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

HCREATE(P)			    POSIX Programmer's Manual			       HCREATE(P)

       hcreate, hdestroy, hsearch - manage hash search table

       #include <search.h>

       int hcreate(size_t nel);
       void hdestroy(void);
       ENTRY *hsearch(ENTRY item, ACTION action);

       The hcreate(), hdestroy(), and hsearch() functions shall manage hash search tables.

       The  hcreate() function shall allocate sufficient space for the table, and the application
       shall ensure it is called before hsearch() is used. The nel argument is an estimate of the
       maximum number of entries that the table shall contain. This number may be adjusted upward
       by the algorithm in order to obtain certain mathematically favorable circumstances.

       The hdestroy() function shall dispose of the search table, and may be followed by  another
       call  to  hcreate().  After  the  call to hdestroy(), the data can no longer be considered

       The hsearch() function is a hash-table search routine. It shall return a  pointer  into	a
       hash  table indicating the location at which an entry can be found. The item argument is a
       structure of type ENTRY (defined  in  the  <search.h>  header)  containing  two	pointers:
       item.key  points  to the comparison key (a char *), and item.data (a void *) points to any
       other data to be associated with that key. The comparison function used	by  hsearch()  is
       strcmp().  The  action  argument  is a member of an enumeration type ACTION indicating the
       disposition of the entry if it cannot be found in the table. ENTER indicates that the item
       should  be  inserted  in  the  table at an appropriate point. FIND indicates that no entry
       should be made.	Unsuccessful resolution is indicated by the return of a null pointer.

       These functions need not be reentrant. A function that is not required to be reentrant  is
       not required to be thread-safe.

       The  hcreate()  function shall return 0 if it cannot allocate sufficient space for the ta-
       ble; otherwise, it shall return non-zero.

       The hdestroy() function shall not return a value.

       The hsearch() function shall return a null pointer if either the action is  FIND  and  the
       item could not be found or the action is ENTER and the table is full.

       The hcreate() and hsearch() functions may fail if:

       ENOMEM Insufficient storage space is available.

       The following sections are informative.

       The  following  example reads in strings followed by two numbers and stores them in a hash
       table, discarding duplicates. It then reads in strings and finds the matching entry in the
       hash table and prints it out.

	      #include <stdio.h>
	      #include <search.h>
	      #include <string.h>

	      struct info {	   /* This is the info stored in the table */
		  int age, room;   /* other than the key. */

	      #define NUM_EMPL	  5000	  /* # of elements in search table. */

	      int main(void)
		  char string_space[NUM_EMPL*20];   /* Space to store strings. */
		  struct info info_space[NUM_EMPL]; /* Space to store employee info. */
		  char *str_ptr = string_space;     /* Next space in string_space. */
		  struct info *info_ptr = info_space;
						    /* Next space in info_space. */
		  ENTRY item;
		  ENTRY *found_item; /* Name to look for in table. */
		  char name_to_find[30];

		  int i = 0;

		  /* Create table; no error checking is performed. */
		  (void) hcreate(NUM_EMPL);
		  while (scanf("%s%d%d", str_ptr, &info_ptr->age,
			 &info_ptr->room) != EOF && i++ < NUM_EMPL) {

		      /* Put information in structure, and structure in item. */
		      item.key = str_ptr;
		      item.data = info_ptr;
		      str_ptr += strlen(str_ptr) + 1;

		      /* Put item into table. */
		      (void) hsearch(item, ENTER);

		  /* Access table. */
		  item.key = name_to_find;
		  while (scanf("%s", item.key) != EOF) {
		      if ((found_item = hsearch(item, FIND)) != NULL) {

			  /* If item is in the table. */
			  (void)printf("found %s, age = %d, room = %d\n",
			      ((struct info *)found_item->data)->age,
			      ((struct info *)found_item->data)->room);
		      } else
			  (void)printf("no such employee %s\n", name_to_find);
		  return 0;

       The hcreate() and hsearch() functions may use malloc() to allocate space.



       bsearch()  ,  lsearch() , malloc() , strcmp() , tsearch() , the Base Definitions volume of
       IEEE Std 1003.1-2001, <search.h>

       Portions of this text are reprinted and	reproduced  in	electronic  form  from	IEEE  Std
       1003.1,	2003  Edition,	Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  by
       the  Institute  of  Electrical  and  Electronics Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				       HCREATE(P)

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