trusty (3) hcreate.3posix.gz

NAME

       hcreate, hdestroy, hsearch - manage hash search table

SYNOPSIS

       #include <search.h>

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

DESCRIPTION

       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 accessible.

       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.

RETURN VALUE

       The hcreate() function shall return 0 if it cannot allocate sufficient space for the table; 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.

ERRORS

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

       ENOMEM Insufficient storage space is available.

       The following sections are informative.

EXAMPLES

       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;
                      info_ptr++;

                      /* 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",
                              found_item->key,
                              ((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;
              }

APPLICATION USAGE

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

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

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

COPYRIGHT

       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
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .