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)