|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
|
|
|
hsearch(3) Library Functions Manual hsearch(3)
hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r -
hash table management
Standard C library (libc, -lc)
#include <search.h>
int hcreate(size_t nel);
void hdestroy(void);
ENTRY *hsearch(ENTRY item, ACTION action);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <search.h>
int hcreate_r(size_t nel, struct hsearch_data *htab);
void hdestroy_r(struct hsearch_data *htab);
int hsearch_r(ENTRY item, ACTION action, ENTRY **retval,
struct hsearch_data *htab);
The three functions hcreate(), hsearch(), and hdestroy() allow the
caller to create and manage a hash search table containing entries
consisting of a key (a string) and associated data. Using these
functions, only one hash table can be used at a time.
The three functions hcreate_r(), hsearch_r(), hdestroy_r() are
reentrant versions that allow a program to use more than one hash
search table at the same time. The last argument, htab, points to
a structure that describes the table on which the function is to
operate. The programmer should treat this structure as opaque
(i.e., do not attempt to directly access or modify the fields in
this structure).
First a hash table must be created using hcreate(). The argument
nel specifies the maximum number of entries in the table. (This
maximum cannot be changed later, so choose it wisely.) The
implementation may adjust this value upward to improve the
performance of the resulting hash table.
The hcreate_r() function performs the same task as hcreate(), but
for the table described by the structure *htab. The structure
pointed to by htab must be zeroed before the first call to
hcreate_r().
The function hdestroy() frees the memory occupied by the hash
table that was created by hcreate(). After calling hdestroy(), a
new hash table can be created using hcreate(). The hdestroy_r()
function performs the analogous task for a hash table described by
*htab, which was previously created using hcreate_r().
The hsearch() function searches the hash table for an item with
the same key as item (where "the same" is determined using
strcmp(3)), and if successful returns a pointer to it.
The argument item is of type ENTRY, which is defined in <search.h>
as follows:
typedef struct entry {
char *key;
void *data;
} ENTRY;
The field key points to a null-terminated string which is the
search key. The field data points to data that is associated with
that key.
The argument action determines what hsearch() does after an
unsuccessful search. This argument must either have the value
ENTER, meaning insert a copy of item (and return a pointer to the
new hash table entry as the function result), or the value FIND,
meaning that NULL should be returned. (If action is FIND, then
data is ignored.)
The hsearch_r() function is like hsearch() but operates on the
hash table described by *htab. The hsearch_r() function differs
from hsearch() in that a pointer to the found item is returned in
*retval, rather than as the function result.
hcreate() and hcreate_r() return nonzero on success. They return
0 on error, with errno set to indicate the error.
On success, hsearch() returns a pointer to an entry in the hash
table. hsearch() returns NULL on error, that is, if action is
ENTER and the hash table is full, or action is FIND and item
cannot be found in the hash table. hsearch_r() returns nonzero on
success, and 0 on error. In the event of an error, these two
functions set errno to indicate the error.
hcreate_r() and hdestroy_r() can fail for the following reasons:
EINVAL htab is NULL.
hsearch() and hsearch_r() can fail for the following reasons:
ENOMEM action was ENTER, key was not found in the table, and there
was no room in the table to add a new entry.
ESRCH action was FIND, and key was not found in the table.
POSIX.1 specifies only the ENOMEM error.
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────┬───────────────┬────────────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────┼───────────────┼────────────────────────┤
│ hcreate(), hsearch(), │ Thread safety │ MT-Unsafe race:hsearch │
│ hdestroy() │ │ │
├───────────────────────┼───────────────┼────────────────────────┤
│ hcreate_r(), │ Thread safety │ MT-Safe race:htab │
│ hsearch_r(), │ │ │
│ hdestroy_r() │ │ │
└───────────────────────┴───────────────┴────────────────────────┘
hcreate()
hsearch()
hdestroy()
POSIX.1-2008.
hcreate_r()
hsearch_r()
hdestroy_r()
GNU.
hcreate()
hsearch()
hdestroy()
SVr4, POSIX.1-2001.
hcreate_r()
hsearch_r()
hdestroy_r()
GNU.
Hash table implementations are usually more efficient when the
table contains enough free space to minimize collisions.
Typically, this means that nel should be at least 25% larger than
the maximum number of elements that the caller expects to store in
the table.
The hdestroy() and hdestroy_r() functions do not free the buffers
pointed to by the key and data elements of the hash table entries.
(It can't do this because it doesn't know whether these buffers
were allocated dynamically.) If these buffers need to be freed
(perhaps because the program is repeatedly creating and destroying
hash tables, rather than creating a single table whose lifetime
matches that of the program), then the program must maintain
bookkeeping data structures that allow it to free them.
SVr4 and POSIX.1-2001 specify that action is significant only for
unsuccessful searches, so that an ENTER should not do anything for
a successful search. In libc and glibc (before glibc 2.3), the
implementation violates the specification, updating the data for
the given key in this case.
Individual hash table entries can be added, but not deleted.
The following program inserts 24 items into a hash table, then
prints some of them.
#include <search.h>
#include <stdio.h>
#include <stdlib.h>
static char *data[] = { "alpha", "bravo", "charlie", "delta",
"echo", "foxtrot", "golf", "hotel", "india", "juliet",
"kilo", "lima", "mike", "november", "oscar", "papa",
"quebec", "romeo", "sierra", "tango", "uniform",
"victor", "whisky", "x-ray", "yankee", "zulu"
};
int
main(void)
{
ENTRY e;
ENTRY *ep;
hcreate(30);
for (size_t i = 0; i < 24; i++) {
e.key = data[i];
/* data is just an integer, instead of a
pointer to something */
e.data = (void *) i;
ep = hsearch(e, ENTER);
/* there should be no failures */
if (ep == NULL) {
fprintf(stderr, "entry failed\n");
exit(EXIT_FAILURE);
}
}
for (size_t i = 22; i < 26; i++) {
/* print two entries from the table, and
show that two are not in the table */
e.key = data[i];
ep = hsearch(e, FIND);
printf("%9.9s -> %9.9s:%d\n", e.key,
ep ? ep->key : "NULL", ep ? (int) ep->data : 0);
}
hdestroy();
exit(EXIT_SUCCESS);
}
bsearch(3), lsearch(3), malloc(3), tsearch(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 hsearch(3)
Pages that refer to this page: bsearch(3), lsearch(3), tsearch(3)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|