| // Copyright (c) 2014, Jason Choy <[email protected]> |
| // 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. |
| |
| #pragma once |
| |
| // 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" |
| |
| KJ_BEGIN_HEADER |
| |
| namespace kj { |
| |
| #if __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. |
| // |
| // TODO(cleanup): The above comment was written many years ago. Is it still true? Shouldn't the |
| // compiler be smart enough to optimize a thread_local of POD type? |
| |
| #else |
| |
| #define KJ_THREADLOCAL_PTR(type) static thread_local type* |
| |
| #endif // KJ_USE_PTHREAD_TLS |
| |
| } // namespace kj |
| |
| KJ_END_HEADER |
