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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 containing 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 table.  (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 call-
       ing hdestroy() a new hash table can be created using hcreate().  The hdestroy_r() function performs the  analo-
       gous 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 associated 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 non-zero 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 ta-
       ble.  hsearch_r() returns non-zero 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 func-
       tions hcreate_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 table entries.  (It can't do this because it doesn't know whether these buffers were allocated dynami-
       cally.)  If these buffers need to be freed (perhaps because the program is repeatedly creating  and  destroying
       hash  tables, rather than creating a single table whose lifetime matches that of the program), then the program
       must maintain bookkeeping data structures 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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2008-10-06                        HSEARCH(3)