HCREATE(3) Library Functions Manual HCREATE(3)

hcreate, hdestroy, hsearch — manage hash search table

Standard C Library (libc, -lc)

#include <search.h>

int
hcreate(size_t nel);

void
hdestroy(void);

ENTRY *
hsearch(ENTRY item, ACTION action);

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

The () 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 () 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 () 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.

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

The hcreate() function returns 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() 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.

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;
}

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

[]
Insufficient storage space is available.
[]
A table already exists.

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

The hcreate(), hdestroy(), and hsearch() functions conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).

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

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

July 6, 2008 macOS 15.0