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Based on the Windows CreateThread API [1], it doesn't say anything about memory synchronization guarantee. Does it do internally? [1] https://learn.microsoft.

by ww520 3y ago

Based on the Windows CreateThread API [1], it doesn't say anything about memory synchronization guarantee. Does it do internally? [1] https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createthread https://learn.microsoft.com/en-us/windows/win32/api/processt...

cesarb 3y ago
That MSDN documentation is unfortunately silent on this, but the example in the documentation (at https://learn.microsoft.com/en-us/windows/win32/procthread/creating-threads https://learn.microsoft.com/en-us/windows/win32/procthread/c...) only makes sense if the operating system guarantees the ordering. The C++ standard (at least a draft of it I found on a quick web search) is more explicit: it says (https://eel.is/c++draft/thread.thread.constr https://eel.is/c++draft/thread.thread.constr) "The completion of the invocation of the constructor synchronizes with the beginning of the invocation of the copy of f." (see https://eel.is/c++draft/intro.races https://eel.is/c++draft/intro.races for more detail on that "synchronizes with"). Since the code in question is using std::thread, even if the operating system did not have the relevant guarantees, the C++ standard library would have the required memory barriers.
- tom_ 3y ago
  Any time somebody tells you how simple C is by comparison, point them at https://port70.net/~nsz/c/c11/n1570.html#6.2.4p5 https://port70.net/~nsz/c/c11/n1570.html#6.2.4p5: > An object whose identifier is declared with no linkage and without the storage-class specifier static has automatic storage duration, as do some compound literals. The result of attempting to indirectly access an object with automatic storage duration from a thread other than the one with which the object is associated is implementation-defined (I bet in practice almost all implementations behave as an equivalent C++ would, as per your notes, so only the ordering is relevant. But people maintaining C implementations have on occasion shown themselves to be their users' enemies, so don't quote me on this!)

by ww520 3y ago

cesarb 3y ago

That MSDN documentation is unfortunately silent on this, but the example in the documentation (at https://learn.microsoft.com/en-us/windows/win32/procthread/creating-threads https://learn.microsoft.com/en-us/windows/win32/procthread/c...) only makes sense if the operating system guarantees the ordering. The C++ standard (at least a draft of it I found on a quick web search) is more explicit: it says (https://eel.is/c++draft/thread.thread.constr https://eel.is/c++draft/thread.thread.constr) "The completion of the invocation of the constructor synchronizes with the beginning of the invocation of the copy of f." (see https://eel.is/c++draft/intro.races https://eel.is/c++draft/intro.races for more detail on that "synchronizes with"). Since the code in question is using std::thread, even if the operating system did not have the relevant guarantees, the C++ standard library would have the required memory barriers.

tom_ 3y ago
Any time somebody tells you how simple C is by comparison, point them at https://port70.net/~nsz/c/c11/n1570.html#6.2.4p5 https://port70.net/~nsz/c/c11/n1570.html#6.2.4p5: > An object whose identifier is declared with no linkage and without the storage-class specifier static has automatic storage duration, as do some compound literals. The result of attempting to indirectly access an object with automatic storage duration from a thread other than the one with which the object is associated is implementation-defined (I bet in practice almost all implementations behave as an equivalent C++ would, as per your notes, so only the ordering is relevant. But people maintaining C implementations have on occasion shown themselves to be their users' enemies, so don't quote me on this!)