hsearch(3C)               AT&T SYSTEM V               hsearch(3C)



NAME
     hsearch, hcreate, hdestroy - manage hash search tables

SYNOPSIS
     #include <search.h>

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

     int hcreate (nel)
     unsigned nel;

     void hdestroy ( )

DESCRIPTION
     hsearch 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 reso-
     lution 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 will contain.  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.

NOTES
     hsearch uses open addressing with a multiplicative hash
     function.  However, its source code has many other options
     available which the user may select by compiling the hsearch
     source with the following symbols defined to the preproces-
     sor:

          DIV      Use the remainder modulo table size as the
                   hash function instead of the multiplicative
                   algorithm.

          USCR     Use a User Supplied Comparison Routine for



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hsearch(3C)               AT&T SYSTEM V               hsearch(3C)



                   ascertaining table membership.  The routine
                   should be named hcompar and should behave in a
                   mannner similar to strcmp (see string(3C)).

          CHAINED  Use a linked list to resolve collisions.  If
                   this option is selected, the following other
                   options become available.

                   START     Place new entries at the beginning
                             of the linked list (default is at
                             the end).

                   SORTUP    Keep the linked list sorted by key
                             in ascending order.

                   SORTDOWN  Keep the linked list sorted by key
                             in descending order.

     Additionally, there are preprocessor flags for obtaining
     debugging printout (-DDEBUG) and for including a test driver
     in the calling routine (-DDRIVER).  The source code should
     be consulted for further details.

EXAMPLE
     The following example will read in strings followed by two
     numbers and store them in a hash table, discarding dupli-
     cates.  It will then read in strings and find the matching
     entry in the hash table and print 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;




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hsearch(3C)               AT&T SYSTEM V               hsearch(3C)



     /* 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)
         }
     }
}

SEE ALSO
     bsearch(3C), lsearch(3C), malloc(3C), malloc(3X),
     string(3C), tsearch(3C).

DIAGNOSTICS
     hsearch 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.

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

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










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