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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
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HSEARCH(3) Linux Programmer's Manual HSEARCH(3)
hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash
table management
#include <search.h>
int hcreate(size_t nel);
ENTRY *hsearch(ENTRY item, ACTION action);
void hdestroy(void);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <search.h>
int hcreate_r(size_t nel, struct hsearch_data *htab);
int hsearch_r(ENTRY item, ACTION action, ENTRY **retval,
struct hsearch_data *htab);
void hdestroy_r(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 unsuc‐
cessful 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 cause of 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 cause of 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(), hsearch_r(), │ Thread safety │ MT-Safe race:htab │
│hdestroy_r() │ │ │
└──────────────────────────┴───────────────┴────────────────────────┘
The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and
are described in POSIX.1-2001 and POSIX.1-2008.
The functions hcreate_r(), hsearch_r(), and hdestroy_r() are GNU
extensions.
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 version 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 <stdio.h>
#include <stdlib.h>
#include <search.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, *ep;
int i;
hcreate(30);
for (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 (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 release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 HSEARCH(3)
Pages that refer to this page: bsearch(3), lsearch(3), tsearch(3)
Copyright and license for this manual page
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