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.\" Copyright 1993 Ulrich Drepper (drepper@karlsruhe.gmd.de)
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
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.\" References consulted:
.\"     SunOS 4.1.1 man pages
.\" Modified Sat Sep 30 21:52:01 1995 by Jim Van Zandt <jrv@vanzandt.mv.com>
.\" Remarks from dhw@gamgee.acad.emich.edu Fri Jun 19 06:46:31 1998
.\" Modified 2001-12-26, 2003-11-28, 2004-05-20, aeb
.\" 2008-09-02, mtk: various additions and rewrites
.\" 2008-09-03, mtk, restructured somewhat, in part after suggestions from
.\"     Timothy S. Nelson <wayland@wayland.id.au>
.\"
.TH HSEARCH 3 2019-03-06 "GNU" "Linux Programmer's Manual"
.SH NAME
hcreate, hdestroy, hsearch, hcreate_r, hdestroy_r,
hsearch_r \- hash table management
.SH SYNOPSIS
.nf
.B #include <search.h>
.PP
.BI "int hcreate(size_t " nel );
.PP
.BI "ENTRY *hsearch(ENTRY " item ", ACTION " action );
.PP
.B "void hdestroy(void);"
.PP
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <search.h>
.PP
.BI "int hcreate_r(size_t " nel ", struct hsearch_data *" htab );
.PP
.BI "int hsearch_r(ENTRY " item ", ACTION " action ", ENTRY **" retval ,
.BI "              struct hsearch_data *" htab );
.PP
.BI "void hdestroy_r(struct hsearch_data *" htab );
.fi
.SH DESCRIPTION
The three functions
.BR hcreate (),
.BR hsearch (),
and
.BR 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.
.PP
The three functions
.BR hcreate_r (),
.BR hsearch_r (),
.BR hdestroy_r ()
are reentrant versions that allow a program to use
more than one hash search table at the same time.
The last argument,
.IR 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).
.PP
First a hash table must be created using
.BR hcreate ().
The argument \fInel\fP 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.
.\" e.g., in glibc it is raised to the next higher prime number
.PP
The
.BR hcreate_r ()
function performs the same task as
.BR hcreate (),
but for the table described by the structure
.IR *htab .
The structure pointed to by
.I htab
must be zeroed before the first call to
.BR hcreate_r ().
.PP
The function
.BR hdestroy ()
frees the memory occupied by the hash table that was created by
.BR hcreate ().
After calling
.BR hdestroy (),
a new hash table can be created using
.BR hcreate ().
The
.BR hdestroy_r ()
function performs the analogous task for a hash table described by
.IR *htab ,
which was previously created using
.BR hcreate_r ().
.PP
The
.BR hsearch ()
function searches the hash table for an
item with the same key as \fIitem\fP (where "the same" is determined using
.BR strcmp (3)),
and if successful returns a pointer to it.
.PP
The argument \fIitem\fP is of type \fIENTRY\fP, which is defined in
\fI<search.h>\fP as follows:
.PP
.in +4n
.EX
typedef struct entry {
    char *key;
    void *data;
} ENTRY;
.EE
.in
.PP
The field \fIkey\fP points to a null-terminated string which is the
search key.
The field \fIdata\fP points to data that is associated with that key.
.PP
The argument \fIaction\fP determines what
.BR hsearch ()
does after an unsuccessful search.
This argument must either have the value
.BR ENTER ,
meaning insert a copy of
.IR item
(and return a pointer to the new hash table entry as the function result),
or the value
.BR FIND ,
meaning that NULL should be returned.
(If
.I action
is
.BR FIND ,
then
.I data
is ignored.)
.PP
The
.BR hsearch_r ()
function is like
.BR hsearch ()
but operates on the hash table described by
.IR *htab .
The
.BR hsearch_r ()
function differs from
.BR hsearch ()
in that a pointer to the found item is returned in
.IR *retval ,
rather than as the function result.
.SH RETURN VALUE
.BR hcreate ()
and
.BR hcreate_r ()
return nonzero on success.
They return 0 on error, with
.I errno
set to indicate the cause of the error.
.PP
On success,
.BR hsearch ()
returns a pointer to an entry in the hash table.
.BR hsearch ()
returns NULL on error, that is,
if \fIaction\fP is \fBENTER\fP and
the hash table is full, or \fIaction\fP is \fBFIND\fP and \fIitem\fP
cannot be found in the hash table.
.BR hsearch_r ()
returns nonzero on success, and 0 on error.
In the event of an error, these two functions set
.I errno
to indicate the cause of the error.
.SH ERRORS
.PP
.BR hcreate_r ()
and
.BR hdestroy_r ()
can fail for the following reasons:
.TP
.B EINVAL
.I htab
is NULL.
.PP
.BR hsearch ()
and
.BR hsearch_r ()
can fail for the following reasons:
.TP
.B ENOMEM
.I action
was
.BR ENTER ,
.I key
was not found in the table,
and there was no room in the table to add a new entry.
.TP
.B ESRCH
.I action
was
.BR FIND ,
and
.I key
was not found in the table.
.PP
POSIX.1 specifies only the
.\" PROX.1-2001, POSIX.1-2008
.B ENOMEM
error.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw25 lb lb
l l l.
Interface	Attribute	Value
T{
.BR hcreate (),
.BR hsearch (),
.br
.BR hdestroy ()
T}	Thread safety	MT-Unsafe race:hsearch
T{
.BR hcreate_r (),
.BR hsearch_r (),
.br
.BR hdestroy_r ()
T}	Thread safety	MT-Safe race:htab
.TE
.SH CONFORMING TO
The functions
.BR hcreate (),
.BR hsearch (),
and
.BR hdestroy ()
are from SVr4, and are described in POSIX.1-2001 and POSIX.1-2008.
.PP
The functions
.BR hcreate_r (),
.BR hsearch_r (),
and
.BR hdestroy_r ()
are GNU extensions.
.SH NOTES
Hash table implementations are usually more efficient when the
table contains enough free space to minimize collisions.
Typically, this means that
.I nel
should be at least 25% larger than the maximum number of elements
that the caller expects to store in the table.
.PP
The
.BR hdestroy ()
and
.BR hdestroy_r ()
functions do not free the buffers pointed to by the
.I key
and
.I 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.
.SH BUGS
SVr4 and POSIX.1-2001 specify that \fIaction\fP
is significant only for unsuccessful searches, so that an \fBENTER\fP
should not do anything for a successful search.
In libc and glibc (before version 2.3), the
implementation violates the specification,
updating the \fIdata\fP for the given \fIkey\fP in this case.
.PP
Individual hash table entries can be added, but not deleted.
.SH EXAMPLE
.PP
The following program inserts 24 items into a hash table, then prints
some of them.
.PP
.EX
#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;

    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 = (void *) i;
        ep = hsearch(e, ENTER);
        /* there should be no failures */
        if (ep == NULL) {
            fprintf(stderr, "entry failed\en");
            exit(EXIT_FAILURE);
        }
    }

    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\en", e.key,
               ep ? ep\->key : "NULL", ep ? (int)(ep\->data) : 0);
    }
    hdestroy();
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR bsearch (3),
.BR lsearch (3),
.BR malloc (3),
.BR tsearch (3)