hsearch(3C)hsearch(3C)NAME
hsearch, hcreate, hdestroy - manage hash search tables
SYNOPSIS
#include <search.h>
ENTRY *hsearch (ENTRY item, ACTION action);
int hcreate (size_t nel);
void hdestroy (void);
DESCRIPTION
hsearch is a hash-table search routine generalized 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)]. item is a structure of type ENTRY
(defined in the search.h header file) 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.) action 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 indicated by the return of a null pointer.
hcreate allocates sufficient space for the table, and must be called
before hsearch is used. nel 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 mathematically favorable
circumstances.
hdestroy destroys the search table, and may be followed by another call
to hcreate.
BUGS
Hsearch is compiled by Silicon Graphics with none of the flags named in
NOTES defined.
NOTES
hsearch uses open addressing with a multiplicative hash function.
However, its source code has many other options available which the user
may select by compiling the hsearch source with the following symbols
defined to the preprocessor:
DIV Use the remainder modulo table size as the hash function
instead of the multiplicative algorithm.
Page 1
hsearch(3C)hsearch(3C)
USCR Use a User Supplied Comparison Routine for ascertaining
table membership. The routine should be named hcompar and
should behave in a manner similar to strcmp [see
string(3C)].
CHAINED Use a linked list to resolve collisions. If this option is
selected, the following other options become available.
START Place new entries at the beginning of the linked
list (default is at the end).
SORTUP Keep the linked list sorted by key in ascending
order.
SORTDOWN Keep the linked list sorted by key in descending
order.
The source code should be consulted for further details.
EXAMPLE
The following example will read in strings followed by two numbers and
store them in a hash table, discarding duplicates. It will then read in
strings and find the matching entry in the hash table and print it out.
#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 */
Page 2
hsearch(3C)hsearch(3C)
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;
}
Page 3
hsearch(3C)hsearch(3C)SEE ALSObsearch(3C), lsearch(3C), malloc(3C), malloc(3X), string(3C),
tsearch(3C).
DIAGNOSTICS
hsearch 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.
hcreate returns zero if it cannot allocate sufficient space for the
table.
NOTES
hsearch and hcreate use malloc(3C) to allocate space.
Only one hash search table may be active at any given time.
Page 4
