{"id":103840,"date":"2020-06-05T07:00:00","date_gmt":"2020-06-05T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103840"},"modified":"2020-06-11T06:18:45","modified_gmt":"2020-06-11T13:18:45","slug":"20200605-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200605-00\/?p=103840","title":{"rendered":"Using fibers to expand a thread’s stack at runtime, part 4"},"content":{"rendered":"

Last time<\/a>, we transported some ephemeral error state from the temporary fiber to the originating thread. A common way of reporting an error in C++ is to use an exception, so let’s use that.<\/p>\n

[[noreturn]] void ThrowWin32Error(DWORD error);<\/span>\r\n\r\ntemplate<typename RetType>\r\nRetType RunOnFiberWorker(\r\n    RetType (*callback)(void*),\r\n    void* parameter)\r\n    \/\/ RetType failureValue<\/span><\/span>\r\n{\r\n  \/\/ static_assert(std::is_trivially_copyable_v<RetType>);<\/span>\r\n  \/\/ static_assert(std::is_trivially_destructible_v<RetType>);<\/span><\/span>\r\n\r\n  struct State\r\n  {\r\n    RetType (*callback)(void*);\r\n    void* parameter;\r\n    std::variant<std::exception_ptr, RetType>\r\n      capturedValue;<\/span>\r\n    HANDLE originalFiber;\r\n\r\n    void FiberProc()\r\n    {\r\n      try {\r\n        capturedValue.template emplace<1>\r\n          (callback(parameter));\r\n      } catch (...) {\r\n        capturedValue.template emplace<0>\r\n          (std::current_exception());\r\n      }<\/span>\r\n      SwitchToFiber(originalFiber);\r\n    }\r\n\r\n  } state{ callback, parameter };\r\n  \/\/ std::move(failureValue), errno<\/span><\/span>\r\n\r\n  DWORD error = RunOnFiberTypeNeutral(\r\n    [](void* parameter)\r\n    {\r\n      reinterpret_cast<State*>(parameter)->FiberProc();\r\n    }, &state, &state.originalFiber);\r\n  if (error != ERROR_SUCCESS) {\r\n    ThrowWin32Error(error);<\/span>\r\n  }\r\n\r\n  auto e = std::get_if<0>(&state.capturedValue);\r\n  if (e) std::rethrow_exception(*e);\r\n  return std::get<1>(std::move(state.capturedValue));<\/span>\r\n}\r\n\r\ntemplate<typename Lambda>\r\nauto<\/span> RunOnFiber(Lambda&& lambda)\r\n{\r\n  using Type = std::remove_reference_t<Lambda>;\r\n  using RetType = decltype(lambda());\r\n  return RunOnFiberWorker<RetType>(\r\n    [](void* parameter)\r\n    {\r\n      return (*reinterpret_cast<Type*>(parameter))();\r\n    }, &lambda);\r\n}\r\n<\/pre>\n
In this case, we capture the result of the callback with a std::variant<\/code> of an exception_ptr<\/code> or the formal return value. The exception_ptr<\/code> is used if the callback threw an exception.<\/p>\n
The exception_ptr<\/code> is the first type in the variant because the RetType<\/code> may not be default-constructible, and even if it has a default constructor, that default constructor may be heavy with unwanted side-effects. Putting the exception_ptr<\/code> as the first type in the variant means that the default constructor for the variant creates an exception_ptr<\/code>, which is default-constructible.<\/p>\n
We then ask Run\u00adOn\u00adFiber\u00adType\u00adNeutral<\/code> to do the fiber magic. If it failed, then we use some program-specific Throw\u00adWin32\u00adError()<\/code> function to transform the Win32 error into some kind of an exception.<\/p>\n
Otherwise, we know that the fiber ran to completion. In the fiber procedure, we call the callback and we save the result in the capturedValue<\/code> variable as the RetType<\/code>. If the callback threw an exception, we catch the exception and stow it in the capturedValue<\/code> as an exception_ptr<\/code>.<\/p>\n
After returning to the original thread, we keep inside the variant to see whether it holds an exception or a value. If it holds an exception, we rethrow it. Otherwise, we return the value.<\/p>\n
We must use the explicit index versions of variant::<\/code>emplace<\/code>, get<\/code>, and get_if<\/code> rather than the more readable type-based versions because the type-based version won’t work if the RetType<\/code> is exception_ptr<\/code>!<\/p>\n
Observe that the RetType<\/code> is always moved. There is no requirement that it be default-constructible or copyable.<\/p>\n
Note that the above code does not work if the lambda returns a reference. I’ll leave that as an exercise.<\/p>\n
Even if you don’t plan on working with fibers, this series showed how to transport state between threads, which is still useful.<\/p>\n
But wait, our discussion of using fibers to expand a stack dynamically isn’t over. We’ll pick up additional topics next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
Moving an exception from a fiber to the main thead.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-103840","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Moving an exception from a fiber to the main thead.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103840","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=103840"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103840\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=103840"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=103840"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=103840"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}