On 9/19/2022 3:15 PM, Ken Brown wrote:
On 9/18/2022 5:45 PM, Enrico Forestieri wrote:
Hi,
I think I am experiencing a problem with fifos on cygwin. The attached
C source (fifocomm.c) creates two pipes (/tmp/pipe.{in,out}), expecting
to receive inputs from /tmp/pipe.in and replying to /tmp/pipe.out.
Compiling this source on linux and launching it produces on the terminal
1) Opening pipes
and then the program waits for someone to write to /tmp/pipe.in.
Opening another terminal and launching the fifotest.sh script (also
attached) produces on the first terminal
1) Closing pipes
2) Opening pipes
and the program continues waiting for another input, while the other
terminal shows "You sent: foo"
Instead, on cygwin, after launching the program one gets:
1) Opening pipes
1) Closing pipes
2) Opening pipes
2) Closing pipes
3) Opening pipes
3) Closing pipes
4) Opening pipes
....
....
meaning that the pipes are continually closed and reopened even if
nobody is writing to /tmp/pipe.in.
Seemingly, the problem is the return value of read() on line 55.
As O_NONBLOCK is set, until no data is available for reading,
read() shouldn't block but should return -1 and set errno to EAGAIN.
After a client that opened the pipe for writing, closes it
(and no other client is using the pipe), read() should return 0 and
only at this point the pipes have to be closed and reopened.
However, on cygwin, read() returns 0 also when nobody is writing to the
input pipe causing the above ping pong. As already said, it works as it
should on linux.
I see what's happening, but I don't see why it's a bug, and I don't
understand why the Linux behavior is different.
On Cygwin, the call to 'select' in line 44 returns immediately with
nsel == 1. This seems correct to me, since the man page for 'select'
says, "A file descriptor is ready for reading if a read operation will
not block; in particular, a file descriptor is also ready on
end-of-file." In the present case a read operation will not block for
two reasons: first, O_NONBLOCK has been set; second, we're at EOF
because no process has opened /tmp/pipe.in for writing. Given that
we're at EOF, the return value of 0 for the subsequent 'read' is also
correct.
On Linux, 'select' does not return immediately but instead waits for
someone to write to the FIFO.
Can someone explain why Linux is right and Cygwin is wrong here? I
must be missing something obvious.
Ken
This is how I'm reading this, but I have not actually looked at or tried
the posted code yet:
We use select() specifically when we are using non-blocking I/O. All the
blocking happens in select() so we can track multiple sockets. If
select() returns when there is no data available, it's not really doing
its job, i.e. waiting for data. There are of course particular cases
where something else (opening, closing) causes select() to return, which
is normal and expected, but just because the reader is non-blocking is
not a good reason for select() to return.
--
Problem reports: https://cygwin.com/problems.html
FAQ: https://cygwin.com/faq/
Documentation: https://cygwin.com/docs.html
Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
