comparison win64-msvc/include/kj/threadlocal.h @ 47:d93140aac40b

Current Capnp libs and headers from git

+// Copyright (c) 2014, Jason Choy <[e-mail]>
+// 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_