File descriptors redirecting is stuck

Question

In the daemon class example, which I implement, is used descriptors redirecting. ``` sys.stdout.flush() sys.stderr.flush() si = file(self.stdin, 'r') so = file(self.stdout, 'a+') se = file(self.stderr, 'a+', 0) os.dup2(si.fileno(), sys.stdin.fileno()) os.dup2(so.fileno(), sys.stdout.fileno()) # This line doesn't work os.dup2(se.fileno(), sys.stderr.fileno()) ``` `os.dup2(so.fileno(), sys.stdout.fileno())` is not working. It doesn't raise errors. The code after this line is not executed. I simplified this example to the class contains only the problem area: ``` class Deamon(object): def __init__(self, pidfile, stdout='/dev/null'): self.pidfile = pidfile self.stdout = stdout def get_stdout(self): so = file(self.stdout, 'a+') os.dup2(so.fileno(), sys.stdout.fileno()) print 'executed' ``` After `os.dup2(so.fileno(), sys.stdout.fileno())` code just stuck. Why is it happening? Edit (with @C2H5OH code implementation): ``` try: pid = os.fork() if pid > 0: # exit first parent sys.exit(0) excep…

Accepted Answer

You are mixing Python file handles with the operating system file descriptors, this is asking for trouble.

While you are correct on using `os.dup2()` to redirect `sys.stdout` (because direct assignment may not completely work, if other modules acquired a reference to it), you should open files at the operating system level with `os.open()`.

Here's the daemonization code we use at my workplace:

```
if os.fork() > 0:
    os._exit(0)
os.setsid()
sys.stdin.flush()
sys.stdout.flush()
sys.stderr.flush()
null = os.open(os.devnull, os.O_RDWR)
os.dup2(null, sys.stdin.fileno())
os.dup2(null, sys.stdout.fileno())
os.dup2(null, sys.stderr.fileno())
os.close(null)
```

If you want to redirect stdout to some file, just use a different file name instead of the predefined constant `os.devnull`.

File descriptors redirecting is stuck

Solution

Problem