// Copyright (c) 2014, Jason Choy <jjwchoy@gmail.com> // Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors // Licensed under the MIT License: // // Permission is hereby granted, free of charge, to any person obtaining a copy // of this software and associated documentation files (the "Software"), to deal // in the Software without restriction, including without limitation the rights // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell // copies of the Software, and to permit persons to whom the Software is // furnished to do so, subject to the following conditions: // // The above copyright notice and this permission notice shall be included in // all copies or substantial portions of the Software. // // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN // THE SOFTWARE. #ifndef KJ_THREADLOCAL_H_ #define KJ_THREADLOCAL_H_ #if defined(__GNUC__) && !KJ_HEADER_WARNINGS #pragma GCC system_header #endif // This file declares a macro `KJ_THREADLOCAL_PTR` for declaring thread-local pointer-typed // variables. Use like: // KJ_THREADLOCAL_PTR(MyType) foo = nullptr; // This is equivalent to: // thread_local MyType* foo = nullptr; // This can only be used at the global scope. // // AVOID USING THIS. Use of thread-locals is discouraged because they often have many of the same // properties as singletons: http://www.object-oriented-security.org/lets-argue/singletons // // Also, thread-locals tend to be hostile to event-driven code, which can be particularly // surprising when using fibers (all fibers in the same thread will share the same threadlocals, // even though they do not share a stack). // // That said, thread-locals are sometimes needed for runtime logistics in the KJ framework. For // example, the current exception callback and current EventLoop are stored as thread-local // pointers. Since KJ only ever needs to store pointers, not values, we avoid the question of // whether these values' destructors need to be run, and we avoid the need for heap allocation. #include "common.h" #if !defined(KJ_USE_PTHREAD_THREADLOCAL) && defined(__APPLE__) #include "TargetConditionals.h" #if TARGET_OS_IPHONE // iOS apparently does not support __thread (nor C++11 thread_local). #define KJ_USE_PTHREAD_TLS 1 #endif #endif #if KJ_USE_PTHREAD_TLS #include <pthread.h> #endif namespace kj { #if KJ_USE_PTHREAD_TLS // If __thread is unavailable, we'll fall back to pthreads. #define KJ_THREADLOCAL_PTR(type) \ namespace { struct KJ_UNIQUE_NAME(_kj_TlpTag); } \ static ::kj::_::ThreadLocalPtr< type, KJ_UNIQUE_NAME(_kj_TlpTag)> // Hack: In order to ensure each thread-local results in a unique template instance, we declare // a one-off dummy type to use as the second type parameter. namespace _ { // private template <typename T, typename> class ThreadLocalPtr { // Hacky type to emulate __thread T*. We need a separate instance of the ThreadLocalPtr template // for every thread-local variable, because we don't want to require a global constructor, and in // order to initialize the TLS on first use we need to use a local static variable (in getKey()). // Each template instance will get a separate such local static variable, fulfilling our need. public: ThreadLocalPtr() = default; constexpr ThreadLocalPtr(decltype(nullptr)) {} // Allow initialization to nullptr without a global constructor. inline ThreadLocalPtr& operator=(T* val) { pthread_setspecific(getKey(), val); return *this; } inline operator T*() const { return get(); } inline T& operator*() const { return *get(); } inline T* operator->() const { return get(); } private: inline T* get() const { return reinterpret_cast<T*>(pthread_getspecific(getKey())); } inline static pthread_key_t getKey() { static pthread_key_t key = createKey(); return key; } static pthread_key_t createKey() { pthread_key_t key; pthread_key_create(&key, 0); return key; } }; } // namespace _ (private) #elif __GNUC__ #define KJ_THREADLOCAL_PTR(type) static __thread type* // GCC's __thread is lighter-weight than thread_local and is good enough for our purposes. #else #define KJ_THREADLOCAL_PTR(type) static thread_local type* #endif // KJ_USE_PTHREAD_TLS } // namespace kj #endif // KJ_THREADLOCAL_H_