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HCREATE(3)               BSD Library Functions Manual               HCREATE(3)

     hcreate, hcreate_r, hdestroy, hdestroy_r, hsearch, hsearch_r -- manage
     hash search table

     Standard C Library (libc, -lc)

     #include <search.h>

     hcreate(size_t nel);

     hcreate_r(size_t nel, struct hsearch_data *table);


     hdestroy_r(struct hsearch_data *table);

     ENTRY *
     hsearch(ENTRY item, ACTION action);

     hsearch_r(ENTRY item, ACTION action, ENTRY ** itemp,
         struct hsearch_data *table);

     The hcreate(), hcreate_r(), hdestroy(), hdestroy_r() hsearch(), and
     hsearch_r() functions manage hash search tables.

     The hcreate() function allocates and initializes the table.  The nel
     argument specifies an estimate of the maximum number of entries to be
     held by the table.  Unless further memory allocation fails, supplying an
     insufficient nel value will not result in functional harm, although a
     performance degradation may occur.  Initialization using the hcreate()
     function is mandatory prior to any access operations using hsearch().

     The hdestroy() function destroys a table previously created using
     hcreate().  After a call to hdestroy(), the data can no longer be

     The hsearch() function is used to search to the hash table.  It returns a
     pointer into the hash table indicating the address of an item.  The item
     argument is of type ENTRY, defined in the <search.h> header.  This is a
     structure type that contains two pointers:

           char *key    comparison key
           void *data   pointer to data associated with key

     The key comparison function used by hsearch() is strcmp(3).

     The action argument is of type ACTION, an enumeration type which defines
     the following values:

           ENTER    Insert item into the hash table.  If an existing item with
                    the same key is found, it is not replaced.  Note that the
                    key and data elements of item are used directly by the new
                    table entry.  The storage for the key must not be modified
                    during the lifetime of the hash table.

           FIND     Search the hash table without inserting item.

     Note that the comparison key must be allocated using malloc(3) or
     calloc(3) if action is ENTER and hdestroy() will be called.  This is
     because hdestroy() will call free(3) for each comparison key (but not
     data).  Typically the comparison key is allocated by using strdup(3).

     The hcreate_r(), hdestroy_r(), and hsearch_r() functions are re-entrant
     versions of the above functions that can operate on a table supplied by
     the user.  The hsearch_r() function returns 0 if the action is ENTER and
     the element cannot be created, 1 otherwise.  If the element exists or can
     be created, it will be placed in itemp, otherwise itemp will be set to

     If successful, the hcreate() and hcreate_r() functions return a non-zero
     value.  Otherwise, a value of 0 is returned and errno is set to indicate
     the error.

     The hdestroy() and hdestroy_r() functions return no value.

     If successful, the hsearch() function returns a pointer to hash table
     entry matching the provided key.  If the action is FIND and the item was
     not found, or if the action is ENTER and the insertion failed, NULL is
     returned and errno is set to indicate the error.  If the action is ENTER
     and an entry already existed in the table matching the given key, the
     existing entry is returned and is not replaced.

     The hsearch_r() function returns 1 unless the table is full, when it
     returns 0.  If hsearch() returns 0 or the element is not found, errno is
     set to indicate the error.

     The hcreate(), hcreate_r(), hsearch() and hsearch_r() functions will fail

     [ENOMEM]           Insufficient memory is available.

     The hsearch() and hsearch_r() functions will also fail if the action is
     SEARCH and the element is not found:

     [ESRCH]            The item given is not found.

     bsearch(3), lsearch(3), malloc(3), strcmp(3)

     The hcreate(), hdestroy() and hsearch() functions conform to X/Open
     Portability Guide Issue 4, Version 2 ("XPG4.2").

     The hcreate(), hdestroy() and hsearch() functions first appeared in AT&T
     System V UNIX.  The hcreate_r(), hdestroy_r() and hsearch_r() functions
     are GNU extensions.

     At least the following limitations can be mentioned:

     +o   The original, non-GNU interface permits the use of only one hash
         table at a time.

     +o   Individual hash table entries can be added, but not deleted.

     +o   The standard is indecipherable about the internal memory usage of the
         functions, mentioning only that "hcreate() and hsearch() functions
         may use malloc() to allocate space".  This limits the portability of
         the functions, given that other implementations may not free(3) the
         buffer pointed by key.

BSD                           September 14, 2011                           BSD