hsearch(S)

hsearch, hcreate, hdestroy -- manage hash search tables

Syntax

cc . . . -lc

#include <search.h>


ENTRY *hsearch (ENTRY item, ACTION action);

int hcreate (size_t nel);

void hdestroy(void);

Description

hsearch- searches a hash table

hcreate- allocates sufficient space for the table

hdestroy- destroys the search table

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. 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 contains. 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.

Diagnostics

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.

hcreate returns zero if it cannot allocate sufficient space for the table.

Note

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

Warning

hsearch and hcreate use malloc(S) to allocate space.

Standards conformance

hcreate, hdestroy and hsearch are conformant with:

X/Open Portability Guide, Issue 3, 1989 .

Example

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>
   
   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);
               }
           }
   }