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