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HSEARCH(3)		   Linux Programmer's Manual		    HSEARCH(3)

       hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash ta‐
       ble management

       #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);

       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.

       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  unsuccess‐
       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.

       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.

       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 specifies only the ENOMEM error.

   Multithreading (see pthreads(7))
       The hcreate(), hsearch(), and hdestroy() functions use a	 global	 space
       for storing the table, so they are not thread-safe.

       The  hcreate_r(),  hsearch_r(),	and hdestroy_r() functions are thread-

       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 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.

       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	imple‐
       mentation  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 */
	       e.data = (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)

       This  page  is  part of release 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

GNU				  2013-07-22			    HSEARCH(3)

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