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Date: Tue, 11 Dec 2012 11:47:03 +0000
Subject: Re: forking and avoiding zombies!
From: andrea crotti <andrea.crotti.0@gmail.com>
To: peter <pjmakey2@gmail.com>
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Xref: csiph.com comp.lang.python:34613

Yes I wanted to avoid to do something too complex, anyway I'll just
comment it well and add a link to the original code..

But this is now failing to me:

def daemonize(stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):
    # Perform first fork.
    try:
        pid = os.fork()
        if pid > 0:
            sys.exit(0) # Exit first parent.
    except OSError as e:
        sys.stderr.write("fork #1 failed: (%d) %s\n" % (e.errno, e.strerror))
        sys.exit(1)

    # Decouple from parent environment.
    os.chdir("/")
    os.umask(0)
    os.setsid()

    # Perform second fork.
    try:
        pid = os.fork()
        if pid > 0:
            sys.exit(0) # Exit second parent.
    except OSError, e:
        sys.stderr.write("fork #2 failed: (%d) %s\n" % (e.errno, e.strerror))
        sys.exit(1)

    # The process is now daemonized, redirect standard file descriptors.
    sys.stdout.flush()
    sys.stderr.flush()

    si = file(stdin, 'r')
    so = file(stdout, 'a+')
    se = file(stderr, 'a+', 0)
    os.dup2(si.fileno(), sys.stdin.fileno())
    os.dup2(so.fileno(), sys.stdout.fileno())
    os.dup2(se.fileno(), sys.stderr.fileno())


if __name__ == '__main__':
    daemonize(stdout='sample_file', stderr='sample')
    print("hello world, now should be the child!")


[andrea@andreacrotti experiments]$ python2 daemon.py
Traceback (most recent call last):
  File "daemon.py", line 49, in <module>
    daemonize(stdout='sample_file', stderr='sample')
  File "daemon.py", line 41, in daemonize
    so = file(stdout, 'a+')
IOError: [Errno 13] Permission denied: 'sample_file'

The parent process can write to that file easily, but the child can't,
why is it working for you and not for me though?
(Running this on Linux with a non-root user)