HSEARCH

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



NAME
       hcreate, hdestroy, hsearch - hash table management

SYNOPSIS
       #include <search.h>

       int hcreate(size_t nel);

       ENTRY *hsearch(ENTRY item, ACTION action);

       void hdestroy(void);


       #define _GNU_SOURCE
       #include <search.h>

       int hcreate_r(size_t nel, struct hsearch_data *tab);

       int   *hsearch_r(ENTRY   item,   ACTION  action,  ENTRY  **ret,  struct
       hsearch_data *tab);

       void hdestroy_r(struct hsearch_data *tab);

DESCRIPTION
       The three functions hcreate, hsearch, and hdestroy allow  the  user  to
       create  a  hash  table (only one at a time) which associates a key with
       any data.

       First the table must be created with the function hcreate().  The argu-
       ment  nel is an estimate of the maximum number of entries in the table.
       The function hcreate() may adjust this value upward to improve the per-
       formance of the resulting hash table.

       The  corresponding function hdestroy() frees the memory occupied by the
       hash table so that a new table can be constructed.

       The argument item is of type ENTRY,  which  is  a  typedef  defined  in
       <search.h> and includes these elements:

            typedef struct entry {
                char *key;
                void *data;
            } ENTRY;

       The  field  key points to the NUL-terminated string which is the search
       key.  The field data points to the data associated with that key.   The
       function  hsearch()  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 action determines
       what hsearch() does after an unsuccessful search.   A  value  of  ENTER
       instructs  it  to insert a copy of item, while a value of FIND means to
       return NULL.

       The three functions hcreate_r, hsearch_r, hdestroy_r are reentrant ver-
       sions  that  allow  the  use of more than one table.  The last argument
       used identifies the table. The struct  it  points  to  must  be  zeroed
       before the first call to hcreate_r().

RETURN VALUE
       hcreate()  and  hcreate_r()  return 0 when allocation of the memory for
       the hash table fails, nonzero otherwise.

       hsearch() returns NULL 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  0  if action is ENTER and the hash table is full,
       and nonzero otherwise.

ERRORS
       POSIX documents

       ENOMEM Out of memory.

       The glibc implementation will return the following two errors.

       ENOMEM Table full with action set to ENTER.

       ESRCH  The action parameter is FIND and  no  corresponding  element  is
              found in the table.

CONFORMS TO
       The  functions  hcreate,  hsearch,  and hdestroy are from SVID, and are
       described in POSIX 1003.1-2001.  The  functions  hcreate_r,  hsearch_r,
       hdestroy_r are GNU extensions.

BUGS
       SVID  and POSIX 1003.1-2001 specify that action is significant only for
       unsuccessful searches, so that an ENTER should not do  anything  for  a
       successful  search.  The libc and glibc implementations update the data
       for the given key in this case.

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

EXAMPLE
       The following program inserts 24 items in to 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"
           };

           int main() {
             ENTRY e, *ep;
             int i;

             /* starting with small table, and letting it grow does not work */
             hcreate(30);
             for (i = 0; i < 24; i++) {
                 e.key = data[i];
                 /* data is just an integer, instead of a
                    pointer to something */
                 e.data = (char *)i;
                 ep = hsearch(e, ENTER);
                 /* there should be no failures */
                 if (ep == NULL) {
                   fprintf(stderr, "entry failed\n");
                   exit(1);
                 }
             }
             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);
             }
             return 0;
           }


SEE ALSO
       bsearch(3), lsearch(3), tsearch(3), malloc(3)



GNU                               2004-05-20                        HSEARCH(3)