Why can I not read from stdin in this forked proce

问题:

The following code prints nothing, but it should print out "a" repeatedly. The forked process blocks on the os.read(0, 1). The parent process is indeed writing to the stdin_master, but stdin_slave receives nothing. Any ideas?

import os
import pty
import resource
import select
import signal
import time


stdin_master, stdin_slave = pty.openpty()
stdout_master, stdout_slave = pty.openpty()
stderr_master, stderr_slave = pty.openpty()

pid = os.fork()

# child process
if pid == 0:
    os.setsid()
    os.close(stdin_master)
    os.close(stdout_master)
    os.close(stderr_master)

    os.dup2(stdin_slave, 0)
    os.dup2(stdout_slave, 1)
    os.dup2(stderr_slave, 2)

    max_fd = resource.getrlimit(resource.RLIMIT_NOFILE)[0]
    os.closerange(3, max_fd)


    while True:
        char = os.read(0, 1)
        os.write(1, char)

    os._exit(255)

# parent process
else:
    os.close(stdin_slave)
    os.close(stdout_slave)
    os.close(stderr_slave)

    try:
        while True:
            read, write, err = select.select([stdout_master], [stdin_master], [], 0)
            for to_read in read:
                print os.read(to_read, 1)

            for to_write in write:
                os.write(to_write, "a")

            time.sleep(0.1)

    finally:
        os.kill(pid, signal.SIGKILL)

回答1:

I'm not sure your want to be using the pty module here. What you seem to need is basically a pair of os.pipe's.

Here's a solution that works using pipes:

import os
import select
import signal
import time


stdin_slave, stdout_master = os.pipe()
stdin_master,stdout_slave  = os.pipe()

pid = os.fork()

# child process
if pid == 0:

    while True:
        char = os.read(stdin_slave, 1)
        os.write(stdout_slave, char)

    os._exit(255)

# parent process
else:
    try:
        while True:
            read, write, err = select.select([stdin_master], [stdout_master], [], 0)

            for to_read in read:
                print os.read(to_read, 1)

            for to_write in write:
                os.write(to_write, "a")

            time.sleep(0.1)

    finally:
        os.kill(pid, signal.SIGKILL)

回答2:

Thomas Wouters was kind enough to furnish the answer:

PTYs can only be read from the master fd. So the child's os.read(0, 1) was not working because of this. If we change

stdin_master, stdin_slave = pty.openpty()

to

stdin_slave, stdin_master = pty.openpty()

Now the chlid will be reading from the master, and we'll be writing to the slave from the parent. This works.