Question

You are given either an IO::File object or a typeglob (\*STDOUT or Symbol::symbol_to_ref('main::FH')); how would you go about determining if it is a read or write handle? The interface cannot be extended to pass this information (I am overriding close to add calls to flush and sync before the actual close).

Currently I am attempting to flush and sync the filehandle and ignoring the error 'Invalid argument' (which is what I get when I attempt to flush or sync a read filehandle):

eval { $fh->flush; 1 } or do {         #this seems to exclude flushes on read handles         unless ($! =~ /Invalid argument/) {                 croak 'could not flush $fh: $!';         } };  eval { $fh->sync; 1 } or do {         #this seems to exclude syncs on read handles         unless ($! =~ /Invalid argument/) {                 croak 'could not sync $fh: $!';         } }; 

Answer 1

Have a look at the fcntl options. Maybe F_GETFL with O_ACCMODE.

Edit: I did a little googling and playing over lunch and here is some probably non-portable code but it works for my Linux box, and probably any Posix system (perhaps even Cygwin, who knows?).

use strict; use Fcntl; use IO::File;  my $file; my %modes = ( 0 => 'Read only', 1 => 'Write only', 2 => 'Read / Write' );  sub open_type {     my $fh = shift;     my $mode = fcntl($fh, F_GETFL, 0);     print 'File is: ' . $modes{$mode & 3} . '\n'; }  print 'out\n'; $file = new IO::File(); $file->open('> /tmp/out'); open_type($file);  print '\n';  print 'in\n'; $file = new IO::File(); $file->open('< /etc/passwd'); open_type($file);  print '\n';  print 'both\n'; $file = new IO::File(); $file->open('+< /tmp/out'); open_type($file); 

Example output:

$ perl test.pl  out File is: Write only  in File is: Read only  both File is: Read / Write