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This is the backing feature to implement C++11 thread storage duration specified by the thread_local keyword. A destructor for given thread-local object is registered to be executed at the thread termination time using __cxa_thread_atexit(). Libc calls the __cxa_thread_calls_dtors() during exit(3), before finalizers and atexit functions, and libthr calls the function at the thread termination time, after the stack unwinding and thread-specific key destruction. There are several uncertainties in the API which lacks a formal specification. Among them: - is it allowed to register destructors during destructing; we allow, but limiting the nesting level. If too many iterations detected, a diagnostic is issued to stderr and thread forcibly terminates for now. - how to handle destructors which belong to an unloading dso; for now, we ignore destructor calls for such entries, and issue a diagnostic. Linux does prevent dso unload until all threads with destructors from the dso terminated. It is supposed that the diagnostics allow to detect real-world applications relying on the above details and possibly adjust our implementation. Right now the choices were to provide the slim API (but that rarely stands the practice test). Tests are added to check generic functionality and to specify some of the above implementation choices. Submitted by: Mahdi Mokhtari <mokhi64_gmail.com> Reviewed by: theraven Discussed with: dim (detection of -std=c++11 supoort for tests) Sponsored by: The FreeBSD Foundation (my involvement) MFC after: 2 weeks Differential revisions: https://reviews.freebsd.org/D7224, https://reviews.freebsd.org/D7427
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