If a Unix (Posix) process receives a signal, a signal handler will run.

What will happen to it in a multithreaded process? Which thread receives the signal?

In my opinion, the signal API should be extended to handle that (i.e. the thread of the signal handler should be able to be determined), but hunting for infos on the net I only found year long flames on the linux kernel mailing list and on different forums. As I understood, Linus' concept differed from the Posix standard, and first some compat layer was built, but now the Linux follows the posix model.

What is the current state?

  • 3
    Duplicate of stackoverflow.com/questions/11679568/… "pthreads(7) describes that POSIX.1 requires all threads in a process share attributes, including signal dispositions" – steve Aug 26 '15 at 19:17
  • @steve Thanks, but 1) it is on another SE site 2) this spec doesn't specify clearly, what will exactly happen. What it means, the signal handlers will be called on all threads, but it seems a little bit surrealistic to me. 3) That answer doesn't specify what was Linus' model and why/how is it used currently. – peterh Aug 26 '15 at 19:52

The entry in POSIX on "Signal Generation and Delivery" in "Rationale: System Interfaces General Information" says

Signals generated for a process are delivered to only one thread. Thus, if more than one thread is eligible to receive a signal, one has to be chosen. The choice of threads is left entirely up to the implementation both to allow the widest possible range of conforming implementations and to give implementations the freedom to deliver the signal to the "easiest possible" thread should there be differences in ease of delivery between different threads.

From the signal(7) manual on a Linux system:

A signal may be generated (and thus pending) for a process as a whole (e.g., when sent using kill(2)) or for a specific thread (e.g., certain signals, such as SIGSEGV and SIGFPE, generated as a consequence of executing a specific machine-language instruction are thread directed, as are signals targeted at a specific thread using pthread_kill(3)). A process-directed signal may be delivered to any one of the threads that does not currently have the signal blocked. If more than one of the threads has the signal unblocked, then the kernel chooses an arbitrary thread to which to deliver the signal.

And in pthreads(7):

Threads have distinct alternate signal stack settings. However, a new thread's alternate signal stack settings are copied from the thread that created it, so that the threads initially share an alternate signal stack (fixed in kernel 2.6.16).

From the pthreads(3) manual on an OpenBSD system (as an example of an alternate approach):

Signals handlers are normally run on the stack of the currently executing thread.

(I'm currently not aware of how this is handled when multiple threads are executing concurrently on a multi-processor machine)

The older LinuxThread implementation of POSIX threads only allowed distinct single threads to be targeted by signals. From pthreads(7) on a Linux system:

LinuxThreads does not support the notion of process-directed signals: signals may be sent only to specific threads.

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