DESCRIPTION

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.

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:

The field key points to the null-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.

The three functions hcreate_r(), hsearch_r(), hdestroy_r() are reentrant versions that allow the use of more than one table. The last argument used identifies the table. The struct it points to must be zeroed before the first call to hcreate_r().

RETURN VALUE

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.

ERRORS

POSIX documents

The glibc implementation will return the following two errors.

CONFORMING TO

The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and are described in POSIX.1-2001. The functions hcreate_r(), hsearch_r(), hdestroy_r() are GNU extensions.

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. 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.

EXAMPLE

The following program inserts 24 items in to 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;

    /* starting with small table, and letting it grow does not work */
    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);
    }
    exit(EXIT_SUCCESS);
}

SEE ALSO

bsearch(3), lsearch(3), malloc(3), tsearch(3), feature_test_macros(7)