hash table must be created using hcreate(). The argument nel
40 specifies the maximum number of entries in the table. (This maximum
41 cannot be changed later, so choose it wisely.) The implementation may
42 adjust this value upward to improve the performance of the resulting
43 hash table.
44 The hcreate_r() function performs the same task as hcreate(), but for
46 the table described by the structure *htab. The structure pointed to
47 by htab must be zeroed before the first call to hcreate_r().
48 The function hdestroy() frees the memory occupied by the hash table
50 that was created by hcreate(). After calling hdestroy(), a new hash
51 table can be created using hcreate(). The hdestroy_r() function per‐
52 forms the analogous task for a hash table described by *htab, which was
53 previously created using hcreate_r().
54 The hsearch() function searches the hash table for an item with the
56 same key as item (where "the same" is determined using strcmp(3)), and
57 if successful returns a pointer to it.
58 The argument item is of type ENTRY, which is defined in <search.h> as
60 follows:
61 typedef struct entry {
63 char *key;
64 void *data;
65 } ENTRY;
66 The field key points to a null-terminated string which is the search
68 key. The field data points to data that is associated with that key.
69 The argument action determines what hsearch() does after an unsuccess‐
71 ful search. This argument must either have the value ENTER, meaning
72 insert a copy of item (and return a pointer to the new hash table entry
73 as the function result), or the value FIND, meaning that NULL should be
74 returned. (If action is FIND, then data is ignored.)
75 The hsearch_r() function is like hsearch() but operates on the hash ta‐
77 ble described by *htab. The hsearch_r() function differs from
78 hsearch() in that a pointer to the found item is returned in *retval,
79 rather than as the function result.
80
82 hcreate() and hcreate_r() return nonzero on success. They return 0 on
83 error, with errno set to indicate the error.
84 On success, hsearch() returns a pointer to an entry in the hash table.
86 hsearch() returns NULL on error, that is, if action is ENTER and the
87 hash table is full, or action is FIND and item cannot be found in the
88 hash table. hsearch_r() returns nonzero on success, and 0 on error.
89 In the event of an error, these two functions set errno to indicate the
90 error.
91
93 hcreate_r() and hdestroy_r() can fail for the following reasons:
94 EINVAL htab is NULL.
96 hsearch() and hsearch_r() can fail for the following reasons:
98 ENOMEM action was ENTER, key was not found in the table, and there was
100 no room in the table to add a new entry.
101 ESRCH action was FIND, and key was not found in the table.
103 POSIX.1 specifies only the ENOMEM error.
105
107 For an explanation of the terms used in this section, see at‐
108 tributes(7).
109 ┌─────────────────────────────┬───────────────┬────────────────────────┐
111 │Interface │ Attribute │ Value │
112 ├─────────────────────────────┼───────────────┼────────────────────────┤
113 │hcreate(), hsearch(), │ Thread safety │ MT-Unsafe race:hsearch │
114 │hdestroy() │ │ │
115 ├─────────────────────────────┼───────────────┼────────────────────────┤
116 │hcreate_r(), hsearch_r(), │ Thread safety │ MT-Safe race:htab │
117 │hdestroy_r() │ │ │
118 └─────────────────────────────┴───────────────┴────────────────────────┘
119
121 The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and
122 are described in POSIX.1-2001 and POSIX.1-2008.
123 The functions hcreate_r(), hsearch_r(), and hdestroy_r() are GNU exten‐
125 sions.
126
128 Hash table implementations are usually more efficient when the table
129 contains enough free space to minimize collisions. Typically, this
130 means that nel should be at least 25% larger than the maximum number of
131 elements that the caller expects to store in the table.
132 The hdestroy() and hdestroy_r() functions do not free the buffers
134 pointed to by the key and data elements of the hash table entries. (It
135 can't do this because it doesn't know whether these buffers were allo‐
136 cated dynamically.) If these buffers need to be freed (perhaps because
137 the program is repeatedly creating and destroying hash tables, rather
138 than creating a single table whose lifetime matches that of the pro‐
139 gram), then the program must maintain bookkeeping data structures that
140 allow it to free them.
141
143 SVr4 and POSIX.1-2001 specify that action is significant only for un‐
144 successful searches, so that an ENTER should not do anything for a suc‐
145 cessful search. In libc and glibc (before version 2.3), the implemen‐
146 tation violates the specification, updating the data for the given key
147 in this case.
148 Individual hash table entries can be added, but not deleted.
150
152 The following program inserts 24 items into a hash table, then prints
153 some of them.
154 #include <stdio.h>
156 #include <stdlib.h>
157 #include <search.h>
158 static char *data[] = { "alpha", "bravo", "charlie", "delta",
160 "echo", "foxtrot", "golf", "hotel", "india", "juliet",
161 "kilo", "lima", "mike", "november", "oscar", "papa",
162 "quebec", "romeo", "sierra", "tango", "uniform",
163 "victor", "whisky", "x-ray", "yankee", "zulu"
164 };
165 int
167 main(void)
168 {
169 ENTRY e;
170 ENTRY *ep;
171 hcreate(30);
173 for (int i = 0; i < 24; i++) {
175 e.key = data[i];
176 /* data is just an integer, instead of a
177 pointer to something */
178 e.data = (void *) i;
179 ep = hsearch(e, ENTER);
180 /* there should be no failures */
181 if (ep == NULL) {
182 fprintf(stderr, "entry failed\n");
183 exit(EXIT_FAILURE);
184 }
185 }
186 for (int i = 22; i < 26; i++) {
188 /* print two entries from the table, and
189 show that two are not in the table */
190 e.key = data[i];
191 ep = hsearch(e, FIND);
192 printf("%9.9s -> %9.9s:%d\n", e.key,
193 ep ? ep->key : "NULL", ep ? (int)(ep->data) : 0);
194 }
195 hdestroy();
196 exit(EXIT_SUCCESS);
197 }
198
200 bsearch(3), lsearch(3), malloc(3), tsearch(3)
201
203 This page is part of release 5.12 of the Linux man-pages project. A
204 description of the project, information about reporting bugs, and the
205 latest version of this page, can be found at
206 https://www.kernel.org/doc/man-pages/.
207GNU 2021-03-22 HSEARCH(3)