HSEARCH(3) Linux Programmer's Manual HSEARCH(3)
NAME
hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash ta-
ble management
SYNOPSIS
#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);
DESCRIPTION
The three functions hcreate(), hsearch(), and hdestroy() allow the
caller to create and manage a hash search table containing entries con-
sisting of a key (a string) and associated data. Using these func-
tions, only one hash table can be used at a time.
The three functions hcreate_r(), hsearch_r(), hdestroy_r() are reen-
trant 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 pro-
grammer 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 per-
forms 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 unsuccess-
ful 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 ta-
ble 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.
RETURN VALUE
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.
ERRORS
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.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(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() | | |
+--------------------------+---------------+------------------------+
CONFORMING TO
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 exten-
sions.
NOTES
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 allo-
cated 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 pro-
gram), then the program must maintain bookkeeping data structures that
allow it to free them.
BUGS
SVr4 and POSIX.1-2001 specify that action is significant only for un-
successful searches, so that an ENTER should not do anything for a suc-
cessful search. In libc and glibc (before version 2.3), the implemen-
tation violates the specification, updating the data for the given key
in this case.
Individual hash table entries can be added, but not deleted.
EXAMPLES
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);
}
SEE ALSO
bsearch(3), lsearch(3), malloc(3), tsearch(3)
COLOPHON
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GNU 2020-06-09 HSEARCH(3)