HSEARCH(3)

NAME
hsearch, hcreate, hdestroy - manage hash search tables

SYNOPSIS
#include <search.h>

ENTRY *hsearch (item, action)
ENTRY item;
ACTION action;

int hcreate (nel)
unsigned nel;

void hdestroy ( )

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. 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 character should be cast to pointer-to-character.) action is a member of an enumeration type ACTION 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. 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.

NOTES
hsearch() uses open addressing with a multiplicative hash function.

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>
    struct info {		/* this is the info stored in the 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, *hsearch( );
    	/* 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 = (char *)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);
    	    }
    	}
    }

SEE ALSO
bsearch(3), lsearch(3), malloc(3V), string(3), tsearch(3)

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.

WARNING
hsearch() and hcreate() use malloc(3V) to allocate space.

BUGS
Only one hash search table may be active at any given time.