Question

I am trying to setup two way communication between a daemon and a client using named pipes. The code hangs while trying to open the named pipe used for input Why?

class comm(threading.Thread):

def __init__(self):
    self.srvoutf = './tmp/serverout'
    self.srvinf = './tmp/serverin'
    if os.path.exists(self.srvoutf):
        self.pipein = open(self.srvoutf, 'r') 
        #-----------------------------------------------------Hangs here
    else:
        os.mkfifo(self.srvoutf)
        self.pipein = open(self.srvoutf, 'r')
        #-----------------------------------------------------or here
    if os.path.exists(self.srvinf):
        self.pipeout = os.open(self.srvinf, os.O_WRONLY)
    else:
        os.mkfifo(self.srvinf)
        self.pipeout = os.open(self.srvinf, os.O_WRONLY)
        
    threading.Thread.__init__ ( self )

Answer 1

From the specification for open():

When opening a FIFO with O_RDONLY or
O_WRONLY set:

If O_NONBLOCK is set, an
open() for reading-only shall return
without delay. An open() for
writing-only shall return an error if
no process currently has the file open
for reading.

If O_NONBLOCK is clear, an open() for
reading-only shall block the calling
thread until a thread opens the file
for writing. An open() for
writing-only shall block the calling
thread until a thread opens the file
for reading.

In other words, when you open a named pipe for reading, by default the open will block until the other side of the pipe is opened for writing. To fix this, use os.open() and pass os.O_NONBLOCK on the read side of the named pipe.