hcreate(3C)
NAME
hsearch, hcreate, hdestroy - manage hash search tables
SYNOPSIS
#include <search.h>
ENTRY *hsearch(ENTRY item, ACTION action);
int hcreate(size_t mekments);
void hdestroy(void);
DESCRIPTION
The hsearch() function is a hash-table search routine gen-
eralized from Knuth (6.4) Algorithm D. It returns a pointer
into a hash table indicating the location at which an entry
can be found. The comparison function used by hsearch() is
strcmp() (see string(3C)). The item argument is a structure
of type ENTRY (defined in the <search.h> header) containing
two pointers: item.key points to the comparison key, and
item.data points to any other data to be associated with
that key. (Pointers to types other than void should be cast
to pointer-to-void.) The action argument is a member of an
enumeration type ACTION (defined in <search.h>) indicating
the disposition of the entry if it cannot be found in the
table. ENTER indicates that the item should be inserted in
the table at an appropriate point. Given a duplicate of an
existing item, the new item is not entered and hsearch()
returns a pointer to the existing item. FIND indicates that
no entry should be made. Unsuccessful resolution is indi-
cated by the return of a null pointer.
The hcreate() function allocates sufficient space for the
table, and must be called before hsearch() is used. The nel
argument is an estimate of the maximum number of entries
that the table will contain. This number may be adjusted
upward by the algorithm in order to obtain certain mathemat-
ically favorable circumstances.
The hdestroy() function destroys the search table, and may
be followed by another call to hcreate().
RETURN VALUES
The hsearch() function returns a null pointer if either the
action is FIND and the item could not be found or the action
is ENTER and the table is full.
The hcreate() function returns 0 if it cannot allocate suf-
ficient space for the table.
USAGE
The hsearch() and hcreate() functions use malloc(3C) to
allocate space.
Only one hash search table may be active at any given time.
EXAMPLES
Example 1: Example to read in strings.
The following example will read in strings followed by two
numbers and store them in a hash table, discarding dupli-
cates. It will then read in strings and find the matching
entry in the hash table and print it.
#include <stdio.h>
#include <search.h>
#include <string.h>
#include <stdlib.h>
struct info { /* this is the info stored in table */
int age, room; /* other than the key */
};
#define NUM_EMPL 5000 /* # of elements in search table */
main( )
{
/* space to store strings */
char string_space[NUM_EMPL*20];
/* space to store employee info */
struct info info_space[NUM_EMPL];
/* next avail space in string_space */
char *str_ptr = string_space;
/* next avail space in info_space */
struct info *info_ptr = info_space;
ENTRY item, *found_item;
/* name to look for in table */
char name_to_find[30];
int i = 0;
/* create table */
(void) hcreate(NUM_EMPL);
while (scanf("%s%d%d", str_ptr, &info_ptr->age,
&info_ptr->room) != EOF && i++ < NUM_EMPL) {
/* put info in structure, and structure in item */
item.key = str_ptr;
item.data = (void *)info_ptr;
str_ptr += strlen(str_ptr) + 1;
info_ptr++;
/* put item into table */
(void) hsearch(item, ENTER);
}
/* access table */
item.key = name_to_find;
while (scanf("%s", item.key) != EOF) {
if ((found_item = hsearch(item, FIND)) != NULL) {
/* if item is in the table */
(void)printf("found %s, age = %d, room = %d\n",
found_item->key,
((struct info *)found_item->data)->age,
((struct info *)found_item->data)->room);
} else {
(void)printf("no such employee %s\n",
name_to_find)
}
}
return 0;
}
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| MT-Level | Safe |
|_____________________________|_____________________________|
SEE ALSO
bsearch(3C), lsearch(3C), malloc(3C), string(3C),
tsearch(3C), malloc(3MALLOC), attributes(5)
The Art of Computer Programming, Volume 3, Sorting and
Searching by Donald E. Knuth, published by Addison-Wesley
Publishing Company, 1973.
Man(1) output converted with
man2html