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HSEARCH(3) Linux Programmer's Manual HSEARCH(3)
hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash table man-
agement
#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 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.
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.
hcreate() and hcreate_r() can fail for the following reasons:
EINVAL (hcreate_r()) htab is NULL.
ENOMEM Table full with action set to ENTER.
ESRCH The action argument is FIND and no corresponding element is found in
the table.
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.1-2001 only specifies the ENOMEM error.
The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and are
described in POSIX.1-2001. 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>
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 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2008-10-06 HSEARCH(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface