os.pipe() + os.fork()

Sebastian Noack sebastian.noack at googlemail.com
Sat Feb 20 11:36:04 CET 2010


Hi,

I have a problem using os.pipe() together with os.fork(). Usually when
the writing end of the pipe is closed, the reading end gets EOF. So
subsequent attempts to read data will return an empty string. But when
you call os.fork() after you have created a pipe using os.pipe(), and
read data from the pipe in the child process, when the partent process
has already closed the writing end, the reading end does not get EOF.
Instead of the os.read() call in the child process is blocking.

I am using Linux, so I would like to know if this is a bug in Python or
just weird behaviour under Linux and if it is possible work around it.

Regards
Sebastian Noack


---------------------------------------------------------------------------

import os

def test_pipe_sync():
    pipe = os.pipe()

    os.write(pipe[1], 'Spam')
    os.close(pipe[1])

    print repr(os.read(pipe[0], 4))        # 'Spam' is printed.
    print repr(os.read(pipe[0], 4))        # '' is printed, because of EOF is reached.

def test_pipe_forked():
    pipe = os.pipe()
    pid  = os.fork()

    if pid:
        os.write(pipe[1], 'Spam')
        os.close(pipe[1])

        os.waitpid(pid, 0)
    else:
        print repr(os.read(pipe[0], 4))    # 'Spam' is printed.
        print repr(os.read(pipe[0], 4))    # Nothing is printed and os.read()
                                           # is blocking, eventhough the other
                                           # end of the pipe was closed.

if __name__ == '__main__':
    test_pipe_sync()

    print
    print '-' * 80
    print

    test_pipe_forked()
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