Hello, Monks !

I'm relatively new to Perl and still in a phase of learning how things work. My question might sound stupid to you but I searched perldoc back and forth and googled through the web (including your great site) without finding an answer. So here's my question:

Short version: How do I portably and correctly open() a binary file ?

Long version or how I got all confused about that:

My PerlIO documentation states the following about the ":raw" layer:

The ":raw" layer is defined as being identical to calling "binmode($fh)" – the stream is made suitable for passing binary data i.e. each byte is passed as-is. The stream will still be buffered.

Having read that I thought the following two code fragments would be equivalent:

open(my $fh, '<', $filename);
binmode($fh);

[download]
open(my $fh, '<:raw', $filename);

[download]

Because the second one is shorter (and I only have to check one function return value) I used that method in some scripts.

So far so good. A couple of days ago I discovered the PerlIO::get_layers() function and curious and naiv as I am I did the following:

$, = ' ';
$\ = "\n";

open(FH, '<', '/dev/null');
print(PerlIO::get_layers(FH));          # --> unix perlio

[download]

As documented and expected this is the default layer stack on an UNIX system.

open(FH, '<', '/dev/null');
binmode(FH);
print(PerlIO::get_layers(FH));          # --> unix perlio

[download]

So is this one. No problems yet, but look at the next one:

open(FH, '<:raw', '/dev/null');
print(PerlIO::get_layers(FH));          # --> unix

[download]

Big surprise (at least for a dummy like me): The ":perlio" layer mysteriously disappeared. And this mystery is the whole reason for this post.

Obviously, using ":raw" in open() is not the same as calling binmode() afterwards. Having no ":perlio" layer also means that the stream is unbuffered now, right ? (Which explains the results of this PerlIO Benchmark, which I found while searching for answers.)

Btw., My perl version is v5.8.4 but a friend of mine just confirmed the last code snippet on perl version v5.8.8.

Could someone please explain to me what I misunderstood so badly here. (Need not be a lengthy answer, a link or a hint to the right place in some Perl documentation would suffice.)

Thanks so much !

In reply to Using ":raw" layer in open() vs. calling binmode() by Anonymous Monk

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