Check if a process with a given pid is running or not?

[selfboot]

In daemon.py/daemon_stop, there is snippet as follows:

try:
    # query for the pid
    os.kill(pid, 0)
except OSError as e:
    if e.errno == errno.ESRCH:
        break

The usual way to check if a process is still running is to kill() it with signal '0'. It does nothing to a running job and raises an OSError exception with errno=ESRCH if the process does not exist. And kill(pid, 0) can also raise EPERM (access denied) in which case that obviously means a process exists.

def pid_exists(pid):
    """Check whether pid exists in the current process table.  UNIX only.
    """
    if pid < 0:
        return False
    if pid == 0:
        # According to "man 2 kill" PID 0 refers to every process
        # in the process group of the calling process.
        # On certain systems 0 is a valid PID but we have no way to know that in a portable fashion.
        raise ValueError('invalid PID 0')
    try:
        os.kill(pid, 0)
    except OSError as err:
        if err.errno == errno.ESRCH:
            # ESRCH == No such process
            return False
        elif err.errno == errno.EPERM:
            # EPERM clearly means there's a process to deny access to
            return True
        else:
            # According to "man 2 kill" possible error values are (EINVAL, EPERM, ESRCH)
            raise
    else:
        return True

Note: os.kill(pid, 0) can only work on linux and unix, but not windows. Here is the issue about why os.kill on Windows should not accept zero as signal.

Ref

How to check if there exists a process with a given pid in Python?
What is the easiest way to see if a process with a given pid exists in Python?
os.kill on Windows should accept zero as signal