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


     hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r, hsearch_r - hash ta-
     ble 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 con-
     sisting of a key (a string) and associated  data.   Using  these  func-
     tions, only one hash table can be used at a time.
     The  three  functions  hcreate_r(), hsearch_r(), hdestroy_r() are reen-
     trant 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 pro-
     grammer  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 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  per-
     forms 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
         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  unsuccess-
     ful  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 ta-
     ble   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 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.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     allbox;  lbw25  lb  lb  l l l.  Interface Attribute Value T{ hcreate(),
     hdestroy() T}   Thread safety  MT-Unsafe race:hsearch  T{  hcreate_r(),
     hdestroy_r() T}   Thread safety  MT-Safe race:htab


     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 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  allo-
     cated 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 pro-
     gram), 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 imple-
     mentation 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>
     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, *ep;
         int i;
         for (i = 0; i < 24; i++) {
             e.key = data[i];
             /* data is just an integer, instead of a
                pointer to something */
    = (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);
         exit(EXIT_SUCCESS); }


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


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GNU 2017-09-15 HSEARCH(3)

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