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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
hcreate()
hsearch()
hdestroy()
POSIX.1-2008.
hcreate_r()
hsearch_r()
hdestroy_r()
GNU.
HISTORY
hcreate()
hsearch()
hdestroy()
SVr4, POSIX.1-2001.
hcreate_r()
hsearch_r()
hdestroy_r()
GNU.
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)
Linux man-pages 6.9.1 2024-06-15 hsearch(3)