Let me begin with a simplified script that produces strange behaiviour for me. 

perl -we 'open $f, "<", "/etc/passwd" or die 1; for (0..2) { $! = 0; $
+l = readline $f; $s = $!; chomp $l; print qq/"$l", $s\n/; }'

[download]
Update: changed printf to print, which was a mistake and could cause problems if there were % signs in the file read.

This prints the following for me: 

"root:x:0:0:root:/root:/bin/bash", Bad file descriptor
"bin:x:1:1:bin:/bin:/bin/false",
"daemon:x:2:2:daemon:/sbin:/bin/false",

[download]

Thus, readline succeeds as it should, but the first time I call it, it sets $! to EBADF.

It seems that this happens with any file, /etc/passwd is just an example. Also, it happens the first time I readline after a seek, not only after opening the file.

My questions are:

  1. Can any of you duplicate this problem? I am using perl, v5.8.2 built for i686-linux.
  2. Is it indeed readline that sets $!? Note that I've cleared $! before the readline, and I have not defined any signal handles here that could change $!, so I belive it is.
  3. Can readline report an error in $! and return a parital result instead of an undef? I guess no.
  4. Is readline (or any function for that matter) allowed to change $! if it succeeds? I think no. Although it's easy to make a mistake and forget to localize $!, but I think that libc and the perl core should behaive well in this sense. (Update: deleted libc. It shouldn't have much role here, as perlio does all the work.) Update: if it can set $! when it succeeds, can it do the same when it meets the end of file? If it could, I couldn't just do 
    $! = 0; $line = <$file>; !defined($line) && $! and die "error";

    [download]
    as perldoc perlfunc says, because that can report an error even if there isn't any. I'd have to use the eof function instead. Update: diotalevi says that eof() eof returns true after a read error, so one can't use that either. One could use IO::Handle::error, I'm not sure now if that's more reliable than $!.
  5. Summarizing, what does this error mean, and can I safely ignore it?

Update: I've tried the same code in a few other (un*x) machines, but I've only got the error with mine.

In reply to readline succeeds but sets $! = EBADF by ambrus

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