Re: Best way to handle readline errors?

I did some digging (on Windows XP, using perl 5.8.8). I didn't come up with a solution, but I thought I'd post what I found out.

I can get <$fh> to return undef by ejecting the media on which resides the file being read. close reports no error. $! is meaningless (It's "Bad file descriptor"/9 when the read is successful and when the read is not successful.)
I cannot get <$fh> (with $/ = \100) to return when the media on which resides the file being read has been ejected. It just hangs. Reinserting the media doesn't help.
I can get sysread to return undef by ejecting the media on which resides the file being read. close reports no error.
I can get read to return undef by ejecting the media on which resides the file being read. close reports "Bad file descriptor".

In all cases, I can confirm an IO error occured, since the OS pops a dialog asking to reinsert the disk once all read buffers have been emptied.

Test programs:

For <$fh>:

use strict;
use warnings;

my $file_name = 'G:\\I386\\DRIVER.CAB';  # 50MB file.

open(my $fh, '<', $file_name)
   or die("Unable to open file \"$file_name\": $!\n");

# Read 100 bytes at a time.
#binmode($fh);
#$/ = \100;

my $line;

undef $!;
$line = <$fh>;
my $ok_defined = defined($!);
my $ok_str_val = "$!";
my $ok_num_val = 0+$!;

print("Eject the media then press Enter.");
<STDIN>;

undef $!;
for (;;) {
   undef $!;
   last if not defined ($line = <$fh>);
   warn(length($line), "\n");
}
my $bad_defined = defined($!);
my $bad_str_val = "$!";
my $bad_num_val = 0+$!;

close($fh)
   or warn("Unable to read file \"$file_name\": $!\n");

print("On success, ");
if ($ok_defined) {
   print("\$! = \"$ok_str_val\"/$ok_num_val\n");
} else {
   print("\$! is not defined\n");
}

print("On failure, ");
if ($bad_defined) {
   print("\$! = \"$bad_str_val\"/$bad_num_val\n");
} else {
   print("\$! is not defined\n");
}
[download]

For sysread and read:

use strict;
use warnings;

my $file_name = 'G:\\I386\\DRIVER.CAB';  # 50MB file.

open(my $fh, '<', $file_name)
   or die("Unable to open file \"$file_name\": $!\n");

my $buf;
my $rv;

$rv = sysread($fh, $buf='', 1);
print("On success, ");
if (defined($rv)) {
   print("\$rv = $rv\n");
} else {
   print("\$rv is not defined\n");
}

print("Eject the media then press Enter.");
<STDIN>;

1 while $rv = sysread($fh, $buf='', 1);
print("On failure, ");
if (defined($rv)) {
   print("\$rv = $rv\n");
} else {
   print("\$rv is not defined\n");
}

close($fh)
   or warn("Unable to read file \"$file_name\": $!\n");
[download]

Comment on Re: Best way to handle readline errors? Select or Download Code

Replies are listed 'Best First'.
Re^2: Best way to handle readline errors? by jrw (Monk) on Nov 11, 2006 at 21:59 UTC
ikegami, I liked your approach (ejecting a CD to cause a read error). So, here are my results which are somewhat different than yours: All three versions read 99Mb after the disk was ejected, indicating that WinXP must be caching the CD file `sysread` returned `undef` but $! was set to 0! Failures in `sysread` and `readline` did not cause `close` to fail whereas `read` failure did. In my tests, `readline` did fail, similar to `read`, as expected. The results, run on WinXP cygwin using ActiveState Perl for Windows v5.8.7: Read more... (703 Bytes) Here are the results when I set `$initial_read` to 0: Read more... (563 Bytes) Here's my test code: Read more... (2 kB)	[reply] [d/l] [select]
Re^3: Best way to handle readline errors? by ikegami (Patriarch) on Nov 12, 2006 at 23:06 UTC
All three versions read 99Mb after the disk was ejected, indicating that WinXP must be caching the CD file It did that to me when I used Alcohol to mount and unmount a virtual CD instead of ejecting a real CD. Apparently, Alcohol doesn't fully unmount an image with an open file handle. When I switched to using a real CD, WinXP would prompt me to reinsert the disk when all cached data had been read. When I Cancel that dialog, Perl returns an error.	[reply]
Re^3: Best way to handle readline errors? by jrw (Monk) on Nov 13, 2006 at 21:24 UTC
So, can anyone help with my original question? I'm sure I'm not the only perl person who is concerned with thorough error handling in code where error handling is important (such as code run by root). The original question was: what is the Perl Best Practice idiom for handling `readline` errors? Being a new poster to PerlMonks, but not a new perl user, is there anything I can do or any way I can help to get this question answered? Anything I can do to get more attention focused on this question?	[reply] [d/l]
Re^2: Best way to handle readline errors? by jrw (Monk) on Nov 11, 2006 at 14:49 UTC
Thanks, ikegami! That was useful. It's worrying that `<$fh>` won't return when you eject and/or reinsert the media. What's going on down in the bowels of that code? `sysread` and `read` were planned in a way which takes errors into consideration (patterned after `read` in c). But error handling in `readline` seems to be somewhat of an afterthought. It makes me wonder if I have to reimplement `readline` at the perl level using `read` (Ugh!). I started asking this question because it's hard for me to justify writing code which will run as root that doesn't have complete error checking, including checking for input errors. Oh, Damian, why didn't Perl Best Practices should us lowly monks how to handle `readline` errors, too?	[reply] [d/l] [select]