path: root/lib/libc/stdlib/hcreate.3

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.Dd $Mdocdate: January 30 2018 $
.Dt HCREATE 3
.Os
.Sh NAME
.Nm hcreate ,
.Nm hdestroy ,
.Nm hsearch
.Nd manage hash search table
.Sh SYNOPSIS
.In search.h
.Ft int
.Fn hcreate "size_t nel"
.Ft void
.Fn hdestroy "void"
.Ft ENTRY *
.Fn hsearch "ENTRY item" "ACTION action"
.Sh DESCRIPTION
The
.Fn hcreate ,
.Fn hdestroy ,
and
.Fn hsearch
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 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 .
.Sh RETURN VALUES
If successful, the
.Fn hcreate
function returns a non-zero value.
Otherwise, a value of 0 is returned and
.Va errno
is set to indicate the error.
.Pp
If successful, the
.Fn hsearch
function returns a pointer to a 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.
.Sh ERRORS
The
.Fn hcreate
and
.Fn hsearch
functions will fail if:
.Bl -tag -width Er
.It Bq Er ENOMEM
Insufficient memory is available.
.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 .
.Sh CAVEATS
At least the following limitations can be mentioned:
.Bl -bullet
.It
The 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