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For sigwait to work reliably, the signals being waited for must be blocked in all threads, not only in the calling thread, since otherwise the POSIX semantics for signal delivery do not guarantee that it's the thread doing the sigwait that will receive the signal. The best way to achieve this is block those signals before any threads are created, and never unblock them in the program other than by calling sigwait. Signal handling in LinuxThreads departs significantly from the POSIX standard. According to the standard, ``asynchronous'' (external) sig‐ nals are addressed to the whole process (the collection of all threads), which then delivers them to one particular thread. The thread that actually receives the signal is any thread that does not currently block the signal. In LinuxThreads, each thread is actually a kernel process with its own PID, so external signals are always directed to one particular thread. If, for instance, another thread is blocked in sigwait on that signal, it will not be restarted. The LinuxThreads implementation of sigwait installs dummy signal han‐ dlers for the signals in set for the duration of the wait. Since signal handlers are shared between all threads, other threads must not attach their own signal handlers to these signals, or alternatively they should all block these signals (which is recommended anyway -- see the Notes section).