in reply to Writing to DATA

The DATA filehandle is just a an open handle on the current file. The current file is just the $0 variable (so long as you haven't changed it). So: 

# main part of script ...

# then redo the crc:
rewrite_crc('datafile');

sub rewrite_crc {
    my $file = shift;
    open(SELF,"+<$0")||die $!;
    while(<SELF>){last if /^__END__/}
    print SELF new_crc($file),$/;
    truncate(SELF,tell SELF);
    close SELF;
}

sub new_crc {
    my $file = shift;
    my $crc = 0;
    # calculate new crc here
    $crc = 43256;
    return $crc;
}

__END__
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Re: Re: Writing to DATA
by bart (Canon) on Nov 15, 2003 at 07:31 UTC
    In my experience, at least on some platforms, truncate right after print may fail. You need an intermediate seek to be safe. 
        print SELF new_crc($file),$/;
    seek SELF, 0, 1;
    truncate(SELF, tell SELF);

    [download]
    See also perldoc -f seek:
    Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing.

    Perhaps you don't need the truncate — or the seek. CRCs tend to all be the same string length.

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[d/l]
Re: Re: Writing to DATA
by sweetblood (Prior) on Nov 14, 2003 at 17:30 UTC
    That's pretty darn clever, except that I think you meant to do the trucate prior to the print in rewrite_crc(), otherwise it prints the new crc then truncates it. But this is the basic idea I was looing for.

    What a great place our monestary is! Not to mention the fine people!

    Thanks and be well!

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      No. The truncate call works fine where it is, the file will be truncated at its current position (after the write). Of course, you can truncate first and then write too.

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        Huh!? I don't get anything after __END__ if the truncate is before the print. I wonder why it work for you but not I. I'm running on an HP-UX 11.i box, how 'bout you?
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