# ``twisted`.`internet`.`_signals``

module documentation
(source)

This module is used to integrate child process termination into a reactor event loop. This is a challenging feature to provide because most platforms indicate process termination via SIGCHLD and do not provide a way to wait for that signal and arbitrary I/O events at the same time. The naive implementation involves installing a Python SIGCHLD handler; unfortunately this leads to other syscalls being interrupted (whenever SIGCHLD is received) and failing with EINTR (which almost no one is prepared to handle). This interruption can be disabled via siginterrupt(2) (or one of the equivalent mechanisms); however, if the SIGCHLD is delivered by the platform to a non-main thread (not a common occurrence, but difficult to prove impossible), the main thread (waiting on select() or another event notification API) may not wake up leading to an arbitrary delay before the child termination is noticed.

The basic solution to all these issues involves enabling SA_RESTART (ie, disabling system call interruption) and registering a C signal handler which writes a byte to a pipe. The other end of the pipe is registered with the event loop, allowing it to wake up shortly after SIGCHLD is received. See `twisted.internet.posixbase._SIGCHLDWaker` for the implementation of the event loop side of this solution. The use of a pipe this way is known as the self-pipe trick.

From Python version 2.6, `signal.siginterrupt` and `signal.set_wakeup_fd` provide the necessary C signal handler which writes to the pipe to be registered with `SA_RESTART`.

Function | `installHandler` | Install a signal handler which will write a byte to `fd` when SIGCHLD is received.
Function | `isDefaultHandler` | Determine whether the SIGCHLD handler is the default or not.

def installHandler(fd): (source)

Install a signal handler which will write a byte to `fd` when SIGCHLD is received.

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

Parameters
fd:`int` | The file descriptor to which to write when SIGCHLD is received.

def isDefaultHandler(): (source)
Determine whether the SIGCHLD handler is the default or not.