hcreate(3p) — Linux manual page


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

PROLOG         top

       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.

NAME         top

       hcreate, hdestroy, hsearch — manage hash search table

SYNOPSIS         top

       #include <search.h>

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

DESCRIPTION         top

       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         top

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

       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         top

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

       ENOMEM Insufficient storage space is available.

       The following sections are informative.

EXAMPLES         top

       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;

                   /* 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",
                           ((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;


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

RATIONALE         top




SEE ALSO         top

       bsearch(3p), lsearch(3p), malloc(3p), strcmp(3p), tdelete(3p)

       The Base Definitions volume of POSIX.1‐2017, search.h(0p)

COPYRIGHT         top

       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(3P)

Pages that refer to this page: search.h(0p)bsearch(3p)lsearch(3p)tdelete(3p)