How to read named FIFO non-blockingly?

Question

I create a FIFO, and periodically open it in read-only and non-blockingly mode from a.py:

os.mkfifo(cs_cmd_fifo_file, 0777)
io = os.open(fifo, os.O_RDONLY | os.O_NONBLOCK)
buffer = os.read(io, BUFFER_SIZE)

From b.py, open the fifo for writing:

out = open(fifo, 'w')
out.write('sth')

Then a.py will raise an error:

buffer = os.read(io, BUFFER_SIZE)

OSError: [Errno 11] Resource temporarily unavailable

Anyone know what's wrong?

Answer 1

According to the manpage of read(2):

   EAGAIN or EWOULDBLOCK
          The  file  descriptor  fd refers to a socket and has been marked
          nonblocking   (O_NONBLOCK),   and   the   read   would    block.
          POSIX.1-2001  allows  either error to be returned for this case,
          and does not require these constants to have the same value,  so
          a portable application should check for both possibilities.

So what you're getting is that there is no data available for reading. It is safe to handle the error like this:

try:
    buffer = os.read(io, BUFFER_SIZE)
except OSError as err:
    if err.errno == errno.EAGAIN or err.errno == errno.EWOULDBLOCK:
        buffer = None
    else:
        raise  # something else has happened -- better reraise

if buffer is None: 
    # nothing was received -- do something else
else:
    # buffer contains some received data -- do something with it

Make sure you have the errno module imported: import errno.