HCREATE(3)                 POSIX Programmer's Manual                HCREATE(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       hcreate, hdestroy, hsearch -- manage hash search table

SYNOPSIS
       #include <search.h>

       int hcreate(size_t nel);
       void hdestroy(void);
       ENTRY *hsearch(ENTRY item, ACTION action);

DESCRIPTION
       The hcreate(), hdestroy(), and hsearch() functions shall manage hash
       search tables.

       The hcreate() function shall allocate sufficient space for the table,
       and the application shall ensure it is called before hsearch() is used.
       The nel argument is an estimate of the maximum number of entries that
       the table shall contain. This number may be adjusted upward by the
       algorithm in order to obtain certain mathematically favorable
       circumstances.

       The hdestroy() function shall dispose of the search table, and may be
       followed by another call to hcreate().  After the call to hdestroy(),
       the data can no longer be considered accessible.

       The hsearch() function is a hash-table search routine. It shall return
       a pointer into a hash table indicating the location at which an entry
       can be found. 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 (a char **), and item.data (a void **) points to any
       other data to be associated with that key. The comparison function used
       by hsearch() is strcmp().  The action argument 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.

       These functions need not be thread-safe.

RETURN VALUE
       The hcreate() function shall return 0 if it cannot allocate sufficient
       space for the table; otherwise, it shall return non-zero.

       The hdestroy() function shall not return a value.

       The hsearch() function shall return 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.

ERRORS
       The hcreate() and hsearch() functions may fail if:

       ENOMEM Insufficient storage space is available.

       The following sections are informative.

EXAMPLES
       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>
           #include <string.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. */

           int main(void)
           {
               char string_space[NUM_EMPL*20];   /* Space to store strings. */
               struct info info_space[NUM_EMPL]; /* Space to store employee info. */
               char *str_ptr = string_space;     /* Next space in string_space. */
               struct info *info_ptr = info_space;
                                                 /* Next space in info_space. */
               ENTRY item;
               ENTRY *found_item; /* Name to look for in table. */
               char name_to_find[30];

               int i = 0;

               /* Create table; no error checking is performed. */
               (void) hcreate(NUM_EMPL);
               while (scanf("%s%d%d", str_ptr, &info_ptr->age,
                      &info_ptr->room) != EOF && i++ < NUM_EMPL) {

                   /* Put information in structure, and structure in item. */
                   item.key = str_ptr;
                   item.data = 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);
               }
               return 0;
           }

APPLICATION USAGE
       The hcreate() and hsearch() functions may use malloc() to allocate
       space.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       bsearch(), lsearch(), malloc(), strcmp(), tdelete()

       The Base Definitions volume of POSIX.1-2017, <search.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           HCREATE(3)
