Question about threads and signals
Dear POSIX thread experts (i.e. Ingo and Ulrich),
I have a teensy question. Using the new Linux 2.5 threads + glibc/NPTL:
1. If a signal is delivered to a thread, is it masked for the duration of the handler in (a) just that thread or (b) all threads?
In other words, if I have 3 threads and SIGIO is not blocked in any of them, is it possible for my SIGIO handler to be called up to 3 times concurrently? Or is the blocked mask somehow shared?
Is the same thing true of SIGCHLD? SIGSEGV?
2. Is this true of POSIX threads in general, or just Linux?
Thanks,
-- Jamie
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Quoted 2 lines · Jamie Lokier
1. If a signal is delivered to a thread, is it masked for the duration of the handler in (a) just that thread or (b) all threads?
(a)
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In other words, if I have 3 threads and SIGIO is not blocked in any of them, is it possible for my SIGIO handler to be called up to 3 times concurrently? Or is the blocked mask somehow shared?
Masks are never shared.
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Is the same thing true of SIGCHLD? SIGSEGV?
Yes. Up to the point where a fatal signal isn't caught and the process is killed. At that point all threads except the one responsible for the termination is stopped and then terminated.
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2. Is this true of POSIX threads in general, or just Linux?
Well, the above is what POSIX requires and what I think we've implemented. These requirements are essential for programs which do much of their work in signal handlers. Creating more threads which mainly just sit around but can react to signals is a valid programming model.
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Quoted 5 lines · Ulrich Drepper
Quoted 1 line
2. Is this true of POSIX threads in general, or just Linux?
Well, the above is what POSIX requires and what I think we've implemented. These requirements are essential for programs which do much of their work in signal handlers.
So, in a program which uses pthread_create(), a SIGCHLD handler must either (a) be reentrant, or (b) be unblocked (or installed) in one thread only.
Quoted 2 lines
Creating more threads which mainly just sit around but can react to signals is a valid programming model.
Indeed, as Ingo pointed out, some signals are like poor man's threads, and that programming model makes that explicit. I like that, because it means the signal handler can validly do everything normal code can do, including use mutexes and other locking primitives which may sleep.
(Some other signals are intrinsically thread-local though, such as SIGSEGV and SIGFPE - they are synchronous exception handlers rather than asynchronous event handlers).
Thanks for your quick answer,
-- Jamie