Open a scalar as a file in Perl

Question

According to PerlDoc you can direct a file open to a scalar variable https://perldoc.perl.org/functions/open#Opening-a-filehandle-into-an-in-memory-scalar

I am trying to establish if I can use this approach for a temp file that is persistent within the lexical domain. I am doing the following as a test:

my $memfile;
my $filename=\$memfile;

open (OUT, ">", $filename);
print OUT "some data\n";
close (OUT);

I want to be able to read from the scalar later; eg:

open (IN, "<", $filename);
while (my $in=<IN>)
{
  print "$in\n";
}
close (IN);

This doesn't work, so am I barking mad or is there a right way to do it so it does work?

Carhart

perl -wE'open my $fh, ">", \my $mem or die $!; say $fh "hi"; say "var: $mem"' works fine – Petrie

Or, if you insist on another layer of variables (why that?), perl -wE'my $mem; my $f = \$mem; open my $fh, ">", $f or die $!; say $fh "hi"; say "var: $mem"' works as well – Petrie

I made a typo on my actual test code and now I fixe dit, it works. I feel so dumb now. – Carhart

"typo on my actual test code" -- ok, no dumb, happens all the time to everybody :) – Petrie

"use this approach for a temp file that is persistent within the lexical domain." --- btw, what do you mean by "persistent" and by "lexical domain"? That the file itself exists while in the scope and is gone once that scope is left? Perhaps look into File::Temp? – Petrie

Also asked in /r/perl. It's fine to crosspost but tell people you are doing that so they don't spend time answering a question that already has an answer. – Egghead

Answer 1

We can indeed open a filehandle to a variable, and write to it; and then we can use it like any other variable

open my $fhv, '>', \my $memfile or die $!;
say $fhv "hi";
close $fhv;

print $memfile;  # hi\n

One can also open a filehandle to the variable's reference for reading, like in the question, but this usually isn't needed. (A possible use would be if we want to keep writing and reading such that it can be changed between file/variable merely by changing the open statement.)

This works with another "layer" of variables, like in the question, as well,

 my $memfile;
 my $file = \$memfile;
 open my $fhv, '>', $file or die $!;
 say $fhv "hi";
 close $fhv;

 print $memfile;

Can again open a filehandle to $file for reading, as well.

Note that $fhv isn't quite a proper filehandle and some filehandle-uses will not work; see this page for one example.

Petrie

It turns out I made a typo in my test code and of course it works when I fixed it. I feel so dumb right now. – Carhart

Alright, it happens :). Btw, if you need more advanced and flexible (etc) ways to read/write from/to resources try out IPC::Run. It supports all kinds of things – Petrie

@Carhart Edited a little; nothing dramatic but you may want to take a glance – Petrie