This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

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

       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(3POSIX)

       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.

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

       ENOMEM Insufficient storage space is available.

       Thefollowingsectionsareinformative.

       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;
           }

       None.

       hcreate, hdestroy, hsearch — manage hash search table

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

       None.

       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.

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

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

       #include <search.h>

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