def installHandler(fd):
    """
    Install a signal handler which will write a byte to C{fd} when
    I{SIGCHLD} is received.

    This is implemented by installing a SIGCHLD handler that does nothing,
    setting the I{SIGCHLD} handler as not allowed to interrupt system calls,
    and using L{signal.set_wakeup_fd} to do the actual writing.

    @param fd: The file descriptor to which to write when I{SIGCHLD} is
        received.
    @type fd: C{int}
    """
    if fd == -1:
        signal.signal(signal.SIGCHLD, signal.SIG_DFL)
    else:
        def noopSignalHandler(*args):
            pass
        signal.signal(signal.SIGCHLD, noopSignalHandler)
        signal.siginterrupt(signal.SIGCHLD, False)
    return signal.set_wakeup_fd(fd)

    Install a signal handler which will write a byte to C{fd} when
    I{SIGCHLD} is received.

    This is implemented by installing a SIGCHLD handler that does nothing,
    setting the I{SIGCHLD} handler as not allowed to interrupt system calls,
    and using L{signal.set_wakeup_fd} to do the actual writing.

    @param fd: The file descriptor to which to write when I{SIGCHLD} is
        received.
    @type fd: C{int}
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/_signals.py