hsearch(S)                     6 January 1993                     hsearch(S)


 Name

    hsearch, hcreate, hdestroy - manage hash search tables

 Syntax


    cc  . . .  -lc


    #include <search.h>

    ENTRY *hsearch (item, action)
    ENTRY item;
    ACTION action;

    int hcreate (nel)
    int nel;

    void hdestroy ()


 Description

    The hsearch function is a hash-table search routine generalized from
    Knuth (6.4) Algorithm D.  It returns a pointer into a hash table indicat-
    ing 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 cir-
    cumstances.

    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.

 See also

    bsearch(S), lsearch(S), malloc(S), string(S), tsearch(S)

 Standards conformance

    hcreate, hdestroy and hsearch are conformant with:
    AT&T SVID Issue 2;
    and 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);
                   }
               }
       }