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overwrite a file

by jdporter (Canon)
on Apr 25, 2007 at 22:04 UTC ( #612107=snippet: print w/ replies, xml ) Need Help??


This code shows how to completely overwrite the data of a file. (Actually, not sure if it will DWIM on sparse files.)

The technique is illustrated in a sample practical application: "securely" erase and delete a file. (Note: does not securely erase and delete a file.)

use Fcntl;
use strict;
use warnings;

my $blocksize = 2048;

sub overwrite_file($;$)
    my( $filename, $pattern ) = @_;
    -e $filename or die "File '$filename' does not exist.\n";
    my $filesize = -s $filename;

    local *F;
    sysopen F, $filename, O_RDWR|O_BINARY
        or die "Can't overwrite '$filename' - $!\n";

    if ( defined $pattern and length $pattern )
        $pattern x= 1 + $blocksize / length $pattern;
        substr( $pattern, $blocksize ) = '';

        my $written = 0;
        $written += syswrite F, $pattern
            while $written < $filesize;
    {    # use random bytes
        $pattern = "\0" x $blocksize;
        my $written = 0;
        while ( $written < $filesize )
            $pattern =~ s/(.)/ $1 ^ chr(rand 256) /seg;
            $written += syswrite F, $pattern;

    #sysseek F, 0, 0; # rewind
    close F;

sub shred_file($)
    my $filename = shift;

    overwrite_file $filename, $_ for
        # byte patterns:
        '',     # random bytes

    unlink $filename or die "Can't delete '$filename' - $!\n";

@ARGV or die "Usage: $0 file file ...\n";
shred_file $_ for @ARGV;

Comment on overwrite a file
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Re: overwrite a file
by ikegami (Pope) on Apr 25, 2007 at 23:56 UTC

    I'm not familiar with the implementations of OS disk caching, but it seems to me there's no guarantee that any but the last call to overwrite_file makes it to disk from the system's cache. Even then... With a particularly good cache, absolutely nothing would be written to disk.

    Remember, sys* disables user-land caching. It doesn't prevent OS caching. FILE_FLAG_NO_BUFFERING would do that in Windows, but I see no reference to it in win32/win32io.c in perl.git.

      The approach is simple and granular. However I would agree with ikegami that system cache may work against you. In *nx systems you might consider to call sync after each file close call to have the system clean out its disk cache.

      For the really paranoid among us other things that may influence success are i.e. disk controller internal caches, cache issues with remote filesystems mounted over network, parallel read/writes on the file by another process. Besides those caching aspects you need to consider filesystem type, as your data may still live in the journal logs.

      Bottomline: Solid erasure of data on a granular level like files is always tricky.

        Agreed. However, secure shredding on disk level is less tricky, albeit cumbersome, as seen here.
        print map{chr}unpack(q{A3}x24,q{074117115116032097110111116104101114032080101114108032104097099107101114})

      Good point. In Windowsland, if the the device being written is a removable type (such as a USB drive), then one can simply disable disk write caching. This worked just fine for me.


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