HSEARCH(3)                                          Linux Programmer's Manual                                         HSEARCH(3)



NAME
       hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash table management

SYNOPSIS
       #include <search.h>

       int hcreate(size_t nel);

       ENTRY *hsearch(ENTRY item, ACTION action);

       void hdestroy(void);

       #define _GNU_SOURCE
       #include <search.h>

       int hcreate_r(size_t nel, struct hsearch_data *htab);

       int hsearch_r(ENTRY item, ACTION action, ENTRY **retval,
                     struct hsearch_data *htab);

       void hdestroy_r(struct hsearch_data *htab);

DESCRIPTION
       The  three  functions hcreate(), hsearch(), and hdestroy() allow the caller to create and manage a hash search table con-
       taining entries consisting of a key (a string) and associated data.  Using these functions, only one hash  table  can  be
       used at a time.

       The  three  functions hcreate_r(), hsearch_r(), hdestroy_r() are reentrant versions that allow a program to use more than
       one hash search table at the same time.  The last argument, htab, points to a structure that describes the table on which
       the  function  is  to  operate.   The  programmer should treat this structure as opaque (i.e., do not attempt to directly
       access or modify the fields in this structure).

       First a hash table must be created using hcreate().  The argument nel specifies the maximum number of entries in the  ta-
       ble.   (This  maximum  cannot be changed later, so choose it wisely.)  The implementation may adjust this value upward to
       improve the performance of the resulting hash table.

       The hcreate_r() function performs the same task as hcreate(), but for the table described by the  structure  *htab.   The
       structure pointed to by htab must be zeroed before the first call to hcreate_r().

       The  function  hdestroy()  frees the memory occupied by the hash table that was created by hcreate().  After calling hde-
       stroy() a new hash table can be created using hcreate().  The hdestroy_r() function performs the  analogous  task  for  a
       hash table described by *htab, which was previously created using hcreate_r().

       The  hsearch()  function  searches  the  hash table for an item with the same key as item (where "the same" is determined
       using strcmp(3)), and if successful returns a pointer to it.

       The argument item is of type ENTRY, which is defined in <search.h> as follows:

           typedef struct entry {
               char *key;
               void *data;
           } ENTRY;

       The field key points to a null-terminated string which is the search key.  The field data points to data that is  associ-
       ated with that key.

       The  argument  action  determines  what  hsearch() does after an unsuccessful search.  This argument must either have the
       value ENTER, meaning insert a copy of item (and return a pointer to the new hash table entry as the function result),  or
       the value FIND, meaning that NULL should be returned.  (If action is FIND, then data is ignored.)

       The  hsearch_r()  function is like hsearch() but operates on the hash table described by *htab.  The hsearch_r() function
       differs from hsearch() in that a pointer to the found item is returned in *retval, rather than as the function result.

RETURN VALUE
       hcreate() and hcreate_r() return nonzero on success.  They return 0 on error.

       On success, hsearch() returns a pointer to an entry in the hash table.  hsearch() returns NULL  on  error,  that  is,  if
       action  is  ENTER  and the hash table is full, or action is FIND and item cannot be found in the hash table.  hsearch_r()
       returns nonzero on success, and 0 on error.

ERRORS
       hcreate() and hcreate_r() can fail for the following reasons:

       EINVAL (hcreate_r()) htab is NULL.

       ENOMEM Table full with action set to ENTER.

       ESRCH  The action argument is FIND and no corresponding element is found in the table.

       hsearch() and hsearch_r() can fail for the following reasons:

       ENOMEM action was ENTER, key was not found in the table, and there was no room in the table to add a new entry.

       ESRCH  action was FIND, and key was not found in the table.

       POSIX.1-2001 only specifies the ENOMEM error.

CONFORMING TO
       The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and are described in POSIX.1-2001.  The functions hcre-
       ate_r(), hsearch_r(), and hdestroy_r() are GNU extensions.

NOTES
       Hash  table  implementations are usually more efficient when the table contains enough free space to minimize collisions.
       Typically, this means that nel should be at least 25% larger than the maximum number of elements that the caller  expects
       to store in the table.

       The hdestroy() and hdestroy_r() functions do not free the buffers pointed to by the key and data elements of the hash ta-
       ble entries.  (It can't do this because it doesn't know whether these buffers were allocated dynamically.)  If these buf-
       fers  need to be freed (perhaps because the program is repeatedly creating and destroying hash tables, rather than creat-
       ing a single table whose lifetime matches that of the program), then the program must maintain  bookkeeping  data  struc-
       tures that allow it to free them.

BUGS
       SVr4  and  POSIX.1-2001 specify that action is significant only for unsuccessful searches, so that an ENTER should not do
       anything for a successful search.  In libc and glibc (before version 2.3), the implementation violates the specification,
       updating the data for the given key in this case.

       Individual hash table entries can be added, but not deleted.

EXAMPLE
       The following program inserts 24 items into a hash table, then prints some of them.

       #include <stdio.h>
       #include <stdlib.h>
       #include <search.h>

       char *data[] = { "alpha", "bravo", "charlie", "delta",
            "echo", "foxtrot", "golf", "hotel", "india", "juliet",
            "kilo", "lima", "mike", "november", "oscar", "papa",
            "quebec", "romeo", "sierra", "tango", "uniform",
            "victor", "whisky", "x-ray", "yankee", "zulu"
       };

       int
       main(void)
       {
           ENTRY e, *ep;
           int i;

           hcreate(30);

           for (i = 0; i < 24; i++) {
               e.key = data[i];
               /* data is just an integer, instead of a
                  pointer to something */
               e.data = (void *) i;
               ep = hsearch(e, ENTER);
               /* there should be no failures */
               if (ep == NULL) {
                   fprintf(stderr, "entry failed\n");
                   exit(EXIT_FAILURE);
               }
           }

           for (i = 22; i < 26; i++) {
               /* print two entries from the table, and
                  show that two are not in the table */
               e.key = data[i];
               ep = hsearch(e, FIND);
               printf("%9.9s -> %9.9s:%d\n", e.key,
                      ep ? ep->key : "NULL", ep ? (int)(ep->data) : 0);
           }
           hdestroy();
           exit(EXIT_SUCCESS);
       }

SEE ALSO
       bsearch(3), lsearch(3), malloc(3), tsearch(3), feature_test_macros(7)

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                                                        2008-10-06                                                 HSEARCH(3)