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oct function in perl

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Old 05-04-2009
oct function in perl
hi

i came across one program which uses some method to calculate file permissions using lstat in perl and i am not getting how its doing that i am pasting the code below ...

Code:
$ > cat b.pl
use Fcntl':mode';
my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,$blksize,$blocks) = lstat($ARGV[0]);
my $perms = S_IMODE($mode);
print "perm : [$perms]\n" ;
my $a = ~oct('055') ;
my $b = oct('777') ;
my $c = ( $a & $b ) ;
print "[$a][$b][$c]\n" ;
$umask = "055" ;
if ( $umask ) {
                $perms = S_IMODE($perms & (~oct($umask) & oct('777')));
                print "perm now : [$perms]\n" ;
              }


output :

perm : [493]
[4294967250][511][466]
perm now : [448]

i am not getting the part which in red colour...

can any one explain this part to me ??
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LEARN ABOUT DEBIAN

alien::package::rpm

Alien::Package::Rpm(3pm)				User Contributed Perl Documentation				  Alien::Package::Rpm(3pm)

NAME

       Alien::Package::Rpm - an object that represents a rpm package

DESCRIPTION

       This is an object class that represents a rpm package. It is derived from Alien::Package.

FIELDS

       prefixes
	   Relocatable rpm packages have a prefixes field.

METHODS

       checkfile
	   Detect rpm files by their extention.

       install
	   Install a rpm. If RPMINSTALLOPT is set in the environement, the options in it are passed to rpm on its command line.

       scan
	   Implement the scan method to read a rpm file.

       unpack
	   Implement the unpack method to unpack a rpm file. This is a little nasty because it has to handle relocatable rpms and has to do a bit
	   of permissions fixing as well.

       prep
	   Prepare for package building by generating the spec file.

       cleantree
	   Delete the spec file.

       build
	   Build a rpm. If RPMBUILDOPT is set in the environement, the options in it are passed to rpm on its command line.

	   An optional parameter, if passed, can be used to specify the program to use to build the rpm. It defaults to rpmbuild.

       version
	   Set/get version.

	   When retreiving the version, remove any dashes in it.

       postinst
       postrm
       preinst
       prerm
	   Set/get script fields.

	   When retrieving a value, we have to do some truely sick mangling. Since debian/slackware scripts can be anything -- perl programs or
	   binary files -- and rpm is limited to only shell scripts, we need to encode the files and add a scrap of shell script to make it
	   unextract and run on the fly.

	   When setting a value, we do some mangling too. Rpm maintainer scripts are typically shell scripts, but often lack the leading shebang
	   line.  This can confuse dpkg, so add the shebang if it looks like there is no shebang magic already in place.

	   Additionally, it's not uncommon for rpm maintainer scripts to contain bashisms, which can be triggered when they are ran on systems
	   where /bin/sh is not bash. To work around this, the shebang line of the scripts is changed to use bash.

	   Also, if the rpm is relocatable, the script could refer to RPM_INSTALL_PREFIX, which is set by rpm at run time. Deal with this by
	   adding code to the script to set RPM_INSTALL_PREFIX.

       arch
	   Set/get arch field. When the arch field is set, some sanitizing is done first to convert it to the debian format used internally. When
	   it's retreived it's converted back to rpm form from the internal form.

AUTHOR

       Joey Hess <joey@kitenet.net>

perl v5.14.2							    2011-08-05						  Alien::Package::Rpm(3pm)