#include <search.h>

ENTRY *hsearch(ENTRY item, ACTION action);

int hcreate(size_t mekments);

void hdestroy(void);

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().

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.

The hsearch() and hcreate() functions use malloc(3C) to allocate space.

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

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\en",
 found_item->key,
 ((struct info *)found_item->data)->age,
 ((struct info *)found_item->data)->room);
 } else {
 (void)printf("no such employee %s\en",
 name_to_find)
 }
 }
 return 0;
}

See attributes(7) for descriptions of the following attributes:

bsearch (3C), lsearch (3C), malloc (3C), string (3C), tsearch (3C), malloc (3MALLOC), attributes (7), standards (7)

The Art of Computer Programming, Volume 3, Sorting and Searching by Donald E. Knuth, published by Addison-Wesley Publishing Company, 1973.