illumos: manual page: hsearch.3c

HSEARCH(3C) Standard C Library Functions HSEARCH(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 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)). 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 indicated 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
mathematically 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 sufficient 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 duplicates. 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