Using file descriptors to communicate between processes

Question

I have the following python code:

import pty
import subprocess
os=subprocess.os
from subprocess import PIPE
import time
import resource

pipe=subprocess.Popen(["cat"], stdin=PIPE, stdout=PIPE, stderr=PIPE, \
                      close_fds=True)
skip=[f.fileno() for f in (pipe.stdin, pipe.stdout, pipe.stderr)]
pid, child_fd = pty.fork()
if(pid==0):
    max_fd=resource.getrlimit(resource.RLIMIT_NOFILE)[0]
    fd=3
    while fd<max_fd:
        if(fd not in skip):
            try:
                os.close(fd)
            except OSError:
                pass
            fd+=1
        enviroment=os.environ.copy()
        enviroment.update({"FD": str(pipe.stdin.fileno())})
        os.execvpe("zsh", ["-i", "-s"], enviroment)
else:
    os.write(child_fd, "echo a >&$FD\n")
    time.sleep(1)
    print pipe.stdout.read(2)

How can I rewrite it so that it will not use Popen and cat? I need a way to pass data from a shell function running in the interactive shell that will not mix with data created by other functions (so I cannot use stdout or stderr).

Answer 1

Ok, I think I've got a handle on your question now, and see two different approaches you could take.

If you absolutely want to provide the shell in the child process with an already-open file descriptor, then you can replace the Popen() of cat with a call to os.pipe(). That will give you a connected pair of real file descriptors (not Python file objects). Anything written to the second file descriptor can be read from the first, replacing your jury-rigged cat-pipe. (Although "cat-pipe" is fun to say...). A socket pair (socket.socketpair()) can also be used to achieve the same end if you need a bidirectional pair.

Alternatively, you could simplify your life even further by using a named pipe (aka FIFO). If you aren't familiar with the facility, a named pipe is a uni-directional pipe located in the filesystem namespace. The os.mkfifo() function will create the pipe on the filesystem. You can then open the pipe for reading in your primary process and open it for writing / direct output to it from your shell child process. This should simplify your code and open the option of using an existing library like Pexpect to interact with the shell.