Re^2: Temporary text files with File::Temp

[jarich]

I think it's a laudable goal to write code that's 100% portable, and works properly on all platforms without any special casing. Realistically, I don't think that's possible for any perl program of any significant complexity.

To a large extent, I agree. It can be very difficult to write Perl programs of significant complexity that are 100% portable, especially if you're not practiced in doing such.

However, I'm not talking about writing Perl programs of any complexity in this question. I don't think it should be hard to open a temporary text file and be able to print stuff including newlines to it, and have the right thing happen. Regardless of operating system. This is an introductory course to Perl after all.

My students should be able to write:

# Reverses each line in the given file
# and replaces that file with the reversed copy
use File::Temp qw(tempfile);
use File::Copy qw(move);
use Fatal qw(open close move);

my $filename = shift or die "Usage: $0 filename";
open(my $in, "<", $filename);
my ($tmp_fh, $tmp_name) = tempfile();

while(<$in>) {
        chomp;
        print {$tmp_fh} scalar(reverse($_)), "\n";
}

close $in;
close $tmp_fh;
move($tmp_name, $filename);

and expect it to work flawlessly on any operating system that has Perl installed. There's nothing in that code which would suggest to a Perl novice (or me until last week) that that code isn't portable.

I realise that it's fundamentally a question of what the logical default case should be. File::Temp has several choices; always binmode the file, never binmode the file, binmode the file if passed a flag, don't binmode the file when passed a flag. They've chosen the first, I was hoping there might have been an undocumented way to get the last.

Alternately if there is a special layer I can pass to binmode itself to say "treat as text" so that Perl can do it's special text-file magic by translating \n to the appropriate thing; I'd love to hear about that. This isn't achieved by using the :crlf layer, because that layer is Windows specific, and won't do the right thing under the *nixes.

Any other suggestions?

thanks, jarich

[John M. Dlugosz]

The problem is that the default for normal open and the default for temfile is different.

Teach to always specify the mode of a file, text or binary, because on some platforms it matters. For that matter, be clear about the character encoding too.

Maybe you should update the File::Temp module.

Or, write your own tempfile function that calls the regular open. That way you have consistent semantics. That's what I do.

—John

[ack]

jarich, you wrote:

"Alternately if there is a special layer I can pass to binmode itself to say "treat as text" so that Perl can do it's special text-file magic by translating \n to the appropriate thing; I'd love to hear about that."

I was recently investigating the use of the I/O layers and seem to recall that there may be a way to do what you asked about. But I'm by no means an expert on the topic so it isn't something I've got much experience with.

But wouldn't that constitute exactly what you said you didn't want...special processing for each environment? And wouldn't that defeat your goal of keeping it simple for your students?

[jarich]

It depends on what the special layer was. For example if I could write:

binmode($fh, ":text");

And have it do the right thing on all systems, it'd be an okay solution. Of course, the best place to fix this is still in File::Temp, because teaching the students that they'll need two commands isn't quite as cool as just giving them one.

I've looked at the File::Temp code, and it looks pretty easy to patch, so as soon as I have time and access to a Windows box, I'll be working on that. Probably mid-April.