HCREATE(3)

NAME
hcreate, hcreate_r, hdestroy, hdestroy_r, hsearch, hsearch_r - manage hash search table

LIBRARY
libc

SYNOPSIS
#include <search.h>
int
hcreate(size_t nel);
int
hcreate_r(size_t nel, struct hsearch_data *table);
void
hdestroy(void);
void
hdestroy_r(struct hsearch_data *table);
ENTRY *
hsearch(ENTRY item, ACTION action);
int
hsearch_r(ENTRY item, ACTION action, ENTRY ** itemp, struct hsearch_data *table);

DESCRIPTION
The hcreate(), hcreate_r(), hdestroy(), hdestroy_r() hsearch(), and hsearch_r() 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. As this implementation resizes the hash table dynamically, this argument is ignored.

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 hdestroy() function calls free(3) for each comparison key in the search table but not the data item associated with the key.

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) that contains 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.

The comparison key (passed to hsearch() as item.key) must be allocated using malloc(3) if action is ENTER and hdestroy() is called.

The hcreate_r(), hdestroy_r(), and hsearch_r() functions are re-entrant versions of the above functions that can operate on a table supplied by the user. The hsearch_r() function returns 0 if the action is ENTER and the element cannot be created, 1 otherwise. If the element exists or can be created, it will be placed in itemp, otherwise itemp will be set to NULL.

RETURN VALUES
The hcreate() and hcreate_r() functions return 0 if the table creation failed and the global variable errno is set to indicate the error; otherwise, a non-zero value is returned.

The hdestroy() and hdestroy_r() functions return no value.

The hsearch() and hsearch_r() functions return 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.

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>
    #include <stdlib.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 str[BUFSIZ]; /* Space to read string */
    	struct info info_space[NUM_EMPL]; /* Space to store employee info. */
    	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, &info_ptr->age,
    	    &info_ptr->room) != EOF && i++ < NUM_EMPL) {
    		/* Put information in structure, and structure in item. */
    		item.key = strdup(str);
    		item.data = info_ptr;
    		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);
    	}
    	hdestroy();
    	return 0;
    }

ERRORS
The hcreate() hcreate_r(), hsearch() and hsearch_r() functions will fail if:

[ENOMEM]
    Insufficient memory is available.

The hsearch() and hsearch_r() functions will also fail if the action is SEARCH and the element is not found:

[ESRCH]
    The item given is not found.

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. The hcreate_r(), hdestroy_r() and hsearch_r() functions are GNU extensions.

BUGS
The original, non-GNU interface permits the use of only one hash table at a time.