When <> fails

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

Replies are listed 'Best First'.
Re: When <> fails by betterworld (Curate) on Jul 29, 2008 at 19:44 UTC
From perldoc -f readline: `If readline encounters an operating system error, $! will be set with the corresponding error message. It can be helpful to check $! when you are reading from filehandles you don't trust, such as a tty or a socket. The following example uses the operator form of "readline", and takes the necessary steps to ensure that "readline" was successful. for (;;) { undef $!; unless (defined( $line = <> )) { die $! if $!; last; # reached EOF } # ... }` [download] I think "filehandles you don't trust" should be interpreted as all filehandles where a read might fail, especially when reading from a special device or something like that.	[reply] [d/l]
Re: When <> fails by pc88mxer (Vicar) on Jul 29, 2008 at 19:32 UTC
The current file handle that `<>` is using is stored in `ARGV`, but my testing shows that perl has alrady closed `ARGV` when the `while (<>) {...}` loop terminates. You might try checking `$!` to see if that gets set when the NFS server stops serving the file: `while (<>) { ... } # check $! which is probably from the last close` [download] Update: ikegami pointed out that testing `$!` (as in `if ($!) ...` doesn't make sense here. One issue is that `$!` is only set when there is an error. One way around this is to force a known error at the end of the loop so that you can tell if `readline` generated an error. Update 2: One can assign to `$!`, so this should be able to detect if `readline` failed: `while (<>) { ... $! = undef; } if (defined($!)) { # $! contains error from last <c>close` [download] } </c>	[reply] [d/l] [select]
Re: When <> fails by repellent (Priest) on Jul 30, 2008 at 01:16 UTC
While I think that suggestions by pc88mxer and betterworld are the way to go, I can't help but wonder about the philosophical aspects of the issue :) All scripts make certain base assumptions in order to function. A stable filesystem to act on is a good base assumption. If NFS is erratic, there's not much the script can do in terms of handling that kind of catastrophic failure. The partial damage would have already occurred in previous `while (<>)` iterations, anyway. So, would it be useful to put out a warning that an unexpected critical error has just happened? Yes, that would be nice. But how far do we go? Will this succeed as a new Perl idiom: `while (<>) { unless ($!) { # do actual stuff here } }` [download] How far do we go down the path of paranoia just to make the script bullet-proof? Update: Thanks for pointing my mistake out, ikegami. Still, I'd like to know how much is too much when it comes to error checking of this nature.	[reply] [d/l] [select]
Re^2: When <> fails by ikegami (Patriarch) on Jul 30, 2008 at 01:39 UTC
That snippet makes no sense. `<>` returns undef on error, so checking for an error in the loop is nonsense. A true `$!` doesn't mean an error occured. A false `$!` doesn't mean no error occured.	[reply] [d/l] [select]


laziness, impatience, and hubris
	PerlMonks