HCREATE(3)

NAME
hcreate, hdestroy, hsearch - manage hash search table

LIBRARY
libc

SYNOPSIS
#include <search.h>
int
hcreate(size_t nel);
void
hdestroy(void);
ENTRY *
hsearch(ENTRY item, ACTION action);

DESCRIPTION
The hcreate(), hdestroy(), and hsearch() functions manage hash search tables.

The hcreate() function allocates sufficient space for the table, and the application should ensure it is called before hsearch() is used. The nel argument is an estimate of the maximum number of entries that the table should contain. This number may be adjusted upward by the algorithm in order to obtain certain mathematically favorable circumstances.

The hdestroy() function disposes of the search table, and may be followed by another call to hcreate(). After the call to hdestroy(), the data can no longer be considered accessible.

The hsearch() function is a hash-table search routine. It returns a pointer into a hash table indicating the location at which an entry can be found. 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 (a char *), and item.data (a void *) points to any other data to be associated with that key. The comparison function used by hsearch() is strcmp(3). The action argument 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.

RETURN VALUES
The hcreate() function returns 0 if it cannot allocate sufficient space for the table; otherwise, it returns non-zero.

The hdestroy() function does not return a value.

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.

ERRORS
The hcreate() and hsearch() functions may fail if:

[ENOMEM]
    Insufficient storage space is available.

EXAMPLES
The following example reads in strings followed by two numbers and stores them in a hash table, discarding duplicates. It then reads in strings and finds the matching entry in the hash table and prints it out.

    #include <stdio.h>
    #include <search.h>
    #include <string.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. */
    int
    main(void)
    {
    	char string_space[NUM_EMPL*20]; /* Space to store strings. */
    	struct info info_space[NUM_EMPL]; /* Space to store employee info. */
    	char *str_ptr = string_space; /* Next space in string_space. */
    	struct info *info_ptr = info_space; /* Next space in info_space. */
    	ENTRY item;
    	ENTRY *found_item; /* Name to look for in table. */
    	char name_to_find[30];
    	int i = 0;
    	/* Create table; no error checking is performed. */
    	(void) hcreate(NUM_EMPL);
    	while (scanf("%s%d%d", str_ptr, &info_ptr->age,
    	    &info_ptr->room) != EOF && i++ < NUM_EMPL) {
    		/* Put information in structure, and structure in item. */
    		item.key = str_ptr;
    		item.data = 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;
    }

SEE ALSO
bsearch(3), lsearch(3), malloc(3), strcmp(3), tsearch(3)

STANDARDS
The hcreate(), hdestroy(), and hsearch() functions conform to xpg4.2.

HISTORY
The hcreate(), hdestroy(), and hsearch() functions first appeared in AT&T UNIX.

BUGS
The interface permits the use of only one hash table at a time.