NAME¶
IO::LockedFile - supply object methods for locking files
SYNOPSIS¶
use IO::LockedFile;
# create new locked file object. $file will hold a file handle.
# if the file is already locked, the method will not return until the
# file is unlocked
my $file = new IO::LockedFile(">locked1.txt");
# when we close the file - it become unlocked.
$file->close();
# suppose we did not have the line above, we can also delete the
# object, and the file is automatically unlocked and closed.
$file = undef;
DESCRIPTION¶
In its simplistic use, the
IO::LockedFile class gives us the same
interface of the
IO::File class with the unique difference that the
files we deal with are locked using the
Flock mechanism (using the
"flock" function).
If during the running of the process, it crashed - the file will be
automatically unlocked. Actually - if the
IO::LockedFile object goes
out of scope, the file is automatically closed and unlocked.
So, if you are just interested in having locked files with "flock",
you can skip most of the documentation below.
If, on the other hand, you are interested in locking files with other schemes
then
Flock, or you want to control the behavior of the locking (having
non blocking lock for example), read on.
Actually the class
IO::LockedFile is kind of abstract class.
Why abstract? Because methods of this class call the methods "lock"
and "unlock". But those methods are not really implemented in this
class. They suppose to be implemented in the derived classes of
IO::LockedFile.
Why "kind" of abstract? Because the constructor of this class will
return an object!
How abstract class can create objects? This is done by having the constructor
returning object that is actually an object of one of the derived classes of
IO::LockedFile.
So by default the constructor of
IO::LockedFile will return an object of
IO::LockedFile::Flock. For example, the following:
use IO::LockedFile;
$lock = new IO::LockedFile(">bla");
print ref($lock);
Will give:
IO::LockedFile::Flock
So what are the conclusions here?
First of all - do not be surprised to get object of derived class from the
constructor of
IO::LockedFile.
Secondly - by changing the default behavior of the constructor of
IO::LockedFile, we can get object of other class which means that we
have a locked file that is locked with other scheme.
The default behavior of the constructor is determined by the global options.
We can access this global options, or the options per object using the method
"set_option" and "get_option".
We can set the global options in the use line:
use IO::LockedFile 'Flock'; # set the default scheme to be Flock
use IO::LockedFile ( scheme => Flock );
We can also set the options of a new object by passing the options to the
constructor, as we will see below. We can change the options of an existing
object by using the "set_option" method.
Which options are available?
- scheme
- The scheme let us define which derived class we use for the object
we create. See below which derived classes are available. The default
scheme is 'Flock'.
- block
- The block option can be 1 or 0 (true or false). If it is 1, a call
to the "open" method or to the constructor will be blocked if
the file we try to open is already locked. This means that those methods
will not return till the file is unlocked. If the value of the
block option is 0, the "open" and the constructor will
return immediately in any case. If the file is locked, those methods will
return undef. The default value of the block option is 1.
- lock
- The lock option can be 1 or 0 (true or false). It defines if the
file we open when we create the object will be opened locked. Sometimes,
we want to have a file that can be locked, yet we do not want to open it
locked from the beginning. For example if we want to print into a log
file, usually we want to lock that file only when we print into it. Yet,
it might be that when we open the file in the beginning we do not print
into it immediately. In that case we will prefer to open the file as
unlocked, and later we will lock it when needed. The default value of the
lock option is 1.
There might be extra options that are used by one of the derived classes. So
according to the scheme you choose to use, please look in the manual page of
the class that implement that scheme.
Finally, some information that is connected to a certain scheme will be found in
the classes that are derived from this class. For example, compatibility
issues will be discussed in each derived classes.
The classes that currently implement the interface that
IO::LockedFile
defines are:
- •
- IO::LockedFile::Flock
CONSTRUCTOR¶
- new ( FILENAME [,MODE [,PERMS]] )
- Creates an object that belong to one of the derived classes of
"IO::LockedFile". If it receives any parameters, they are passed
to the method "open". if the "open" fails, the object
is destroyed. Otherwise, it is returned to the caller. The object will be
the file handle of that opened file.
- new ( OPTIONS, FILENAME [,MODE [,PERMS]] )
- This version of the constructor is the same as above, with the difference
that we send as the first parameter a reference to a hash - OPTIONS. This
hash let us change for this object only, the options from the default
options. So for example if we want to change the lock option from
its default we can do it as follow:
$file = new IO::LockedFile( { lock => 0 },
">locked_later.txt" );
METHODS¶
- open ( FILENAME [,MODE [,PERMS]] )
- The method let us open the file FILENAME. By default, the file will be
opened as a locked file, and if the file that is opened is already locked,
the method will not return until the file is unlocked. Of course this
default behavior can be controlled by setting other options. The object
will be the file handle of that opened file. The parameters that should be
provided to this method are the same as the parameters that the method
"open" of IO::File accepts. (like
">file.txt" for example). Note that the open method checks if
the file is opened for reading or for writing, and only then calls the
lock method of the derived class that is being used. This way, for
example, when using the Flock scheme, the lock will be a shared
lock for a file that is being read, and exclusive lock for a file that is
opened to be write.
- close ( )
- The file will be closed and unlocked. The method returns the same as the
close method of IO::File.
- lock ( )
- Practically this method does nothing, and returns 1 (true). This method
will be overridden by the derived class that implements the scheme we use.
When it is overridden, the method suppose to lock the file according to
the scheme we use. If the file is already locked, and the block
option is 1 (true), the method will not return until the file is unlocked,
and locked again by the method. If the block option is 0 (false),
the method will return 0 immediately. Besides, the lock method is aware if
the file was opened for reading or for writing. Thus, for example, when
using the Flock scheme, the method will create a shared lock for a
file that is being read, and exclusive lock for a file that is opened to
be write.
- unlock ( )
- Practically this method does nothing, and returns 1 (true). This method
will be overridden by the derived class that implements the scheme we use.
When it is overridden, the method suppose to unlock the file according to
the scheme we use, and return 1 (true) on success and 0 (false) on
failure.
- have_lock ( )
- Will return 1 (true) if the file is already locked by this object. Will
return 0 (false) otherwise. Note that this will not tell us anything about
the situation of the file itself - thus we should not use this method in
order to check if the file is locked by someone else.
- print ( )
- This method is exactly like the "print" method of
IO::Handle, with the difference that when using this method, if the
file is unlocked, then before printing to it, it will be locked and
afterward it will be unlocked.
- truncate ( )
- This method is exactly like the "truncate" method of
IO::Handle, with the difference that when using this method, if the
file is unlocked, then before truncating it, it will be locked and
afterward it will be unlocked.
- is_writable ( )
- This method will return 1 (true) if the file was opened to write. Will
return 0 (false) otherwise.
- should_block ( )
- This method will return 1 (true) if the block option set to 1. Will return
0 (false) otherwise.
- should_lock ( )
- This method will return 1 (true) if the lock option set to 1. Will return
0 (false) otherwise.
- get_scheme ( )
- This method will return the name of the scheme that is currently
used.
AUTHORS¶
Rani Pinchuk, rani@cpan.org
Rob Napier, rnapier@employees.org
COPYRIGHT¶
Copyright (c) 2001-2002 Ockham Technology N.V. & Rani Pinchuk. All rights
reserved. This package is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
SEE ALSO¶
IO::File(3),
IO::LockedFile::Flock(3)