open ($FH,"<named pipe") blocking

gnu@perl has asked for the wisdom of the Perl Monks concerning the following question:

Ok, so I've googled and searched here but have not found the answer, I know I must be missing something simple. I have an open line where I open a Unix named pipe for reading. The problem is the open of the pipe hangs until data is sent into the pipe. I'm not reading it at this time, just trying to open it, here is the sub that opens the pipe

sub open_named_pipe
{
  my $INFILE = shift;
  open (IN_FH,"<$INFILE") || die "Cannot open input file $INFILE for r
+eading!\n";
  IN_FH->autoflush(1);
  return \*IN_FH;
}
[download]

How do I just open this and then continue on?

Comment on open ($FH,"<named pipe") blocking Download Code

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Re: open ($FH,"<named pipe") blocking by polettix (Vicar) on May 12, 2005 at 18:29 UTC
The problem is the open of the pipe hangs until data is sent into the pipe. The open of the FIFO file blocks until another process `open`s the file for writing, not until data is available (which blocks `read/sysread`s). In alternative to previous suggestions, you can `sysopen` using the `O_NONBLOCK` flag provided in Fcntl. Update: I see only now that Fletch already gave you this suggestion, apologies for the repetition. Flavio (perl -e 'print(scalar(reverse("\nti.xittelop\@oivalf")))') Don't fool yourself.	[reply] [d/l] [select]
Re: open ($FH,"<named pipe") blocking by ikegami (Patriarch) on May 12, 2005 at 18:28 UTC
The problem is the open of the pipe hangs until data is sent into the pipe. I've played with names pipes a long time ago on AIX or SunOS/Solaris. If I remember correctly, that's how it behaved for me too, and I wasn't using Perl. Are you sure this isn't how your OS handles opening named pipes?	[reply]
Re: open ($FH,"<named pipe") blocking by dynamo (Chaplain) on May 12, 2005 at 18:18 UTC
Change your open to open(IN_FH,"$INFILE\|"). That tells perl to treat the filehandle as a pipe. UPDATE: Sorry, this is incorrect. see replies.	[reply]
Re^2: open ($FH,"<named pipe") blocking by Fletch (Bishop) on May 12, 2005 at 18:26 UTC
I think you misunderstood what they're asking. They're trying to open a named pipe, which is a special pipe which lives in the filesystem. When you try and open it the `open(2)` system call underneath will block if there's no writer on the other end. They need to look at the `O_NONBLOCK` flag for `sysopen` rather than using `open`. Update: Excepting of course that that will just make the `sysopen` fail with a return code of `EAGAIN` (I believe). That's just the way named pipes work; the call blocks until there's something on the other end.	[reply] [d/l] [select]
Re^3: open ($FH,"<named pipe") blocking by gnu@perl (Pilgrim) on May 12, 2005 at 18:36 UTC
You nailed it. I knew it was something simple, but it has been soooooo long since I had to use pipe for anything I forgot about sysopen. Thanks.	[reply]
Re^2: open ($FH,"<named pipe") blocking by ikegami (Patriarch) on May 12, 2005 at 18:24 UTC
No. That tells `perl`to execute the command stored in `$INFILE` and open `IN_FH` as a pipe to the command's output. That doesn't tell `perl` to open the named pipe whose name is in `$INFILE`.	[reply] [d/l] [select]
Re^2: open ($FH,"<named pipe") blocking by gnu@perl (Pilgrim) on May 12, 2005 at 18:24 UTC
That actually tells perl to exec the thing in $INFILE and read from it as if it were a pipe. like if I were to do: `$INFILE="/usr/bin/cat /var/adm/messages"; open($FH,"$INFILE\|")` [download]	[reply] [d/l]
Re: open ($FH,"<named pipe") blocking by Anonymous Monk on May 13, 2005 at 16:56 UTC
And now the easy way: `open(X,"+<$INFILE");open(IN_FH,"<$INFILE");close(X); # ^please note the +` [download]	[reply] [d/l]