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


       #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);


       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

           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.


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

       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.


       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.


       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.


       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.


       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.


       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"

           ENTRY e, *ep;
           int i;


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

           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);


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


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