HSEARCH(3C)                          SysV                          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 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 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.

EXAMPLES
     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 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)
              }
          }
     }

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.

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 preprocessor:

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

     USCR      Use a User Supplied Comparison Routine for 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.

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

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