minix/lib/libc/stdlib/hcreate.3

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.Dd September 14, 2011
.Dt HCREATE 3
.Os
.Sh NAME
.Nm hcreate ,
.Nm hcreate_r ,
.Nm hdestroy ,
.Nm hdestroy_r ,
.Nm hsearch ,
.Nm hsearch_r
.Nd manage hash search table
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In search.h
.Ft int
.Fn hcreate "size_t nel"
.Ft int
.Fn hcreate_r "size_t nel" "struct hsearch_data *table"
.Ft void
.Fn hdestroy "void"
.Ft void
.Fn hdestroy_r "struct hsearch_data *table"
.Ft ENTRY *
.Fn hsearch "ENTRY item" "ACTION action"
.Ft int
.Fn hsearch_r "ENTRY item" "ACTION action" "ENTRY ** itemp" "struct hsearch_data *table"
.Sh DESCRIPTION
The
.Fn hcreate ,
.Fn hcreate_r ,
.Fn hdestroy ,
.Fn hdestroy_r
.Fn hsearch ,
and
.Fn hsearch_r
functions manage hash search tables.
.Pp
The
.Fn hcreate
function allocates and initializes the table.
The
.Fa nel
argument specifies an estimate of the maximum number of entries to be held
by the table.
Unless further memory allocation fails, supplying an insufficient
.Fa nel
value will not result in functional harm, although a performance degradation
may occur.
Initialization using the
.Fn hcreate
function is mandatory prior to any access operations using
.Fn hsearch .
.Pp
The
.Fn hdestroy
function destroys a table previously created using
.Fn hcreate .
After a call to
.Fn hdestroy ,
the data can no longer be accessed.
.Pp
The
.Fn hsearch
function is used to search to the hash table.
It returns a pointer into the
hash table indicating the address of an item.
The
.Fa item
argument is of type
.Vt ENTRY ,
defined in the
.In search.h
header.
This is a structure type that contains two pointers:
.Pp
.Bl -tag -compact -offset indent -width "void *data "
.It Fa char *key
comparison key
.It Fa void *data
pointer to data associated with
.Fa key
.El
.Pp
The key comparison function used by
.Fn hsearch
is
.Xr strcmp 3 .
.Pp
The
.Fa action
argument is of type
.Vt ACTION ,
an enumeration type which defines the following values:
.Bl -tag -offset indent -width ENTERXX
.It Dv ENTER
Insert
.Fa item
into the hash table.
If an existing item with the same key is found, it is not replaced.
Note that the
.Fa key
and
.Fa data
elements of
.Fa item
are used directly by the new table entry.
The storage for the
key must not be modified during the lifetime of the hash table.
.It Dv FIND
Search the hash table without inserting
.Fa item .
.El
.Pp
Note that the comparison
.Fa key
must be allocated using
.Xr malloc 3
or
.Xr calloc 3
if action is
.Dv ENTER
and
.Fn hdestroy
will be called.
This is because
.Fn hdestroy
will call
.Xr free 3
for each comparison
.Fa key
(but not
.Fa data ) .
Typically the comparison
.Fa key
is allocated by using
.Xr strdup 3 .
.Pp
The
.Fn hcreate_r ,
.Fn hdestroy_r ,
and
.Fn hsearch_r
functions are re-entrant versions of the above functions that can
operate on a table supplied by the user.
The
.Fn hsearch_r
function returns
.Dv 0
if the action is
.Dv ENTER
and the element cannot be created,
.Dv 1
otherwise.
If the element exists or can be created, it will be placed in
.Fa itemp ,
otherwise
.Fa itemp
will be set to
.Dv NULL .
.Sh RETURN VALUES
If successful, the
.Fn hcreate
and
.Fn hcreate_r
functions return a non-zero value.
Otherwise, a value of
.Dv 0
is returned and
.Va errno
is set to indicate the error.
.Pp
The
.Fn hdestroy
and
.Fn hdestroy_r
functions return no value.
.Pp
If successful, the
.Fn hsearch
function returns a pointer to hash table entry matching
the provided key.
If the action is
.Dv FIND
and the item was not found, or if the action is
.Dv ENTER
and the insertion failed,
.Dv NULL
is returned and
.Va errno
is set to indicate the error.
If the action is
.Dv ENTER
and an entry already existed in the table matching the given
key, the existing entry is returned and is not replaced.
.Pp
The
.Fn hsearch_r
function returns
.Dv 1
unless the table is full, when it returns
.Dv 0 .
If
.Fn hsearch
returns
.Dv 0
or the element is not found,
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn hcreate ,
.Fn hcreate_r ,
.Fn hsearch
and
.Fn hsearch_r
functions will fail if:
.Bl -tag -width Er
.It Bq Er ENOMEM
Insufficient memory is available.
.El
.Pp
The
.Fn hsearch
and
.Fn hsearch_r
functions will also fail if the action is
.Dv SEARCH
and the element is not found:
.Bl -tag -width Er
.It Bq Er ESRCH
The
.Fa item
given is not found.
.El
.Sh SEE ALSO
.Xr bsearch 3 ,
.Xr lsearch 3 ,
.Xr malloc 3 ,
.Xr strcmp 3
.Sh STANDARDS
The
.Fn hcreate ,
.Fn hdestroy
and
.Fn hsearch
functions conform to
.St -xpg4.2 .
.Sh HISTORY
The
.Fn hcreate ,
.Fn hdestroy
and
.Fn hsearch
functions first appeared in
.At V .
The
.Fn hcreate_r ,
.Fn hdestroy_r
and
.Fn hsearch_r
functions are
.Tn GNU
extensions.
.Sh CAVEATS
At least the following limitations can be mentioned:
.Bl -bullet
.It
The original,
.Pf non- Tn GNU
interface permits the use of only one hash table at a time.
.It
Individual hash table entries can be added, but not deleted.
.It
The standard is indecipherable about the
internal memory usage of the functions,
mentioning only that
.Do
.Fn hcreate
and
.Fn hsearch
functions may use
.Fn malloc
to allocate space
.Dc .
This limits the portability of the functions,
given that other implementations may not
.Xr free 3
the buffer pointed by
.Fa key .
.El