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1 // Copyright (c) 2014, Jason Choy <jjwchoy@gmail.com>
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2 // Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
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3 // Licensed under the MIT License:
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4 //
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5 // Permission is hereby granted, free of charge, to any person obtaining a copy
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6 // of this software and associated documentation files (the "Software"), to deal
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7 // in the Software without restriction, including without limitation the rights
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8 // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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9 // copies of the Software, and to permit persons to whom the Software is
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10 // furnished to do so, subject to the following conditions:
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11 //
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12 // The above copyright notice and this permission notice shall be included in
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13 // all copies or substantial portions of the Software.
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14 //
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15 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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16 // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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17 // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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18 // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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19 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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20 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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21 // THE SOFTWARE.
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22
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23 #pragma once
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24
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25 // This file declares a macro `KJ_THREADLOCAL_PTR` for declaring thread-local pointer-typed
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26 // variables. Use like:
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27 // KJ_THREADLOCAL_PTR(MyType) foo = nullptr;
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28 // This is equivalent to:
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29 // thread_local MyType* foo = nullptr;
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30 // This can only be used at the global scope.
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31 //
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32 // AVOID USING THIS. Use of thread-locals is discouraged because they often have many of the same
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33 // properties as singletons: http://www.object-oriented-security.org/lets-argue/singletons
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34 //
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35 // Also, thread-locals tend to be hostile to event-driven code, which can be particularly
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36 // surprising when using fibers (all fibers in the same thread will share the same threadlocals,
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37 // even though they do not share a stack).
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38 //
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39 // That said, thread-locals are sometimes needed for runtime logistics in the KJ framework. For
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40 // example, the current exception callback and current EventLoop are stored as thread-local
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41 // pointers. Since KJ only ever needs to store pointers, not values, we avoid the question of
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42 // whether these values' destructors need to be run, and we avoid the need for heap allocation.
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43
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44 #include "common.h"
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45
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46 KJ_BEGIN_HEADER
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47
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48 namespace kj {
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49
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50 #if __GNUC__
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51
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52 #define KJ_THREADLOCAL_PTR(type) static __thread type*
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53 // GCC's __thread is lighter-weight than thread_local and is good enough for our purposes.
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54 //
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55 // TODO(cleanup): The above comment was written many years ago. Is it still true? Shouldn't the
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56 // compiler be smart enough to optimize a thread_local of POD type?
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57
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58 #else
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59
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60 #define KJ_THREADLOCAL_PTR(type) static thread_local type*
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61
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62 #endif // KJ_USE_PTHREAD_TLS
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63
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64 } // namespace kj
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65
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66 KJ_END_HEADER
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