hsearch(3) — Linux manual page


hsearch(3)              Library Functions Manual              hsearch(3)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

       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 zeroed before the first call to

       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 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         top

       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
       functions set errno to indicate the error.

ERRORS         top

       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         top

       For an explanation of the terms used in this section, see
       │ Interface            Attribute     Value                  │
       │ hcreate(),           │ Thread safety │ MT-Unsafe race:hsearch │
       │ hsearch(),           │               │                        │
       │ hdestroy()           │               │                        │
       │ hcreate_r(),         │ Thread safety │ MT-Safe race:htab      │
       │ hsearch_r(),         │               │                        │
       │ hdestroy_r()         │               │                        │

STANDARDS         top



HISTORY         top

              SVr4, POSIX.1-2001.


NOTES         top

       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
       destroying 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         top

       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 glibc 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         top

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

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

           ENTRY e;
           ENTRY *ep;


           for (size_t 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");

           for (size_t 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);

SEE ALSO         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-06-15                     hsearch(3)

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