Select and file handler

c_chipster has asked for the wisdom of the Perl Monks concerning the following question:

Monks, I was reading How do I tell if a handle is open for writing?. Although the thread didn't go down that way, but the guy sort of indicated that select can be used on normal files. I don't think so though. For example, the following code always prints -1 - nothing writable:

use strict;
use warnings;

use IO::Select;

my $file;
open $file, ">", "somefile";
my $sel = IO::Select->new();
$sel->add($file);
my @writables= $sel->can_write();
print $#writables;
[download]

Can anyone demo the opposite?

Comment on Select and file handler Download Code

Replies are listed 'Best First'.
Re: Select and file handler by pg (Canon) on Nov 13, 2005 at 22:18 UTC
Looks like that this is not cross-platform. With Windows XP, the OP's code does return -1. A note that could be related: on Windows, select() is implemented as part of Windows Socket 2. Most likely, select() only works with socket on Windows.	[reply]
Re^2: Select and file handler by ikegami (Patriarch) on Nov 14, 2005 at 13:57 UTC
select only works on sockets in Win32. Update: I guess you said that already. I apparently didn't read your entire post. Well, take it as confirmation, then.	[reply]
Re: Select and file handler by graff (Chancellor) on Nov 13, 2005 at 21:02 UTC
I ran that code as-is, in two conditions: (inadvertently) in a directory where I did not have write permission; the result was consistently "-1" as you reported. (after seeing my initial mistake) in a directory where I did have write permission; the result was "0" -- i.e. there was one element returned by `$sel->can_write();` given that the file had been opened successfully (this is on macosx 10.3/perl 5.8.1) That'll teach us about forgetting to do error checking -- i.e. "open ... or die ..." would have been prudent.	[reply] [d/l]
Re: Select and file handler by Tanktalus (Canon) on Nov 13, 2005 at 20:53 UTC
I actually get "0". But that's confusing. I changed the last line to `print scalar @writables, $/;`, and got "1" as my output. Are you sure that your open succeeded? Try changing the open line to `open $file, ">", "somefile" or die "Can't open somefile for write: $!"` and see if that tells you anything.	[reply] [d/l] [select]
Re: Select and file handler by gaal (Parson) on Nov 13, 2005 at 20:53 UTC
What OS/perl are you on? This works for me on linux and 5.8.7. Updates: 1. make sure your IO isn't failing. Add the usual `or die` guards. 2. `scalar @writables` is a more natural way in Perl to look at the size of an array.	[reply] [d/l] [select]