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Signals and Threads

Q: I have a multi-threaded process on Mac OS X. If my process receives a signal, which thread executes the signal handler?

A: Synchronous signals, those caused by the thread itself (like SIGPIPE and SIGBUS), are delivered to the thread that caused them. Asynchronous signals (signals sent to the process by an external source) are delivered to an arbitrary thread within the process.

At the time of writing, Mac OS X chooses the thread to execute asynchronous signal handlers using the following criteria.

  • The thread must still be running (that is, not terminated).
  • On Mac OS X 10.2 and later, the thread must not have the signal masked (using pthread_sigmask).
  • An older thread is preferred to newer thread.

Thus an asynchronous signal is sent to the oldest non-terminated thread that does not have it masked. Do not rely on this exact behaviour. The thread chosen to handle an asynchronous signal is not guaranteed (except in the case of pthread_kill, described below) and could change an future systems.

Q: I have a multi-threaded process on Mac OS X. If one thread receives a signal, will the other threads stop?

A: No. All other threads continue to run during signal handler invocation.

Q: Has there been any change in thread signal handling in Mac OS X 10.2?

A: The above answers still hold. The most significant changes in thread signal handling in Mac OS X 10.2 are that you can now send an asynchronous signal to a specific thread within your process (pthread_kill), set a per-thread signal mask (pthread_sigmask), and wait for a specific signal (sigwait).

Q: If one of my threads is waiting in sigwait, is it guaranteed to be the thread that executes an asynchronous signal handler?

A: No. The choice of thread to execute an asynchronous signal handler is described above, and it is not affected by calls to sigwait. This is in line with POSIX standard. The only way to guarantee that a specific thread will handle an asynchronous signal is to mask that signal in all other threads.

[Aug 27 2002]

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