Question

The Linux Programming Interface Book has mentioned a method for dealing with asynchronous signals in a multi threaded program:

• All threads block all of the asynchronous signals that the process
  might receive. The simplest way to do
  this is to block the signals in the
  main thread before any other thread
  are created. Each subsequently created
  thread will inherit a copy of the main
  thread’s signal mask.
• create a single dedicated thread that accepts incoming signals using
  sigwaitinfo(), sigtimedwait() or
  sigwait().

The advantage of this approach is that
asynchronously generated signals are
received synchronously. As it accepts
incoming signals, the dedicated thread
can safely modify shared variables
(under mutex control) and call
non-async-safe functions. It can also
signal condition variables, and emply
other thread and process communication
and synchronization mechanisms.

Now the questions:

1. when kernel wants to deliver signals it choose one of the threads inside process arbitrary. from where it can know to deliver signal to the dedicated thread?
2. pthread API is non-aync-safe functions. so how we can use them inside signal handler?

Answer 1

When the kernel delivers a process-directed signal, it chooses one of the threads that does not have the signal blocked. This means that it never chooses any of the threads apart from the signal-handling thread (which acts like it has the signal unblocked while it is blocked in sigwaitinfo() or similar). In other words: the kernel knows where to deliver the signal, because you have arranged things such that the signal-handling thread is the only thread that is ever allowed to deliver the signal to.

You do not use the pthreads API, or any non-async-signal-safe functions in a signal handler. The solution outlined does not handle the signals within signal handlers – it handles the signals within the normal execution flow of the signal-handling thread, after sigwaitinfo() returns. This allows it to access non-async-signal-safe functions, which is the whole point.