How to read named FIFO non-blockingly?

python nonblocking fifo
chaonin picture chaonin · Jan 15, 2013 · Viewed 17.7k times · Source

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

Jonas Schäfer picture Jonas Schäfer · Jan 15, 2013

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.

Related questions

A non-blocking read on a subprocess.PIPE in Python

I'm using the subprocess module to start a subprocess and connect to its output stream (standard output). I want to be able to execute non-blocking reads on its standard output. Is there a way to make .readline non-blocking or to …

Read More
What does Python's socket.recv() return for non-blocking sockets if no data is received until a timeout occurs?

Basically, I've read in several places that socket.recv() will return whatever it can read, or an empty string signalling that the other side has shut down (the official docs don't even mention what it returns when the connection is …

Read More
Polling the keyboard (detect a keypress) in python

How can I poll the keyboard from a console python app? Specifically, I would like to do something akin to this in the midst of a lot of other I/O activities (socket selects, serial port access, etc.): while 1: # doing …

Read More