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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         /* 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 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
       calling hdestroy(), 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 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 nonzero on success.  They return 0 on error, with errno set to  indicate
       the cause of the 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.  In the event of an error, these two
       functions set errno to indicate the cause of the 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 specifies only the ENOMEM error.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────┬───────────────┬────────────────────────┐
       │InterfaceAttributeValue                  │
       ├──────────────────────────┼───────────────┼────────────────────────┤
       │hcreate(), hsearch(),     │ Thread safety │ MT-Unsafe race:hsearch │
       │hdestroy()                │               │                        │
       ├──────────────────────────┼───────────────┼────────────────────────┤
       │hcreate_r(), hsearch_r(), │ Thread safety │ MT-Safe race:htab      │
       │hdestroy_r()              │               │                        │
       └──────────────────────────┴───────────────┴────────────────────────┘

CONFORMING TO

       The  functions  hcreate(), hsearch(), and hdestroy() are from SVr4, and are described in POSIX.1-2001 and
       POSIX.1-2008.

       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
       allocated 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 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.

EXAMPLES

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

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

       static 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;
           ENTRY *ep;

           hcreate(30);

           for (int 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 (int 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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