jpayne@69: // Copyright (c) 2014, Jason Choy <jjwchoy@gmail.com>
jpayne@69: // Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
jpayne@69: // Licensed under the MIT License:
jpayne@69: //
jpayne@69: // Permission is hereby granted, free of charge, to any person obtaining a copy
jpayne@69: // of this software and associated documentation files (the "Software"), to deal
jpayne@69: // in the Software without restriction, including without limitation the rights
jpayne@69: // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
jpayne@69: // copies of the Software, and to permit persons to whom the Software is
jpayne@69: // furnished to do so, subject to the following conditions:
jpayne@69: //
jpayne@69: // The above copyright notice and this permission notice shall be included in
jpayne@69: // all copies or substantial portions of the Software.
jpayne@69: //
jpayne@69: // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
jpayne@69: // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
jpayne@69: // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
jpayne@69: // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
jpayne@69: // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
jpayne@69: // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
jpayne@69: // THE SOFTWARE.
jpayne@69: 
jpayne@69: #pragma once
jpayne@69: 
jpayne@69: // This file declares a macro `KJ_THREADLOCAL_PTR` for declaring thread-local pointer-typed
jpayne@69: // variables.  Use like:
jpayne@69: //     KJ_THREADLOCAL_PTR(MyType) foo = nullptr;
jpayne@69: // This is equivalent to:
jpayne@69: //     thread_local MyType* foo = nullptr;
jpayne@69: // This can only be used at the global scope.
jpayne@69: //
jpayne@69: // AVOID USING THIS.  Use of thread-locals is discouraged because they often have many of the same
jpayne@69: // properties as singletons: http://www.object-oriented-security.org/lets-argue/singletons
jpayne@69: //
jpayne@69: // Also, thread-locals tend to be hostile to event-driven code, which can be particularly
jpayne@69: // surprising when using fibers (all fibers in the same thread will share the same threadlocals,
jpayne@69: // even though they do not share a stack).
jpayne@69: //
jpayne@69: // That said, thread-locals are sometimes needed for runtime logistics in the KJ framework.  For
jpayne@69: // example, the current exception callback and current EventLoop are stored as thread-local
jpayne@69: // pointers.  Since KJ only ever needs to store pointers, not values, we avoid the question of
jpayne@69: // whether these values' destructors need to be run, and we avoid the need for heap allocation.
jpayne@69: 
jpayne@69: #include "common.h"
jpayne@69: 
jpayne@69: KJ_BEGIN_HEADER
jpayne@69: 
jpayne@69: namespace kj {
jpayne@69: 
jpayne@69: #if __GNUC__
jpayne@69: 
jpayne@69: #define KJ_THREADLOCAL_PTR(type) static __thread type*
jpayne@69: // GCC's __thread is lighter-weight than thread_local and is good enough for our purposes.
jpayne@69: //
jpayne@69: // TODO(cleanup): The above comment was written many years ago. Is it still true? Shouldn't the
jpayne@69: //   compiler be smart enough to optimize a thread_local of POD type?
jpayne@69: 
jpayne@69: #else
jpayne@69: 
jpayne@69: #define KJ_THREADLOCAL_PTR(type) static thread_local type*
jpayne@69: 
jpayne@69: #endif // KJ_USE_PTHREAD_TLS
jpayne@69: 
jpayne@69: }  // namespace kj
jpayne@69: 
jpayne@69: KJ_END_HEADER