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hcreate, hdestroy, hsearch - hash table management




#include <search.h>

"int hcreate(size_t "nel);

"ENTRY *hsearch(ENTRY "item", ACTION "action);

"void hdestroy(void);"

#define _GNU_SOURCE
#include <search.h>

"int hcreate_r(size_t "nel", struct hsearch_data *"tab);

"int *hsearch_r(ENTRY "item", ACTION "action, "ENTRY **"ret", struct hsearch_data *"tab);

"void hdestroy_r(struct hsearch_data *"tab);


The three functions hcreate, hsearch, and hdestroy allow the user to create a hash table (only one at a time) which associates a key with any data. The three functions hcreate_r, hsearch_r, hdestroy_r are reentrant versions that allow the use of more than one table.

First the table must be created with the function hcreate(). The argument nel is an estimate of the maximum number of entries in the table. The function hcreate() may adjust this value upward to improve the performance of the resulting hash table.

The corresponding function hdestroy() frees the memory occupied by the hash table so that a new table can be constructed.

The argument item is of type ENTRY, which is a typedef defined in <search.h> and includes these elements:

typedef struct entry {
char *key;
void *data;

The field key points to the NUL-terminated string which is the search key. The field data points to the data associated with that key. The function hsearch() 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 action determines what hsearch() does after an unsuccessful search. A value of ENTER instructs it to insert a copy of item, while a value of FIND means to return NULL.


hcreate() and hcreate_r() return 0 when allocation of the memory for the hash table fails, nonzero otherwise.

hsearch() returns NULL 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 0 if action is ENTER and the hash table is full, and nonzero otherwise.


ENOMEM Out of memory.


The functions hcreate, hsearch, and hdestroy are from SVID, and are described in POSIX 1003.1-2001. The functions hcreate_r, hsearch_r, hdestroy_r are GNU extensions.


SVID and POSIX 1003.1-2001 specify that action is significant only for unsuccessful searches, so that an ENTER should not do anything for a successful search. The libc and glibc implementations update 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 in to a hash table, then prints some of them.

#include <stdio.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() {
ENTRY e, *ep;
int i;

/* starting with small table, and letting it grow does not work */
for (i = 0; i < 24; i++) {
e.key = data[i];
/* data is just an integer, instead of a
pointer to something */
e.data = (char *)i;
ep = hsearch(e, ENTER);
/* there should be no failures */
if (ep == NULL) {
fprintf(stderr, "entry failed\n");
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);
return 0;


bsearch(3), lsearch(3), tsearch(3), malloc(3)

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