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Linux 2.6 - man page for hsearch (linux section 3)

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

       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	   /* 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 consisting of a key (a string)  and  associ-
       ated 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  argu-
       ment,  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 maxi-
       mum 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 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  ta-
	      ble 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 ta-
       ble, so they are not thread-safe.

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

       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 exten-

       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 */
	       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 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

GNU					    2013-07-22				       HSEARCH(3)

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