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

NAME
       hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash table management

SYNOPSIS
       #include <search.h>

       int hcreate(size_t nel);
       void hdestroy(void);

       ENTRY *hsearch(ENTRY item, ACTION action);

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <search.h>

       int hcreate_r(size_t nel, struct hsearch_data *htab);
       void hdestroy_r(struct hsearch_data *htab);

       int hsearch_r(ENTRY item, ACTION action, ENTRY **retval,
                     struct hsearch_data *htab);

DESCRIPTION
       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 associated 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  argument,  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 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 ze‐
       roed 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 us‐
       ing 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 it.

       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.

RETURN VALUE
       hcreate() and hcreate_r() return nonzero on success.  They return 0 on error, with errno set to indicate 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 func‐
       tions set errno to indicate the error.

ERRORS
       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 specifies only the ENOMEM error.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                              β”‚ Attribute     β”‚ Value                  β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚hcreate(), hsearch(), hdestroy()                                                                                       β”‚ Thread safety β”‚ MT-Unsafe race:hsearch β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚hcreate_r(), hsearch_r(), hdestroy_r()                                                                                 β”‚ Thread safety β”‚ MT-Safe race:htab      β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

CONFORMING TO
       The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and are described in POSIX.1-2001 and POSIX.1-2008.

       The functions hcreate_r(), hsearch_r(), and hdestroy_r() are GNU extensions.

NOTES
       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 de‐
       stroying 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.

BUGS
       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.

EXAMPLES
       The following program inserts 24 items into a hash table, then prints some of them.

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

       static 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(void)
       {
           ENTRY e;
           ENTRY *ep;

           hcreate(30);

           for (int 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");
                   exit(EXIT_FAILURE);
               }
           }

           for (int 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);
           }
           hdestroy();
           exit(EXIT_SUCCESS);
       }

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

GNU                                                                            2021-03-22                                                                     HSEARCH(3)