file handling in subroutines

[rvBasic]

I have a main program that could open an error log file.
That program calls a subroutine with the file handle as one of the optional arguments.

What I like to do in the subroutine is as follows:

- If the argument does not exist, use STDOUT as a log file
- If the argument exists, check if the file is open. If it is, use it. If it is not open, then open it (append) and use it.

The problems I have to implement this are essentially:
- How can I check if a file is open?
- Can I have something like:
print $FH, "xxx";
where $FH is either STDOUT or the FileHandle that was passed as an argument?



_________________________________
In theory, there is no difference between theory and practice. In practice, there is. [attributed to Yogi Berra]

 

[travs69]

maybe not the best way.. but it works

chomp @ARGV;
if ($ARGV[0]) {
	open(LOG, ">>$ARGV[0]") or die "can't open $ARGV[0]\n";
	$OUT = LOG;
}
else {
	$OUT = STDOUT;
}


print $OUT "Test\n";

close $OUT;

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Travis - Those who say it cannot be done are usually interrupted by someone else doing it; Give the wrong symptoms, get the wrong solutions;

 

[rvBasic]

two remarks:

- you are probably passing the filename as an argument. I would have preferred a file handle (the file could be already opened by the main program)
- use strict considered LOG and STDOUT as barewords.

_________________________________
In theory, there is no difference between theory and practice. In practice, there is. [attributed to Yogi Berra]

 

[ishnid]

A strict-safe alternative:

chomp @ARGV;
my $OUT;
if ($ARGV[0]) {
    open($OUT, ">>$ARGV[0]") or die "can't open $ARGV[0]\n";
}
else {
    $OUT = *STDOUT;
}


print $OUT "Test\n";

close $OUT;

 

[rvBasic]

Thanks for your replies. I finally managed to get the following code almost to work:

#!/usr/bin/perl
use strict;

my $MyTestFile = 'C:\Perl\TestFile.txt';
my $LogFile = 'C:\Perl\MSGLOG.txt';

my $LOGH = undef;

#Entering a First time - write to standard log
&C($MyTestFile,'XXX',2);

#Entering a Second time - write to predefined log file
open ($LOGH, '>>', $LogFile);
print $LOGH "\nEntering Sub\n";
&C($MyTestFile,'XXX',2,*$LOGH);
print $LOGH "Quiting Sub\n";
[COLOR=red]close $LOGH;[/color]

#Entering a Last time - write to standard log
&C($MyTestFile,'XXX',2,*$LOGH);


sub C {
my $LOG=undef;
my ($Source,$Target,$Mode,$LogFile)=@_;
if ($LogFile){$LOG=$LogFile;
print $LOG "Message to existing log\n";
}
else{
$LOG=*STDOUT;
print $LOG "Message to standard log\n";
}
}

As I said: it almost worked! Of course it bumped on the third call, because I closed the File. But part of the file handle seems to subsist. Is there a possibility within the sub to check if the filehandle is closed?

I prefer not to use the Filename as an argument to the sub, because eventually this sub will go into a package module and I want it to be as independent as possible. I then also realized that I have to drop my initial requirement to open the file from within the sub. Without filename, no open.

So I settled to the simplified requirement: is there an opened log then use it, otherwise go to STDOUT. But there seems to be no simple way to verify if there exists an opened logfile. Or is there?

_________________________________
In theory, there is no difference between theory and practice. In practice, there is. [attributed to Yogi Berra]

 

[Kirsle]

Take a look at other code that works with filehandle references and see how they work. A lot of cryptography modules do this (the one that comes to mind is Crypt::CipherSaber).

http://search.cpan.org/~chromatic/Crypt-CipherSaber-1.00/lib/Crypt/CipherSaber.pm

Relevant bit of code:

sub fh_crypt
{
	my ( $self, $in, $out, $iv ) = @_;

	for my $glob ($in, $out)
	{
		my $reftype = reftype( $glob ) || '';
		unless ($reftype eq 'GLOB')
		{
			require Carp;
			Carp::carp( 'Non-filehandle passed to fh_crypt()' );
			return;
		}
	}

	local *OUT = $out;
	if ( defined($iv) )
	{
		$iv = $self->_gen_iv() if length($iv) == 1;
		$self->_setup_key($iv);
		print OUT $iv;
	}

	my $state = $self->[1];

	my ( $buf, @vars );

	while (<$in>)
	{
		unless ($iv)
		{
			( $iv, $_ ) = unpack( "a10a*", $_ );
			$self->_setup_key($iv);
		}
		my $line;
		( $line, $state, @vars ) = _do_crypt( $state, $_, @vars );
		print OUT $line;
	}
	$self->[1] = [ 0 .. 255 ];
	return 1;
}

-------------
Cuvou.com | My personal homepage

perl -e '$|=$i=1;print" oo\n<|>\n_|_";x:sleep$|;print"\b",$i++%2?"/":"_";goto x;'

 

[rvBasic]

Thanks Kirsle for guiding me in the right direction. I had a hard time to understand the code you supplied, but by googling for reftype, I hit in the same package upon the function openhandle that does exactly what I want: If the handle is open it supplies a reference to it, otherwise it returns undef.

The final code then was easy (as always if you know the solution)

#!/usr/bin/perl
use strict;

my $MyTestFile = 'C:\Perl\TestFile.txt';
my $LogFile = 'C:\Perl\MSGLOG.txt';

my $LOGH = undef;

#Entering a First time - write to standard log
&C($MyTestFile,'XXX',2);

#Entering a Second time - write to predefined log file
open ($LOGH, '>>', $LogFile);
print $LOGH "\nEntering Sub\n";
&C($MyTestFile,'XXX',2,*$LOGH);
print $LOGH "Quiting Sub\n";
close $LOGH;

#Entering a Last time - write to standard log
&C($MyTestFile,'XXX',2,*$LOGH);


sub C {

[COLOR=blue]use Scalar::Util qw(openhandle);[/color]

my $LOG=undef;
my ($Source,$Target,$Mode,$LogFile)=@_;
if ([COLOR=blue]openhandle($LogFile)[/color]){$LOG=$LogFile;
print $LOG "Message to existing log\n";
}
else{
$LOG=*STDOUT;
print $LOG "Message to standard log\n";
}
}

Thanks again to anyone who spent time and effort on this nasty problem!

_________________________________
In theory, there is no difference between theory and practice. In practice, there is. [attributed to Yogi Berra]