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NAME
hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash table management
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
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.
RETURN VALUE
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.
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 attributes(7).
┌────────────────────────────────────────────────┬───────────────┬────────────────────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────────┼───────────────┼────────────────────────┤
│hcreate(), hsearch(), hdestroy() │ Thread safety │ MT-Unsafe race:hsearch │
├────────────────────────────────────────────────┼───────────────┼────────────────────────┤
│hcreate_r(), hsearch_r(), hdestroy_r() │ Thread safety │ MT-Safe race:htab │
└────────────────────────────────────────────────┴───────────────┴────────────────────────┘
STANDARDS
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.
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 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.
BUGS
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.
EXAMPLES
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);
}
SEE ALSO
bsearch(3), lsearch(3), malloc(3), tsearch(3)