Tribblix: 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