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


       #include <search.h>

       int hcreate(size_t nel);
       void hdestroy(void);

       ENTRY *hsearch(ENTRY item, ACTION action);

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <search.h>

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

       int hsearch_r(ENTRY item, ACTION action, ENTRY **retval,
                     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

       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.


       hcreate()  and  hcreate_r() return nonzero on success.  They return 0 on error, with errno
       set to indicate 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 error.


       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.


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

       │InterfaceAttributeValue                  │
       │hcreate(), hsearch(), hdestroy()                │ Thread safety │ MT-Unsafe race:hsearch │
       │hcreate_r(), hsearch_r(), hdestroy_r()          │ Thread safety │ MT-Safe race:htab      │


       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.


       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>

       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"

           ENTRY e;
           ENTRY *ep;


           for (int 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 (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);


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


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