perltie.1

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

in some special way.  See nvi and the Apache module for examples.
.PP
In our example we're going to create a shouting handle.
.PP
.Vb 1
\&    package Shout;
.Ve
.IP "\s-1TIEHANDLE\s0 classname, \s-1LIST\s0" 4
.IX Xref "TIEHANDLE"
.IX Item "TIEHANDLE classname, LIST"
This is the constructor for the class.  That means it is expected to
return a blessed reference of some sort. The reference can be used to
hold some internal information.
.Sp
.Vb 1
\&    sub TIEHANDLE { print "<shout>\en"; my $i; bless \e$i, shift }
.Ve
.IP "\s-1WRITE\s0 this, \s-1LIST\s0" 4
.IX Xref "WRITE"
.IX Item "WRITE this, LIST"
This method will be called when the handle is written to via the
\&\f(CW\*(C`syswrite\*(C'\fR function.
.Sp
.Vb 5
\&    sub WRITE {
\&        $r = shift;
\&        my($buf,$len,$offset) = @_;
\&        print "WRITE called, \e$buf=$buf, \e$len=$len, \e$offset=$offset";
\&    }
.Ve
.IP "\s-1PRINT\s0 this, \s-1LIST\s0" 4
.IX Xref "PRINT"
.IX Item "PRINT this, LIST"
This method will be triggered every time the tied handle is printed to
with the \f(CW\*(C`print()\*(C'\fR function.
Beyond its self reference it also expects the list that was passed to
the print function.
.Sp
.Vb 1
\&    sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\e }
.Ve
.IP "\s-1PRINTF\s0 this, \s-1LIST\s0" 4
.IX Xref "PRINTF"
.IX Item "PRINTF this, LIST"
This method will be triggered every time the tied handle is printed to
with the \f(CW\*(C`printf()\*(C'\fR function.
Beyond its self reference it also expects the format and list that was
passed to the printf function.
.Sp
.Vb 5
\&    sub PRINTF {
\&        shift;
\&        my $fmt = shift;
\&        print sprintf($fmt, @_);
\&    }
.Ve
.IP "\s-1READ\s0 this, \s-1LIST\s0" 4
.IX Xref "READ"
.IX Item "READ this, LIST"
This method will be called when the handle is read from via the \f(CW\*(C`read\*(C'\fR
or \f(CW\*(C`sysread\*(C'\fR functions.
.Sp
.Vb 8
\&    sub READ {
\&        my $self = shift;
\&        my $bufref = \e$_[0];
\&        my(undef,$len,$offset) = @_;
\&        print "READ called, \e$buf=$bufref, \e$len=$len, \e$offset=$offset";
\&        # add to $$bufref, set $len to number of characters read
\&        $len;
\&    }
.Ve
.IP "\s-1READLINE\s0 this" 4
.IX Xref "READLINE"
.IX Item "READLINE this"
This method will be called when the handle is read from via <\s-1HANDLE\s0>.
The method should return undef when there is no more data.
.Sp
.Vb 1
\&    sub READLINE { $r = shift; "READLINE called $$r times\en"; }
.Ve
.IP "\s-1GETC\s0 this" 4
.IX Xref "GETC"
.IX Item "GETC this"
This method will be called when the \f(CW\*(C`getc\*(C'\fR function is called.
.Sp
.Vb 1
\&    sub GETC { print "Don\*(Aqt GETC, Get Perl"; return "a"; }
.Ve
.IP "\s-1CLOSE\s0 this" 4
.IX Xref "CLOSE"
.IX Item "CLOSE this"
This method will be called when the handle is closed via the \f(CW\*(C`close\*(C'\fR
function.
.Sp
.Vb 1
\&    sub CLOSE { print "CLOSE called.\en" }
.Ve
.IP "\s-1UNTIE\s0 this" 4
.IX Xref "UNTIE"
.IX Item "UNTIE this"
As with the other types of ties, this method will be called when \f(CW\*(C`untie\*(C'\fR happens.
It may be appropriate to \*(L"auto \s-1CLOSE\s0\*(R" when this occurs.  See
"The \f(CW\*(C`untie\*(C'\fR Gotcha" below.
.IP "\s-1DESTROY\s0 this" 4
.IX Xref "DESTROY"
.IX Item "DESTROY this"
As with the other types of ties, this method will be called when the
tied handle is about to be destroyed. This is useful for debugging and
possibly cleaning up.
.Sp
.Vb 1
\&    sub DESTROY { print "</shout>\en" }
.Ve
.PP
Here's how to use our little example:
.PP
.Vb 5
\&    tie(*FOO,\*(AqShout\*(Aq);
\&    print FOO "hello\en";
\&    $a = 4; $b = 6;
\&    print FOO $a, " plus ", $b, " equals ", $a + $b, "\en";
\&    print <FOO>;
.Ve
.Sh "\s-1UNTIE\s0 this"
.IX Xref "UNTIE"
.IX Subsection "UNTIE this"
You can define for all tie types an \s-1UNTIE\s0 method that will be called
at \fIuntie()\fR.  See "The \f(CW\*(C`untie\*(C'\fR Gotcha" below.
.ie n .Sh "The ""untie"" Gotcha"
.el .Sh "The \f(CWuntie\fP Gotcha"
.IX Xref "untie"
.IX Subsection "The untie Gotcha"
If you intend making use of the object returned from either \fItie()\fR or
\&\fItied()\fR, and if the tie's target class defines a destructor, there is a
subtle gotcha you \fImust\fR guard against.
.PP
As setup, consider this (admittedly rather contrived) example of a
tie; all it does is use a file to keep a log of the values assigned to
a scalar.
.PP
.Vb 1
\&    package Remember;
\&
\&    use strict;
\&    use warnings;
\&    use IO::File;
\&
\&    sub TIESCALAR {
\&        my $class = shift;
\&        my $filename = shift;
\&        my $handle = IO::File\->new( "> $filename" )
\&                         or die "Cannot open $filename: $!\en";
\&
\&        print $handle "The Start\en";
\&        bless {FH => $handle, Value => 0}, $class;
\&    }
\&
\&    sub FETCH {
\&        my $self = shift;
\&        return $self\->{Value};
\&    }
\&
\&    sub STORE {
\&        my $self = shift;
\&        my $value = shift;
\&        my $handle = $self\->{FH};
\&        print $handle "$value\en";
\&        $self\->{Value} = $value;
\&    }
\&
\&    sub DESTROY {
\&        my $self = shift;
\&        my $handle = $self\->{FH};
\&        print $handle "The End\en";
\&        close $handle;
\&    }
\&
\&    1;
.Ve
.PP
Here is an example that makes use of this tie:
.PP
.Vb 2
\&    use strict;
\&    use Remember;
\&
\&    my $fred;
\&    tie $fred, \*(AqRemember\*(Aq, \*(Aqmyfile.txt\*(Aq;
\&    $fred = 1;
\&    $fred = 4;
\&    $fred = 5;
\&    untie $fred;
\&    system "cat myfile.txt";
.Ve
.PP
This is the output when it is executed:
.PP
.Vb 5
\&    The Start
\&    1
\&    4
\&    5
\&    The End
.Ve
.PP
So far so good.  Those of you who have been paying attention will have
spotted that the tied object hasn't been used so far.  So lets add an
extra method to the Remember class to allow comments to be included in
the file \*(-- say, something like this:
.PP
.Vb 6
\&    sub comment {
\&        my $self = shift;
\&        my $text = shift;
\&        my $handle = $self\->{FH};
\&        print $handle $text, "\en";
\&    }
.Ve
.PP
And here is the previous example modified to use the \f(CW\*(C`comment\*(C'\fR method
(which requires the tied object):
.PP
.Vb 2
\&    use strict;
\&    use Remember;
\&
\&    my ($fred, $x);
\&    $x = tie $fred, \*(AqRemember\*(Aq, \*(Aqmyfile.txt\*(Aq;
\&    $fred = 1;
\&    $fred = 4;
\&    comment $x "changing...";
\&    $fred = 5;
\&    untie $fred;
\&    system "cat myfile.txt";
.Ve
.PP
When this code is executed there is no output.  Here's why:
.PP
When a variable is tied, it is associated with the object which is the
return value of the \s-1TIESCALAR\s0, \s-1TIEARRAY\s0, or \s-1TIEHASH\s0 function.  This
object normally has only one reference, namely, the implicit reference
from the tied variable.  When \fIuntie()\fR is called, that reference is
destroyed.  Then, as in the first example above, the object's
destructor (\s-1DESTROY\s0) is called, which is normal for objects that have
no more valid references; and thus the file is closed.
.PP
In the second example, however, we have stored another reference to
the tied object in \f(CW$x\fR.  That means that when \fIuntie()\fR gets called
there will still be a valid reference to the object in existence, so
the destructor is not called at that time, and thus the file is not
closed.  The reason there is no output is because the file buffers
have not been flushed to disk.
.PP
Now that you know what the problem is, what can you do to avoid it?
Prior to the introduction of the optional \s-1UNTIE\s0 method the only way
was the good old \f(CW\*(C`\-w\*(C'\fR flag. Which will spot any instances where you call
\&\fIuntie()\fR and there are still valid references to the tied object.  If
the second script above this near the top \f(CW\*(C`use warnings \*(Aquntie\*(Aq\*(C'\fR
or was run with the \f(CW\*(C`\-w\*(C'\fR flag, Perl prints this
warning message:
.PP
.Vb 1
\&    untie attempted while 1 inner references still exist
.Ve
.PP
To get the script to work properly and silence the warning make sure
there are no valid references to the tied object \fIbefore\fR \fIuntie()\fR is
called:
.PP
.Vb 2
\&    undef $x;
\&    untie $fred;
.Ve
.PP
Now that \s-1UNTIE\s0 exists the class designer can decide which parts of the
class functionality are really associated with \f(CW\*(C`untie\*(C'\fR and which with
the object being destroyed. What makes sense for a given class depends
on whether the inner references are being kept so that non-tie-related
methods can be called on the object. But in most cases it probably makes
sense to move the functionality that would have been in \s-1DESTROY\s0 to the \s-1UNTIE\s0
method.
.PP
If the \s-1UNTIE\s0 method exists then the warning above does not occur. Instead the
\&\s-1UNTIE\s0 method is passed the count of \*(L"extra\*(R" references and can issue its own
warning if appropriate. e.g. to replicate the no \s-1UNTIE\s0 case this method can
be used:
.PP
.Vb 5
\&    sub UNTIE
\&    {
\&     my ($obj,$count) = @_;
\&     carp "untie attempted while $count inner references still exist" if $count;
\&    }
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
See DB_File or Config for some interesting \fItie()\fR implementations.
A good starting point for many \fItie()\fR implementations is with one of the
modules Tie::Scalar, Tie::Array, Tie::Hash, or Tie::Handle.
.SH "BUGS"
.IX Header "BUGS"
The bucket usage information provided by \f(CW\*(C`scalar(%hash)\*(C'\fR is not
available.  What this means is that using \f(CW%tied_hash\fR in boolean
context doesn't work right (currently this always tests false,
regardless of whether the hash is empty or hash elements).
.PP
Localizing tied arrays or hashes does not work.  After exiting the
scope the arrays or the hashes are not restored.
.PP
Counting the number of entries in a hash via \f(CW\*(C`scalar(keys(%hash))\*(C'\fR
or \f(CW\*(C`scalar(values(%hash)\*(C'\fR) is inefficient since it needs to iterate
through all the entries with \s-1FIRSTKEY/NEXTKEY\s0.
.PP
Tied hash/array slices cause multiple \s-1FETCH/STORE\s0 pairs, there are no
tie methods for slice operations.
.PP
You cannot easily tie a multilevel data structure (such as a hash of
hashes) to a dbm file.  The first problem is that all but \s-1GDBM\s0 and
Berkeley \s-1DB\s0 have size limitations, but beyond that, you also have problems
with how references are to be represented on disk.  One experimental
module that does attempt to address this need is DBM::Deep.  Check your
nearest \s-1CPAN\s0 site as described in perlmodlib for source code.  Note
that despite its name, DBM::Deep does not use dbm.  Another earlier attempt
at solving the problem is \s-1MLDBM\s0, which is also available on the \s-1CPAN\s0, but
which has some fairly serious limitations.
.PP
Tied filehandles are still incomplete.  \fIsysopen()\fR, \fItruncate()\fR,
\&\fIflock()\fR, \fIfcntl()\fR, \fIstat()\fR and \-X can't currently be trapped.
.SH "AUTHOR"
.IX Header "AUTHOR"
Tom Christiansen
.PP
\&\s-1TIEHANDLE\s0 by Sven Verdoolaege <\[e-mail]\fR> and Doug MacEachern <\[e-mail]\fR>
.PP
\&\s-1UNTIE\s0 by Nick Ing-Simmons <\fInick@ing\-simmons.net\fR>
.PP
\&\s-1SCALAR\s0 by Tassilo von Parseval <\fItassilo.von.parseval@rwth\-aachen.de\fR>
.PP
Tying Arrays by Casey West <\[e-mail]\fR>