hdestroy

       ble management

SYNOPSIS
       #include <search.h>

       int hcreate(size_t nel);

       ENTRY *hsearch(ENTRY item, ACTION action);

       void hdestroy(void);

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #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 con-
       sisting of a key (a string) and associated  data.   Using  these  func-
       tions, only one hash table can be used at a time.

       The  three  functions  hcreate_r(), hsearch_r(), hdestroy_r() are reen-
       trant 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 pro-
       grammer  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 calling hdestroy() a new hash ta-
       ble can be created using hcreate().  The hdestroy_r() function performs
       the  analogous task for a hash table described by *htab, which was pre-
       viously 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.

       ful  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 ta-
       ble   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_r() and hdestroy_r() can fail for the following reasons:

       EINVAL htab is NULL.

       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 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 allo-
       cated dynamically.)  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  pro-
       gram),  then the program must maintain bookkeeping data structures that
       allow it to free them.

BUGS
       #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)

COLOPHON
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