mfbt/ThreadLocal.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* Cross-platform lightweight thread local data wrappers. */
   8
   9 #ifndef mozilla_ThreadLocal_h
  10 #define mozilla_ThreadLocal_h
  11
  12 #if defined(XP_WIN)
  13 // This file will get included in any file that wants to add a profiler mark.
  14 // In order to not bring <windows.h> together we could include windef.h and
  15 // winbase.h which are sufficient to get the prototypes for the Tls* functions.
  16 // # include <windef.h>
  17 // # include <winbase.h>
  18 // Unfortunately, even including these headers causes us to add a bunch of ugly
  19 // stuff to our namespace e.g #define CreateEvent CreateEventW
  20 extern "C" {
  21 __declspec(dllimport) void* __stdcall TlsGetValue(unsigned long);
  22 __declspec(dllimport) int __stdcall TlsSetValue(unsigned long, void*);
  23 __declspec(dllimport) unsigned long __stdcall TlsAlloc();
  24 }
  25 #else
  26 #  include <pthread.h>
  27 #  include <signal.h>
  28 #endif
  29
  30 #include "mozilla/Assertions.h"
  31 #include "mozilla/Attributes.h"
  32 #include "mozilla/NullPtr.h"
  33
  34 namespace mozilla {
  35
  36 // sig_safe_t denotes an atomic type which can be read or stored in a single
  37 // instruction.  This means that data of this type is safe to be manipulated
  38 // from a signal handler, or other similar asynchronous execution contexts.
  39 #if defined(XP_WIN)
  40 typedef unsigned long sig_safe_t;
  41 #else
  42 typedef sig_atomic_t sig_safe_t;
  43 #endif
  44
  45 /*
  46  * Thread Local Storage helpers.
  47  *
  48  * Usage:
  49  *
  50  * Only static-storage-duration (e.g. global variables, or static class members)
  51  * objects of this class should be instantiated. This class relies on
  52  * zero-initialization, which is implicit for static-storage-duration objects.
  53  * It doesn't have a custom default constructor, to avoid static initializers.
  54  *
  55  * API usage:
  56  *
  57  * // Create a TLS item.
  58  * //
  59  * // Note that init() should be invoked exactly once, before any usage of set()
  60  * // or get().
  61  * mozilla::ThreadLocal<int> tlsKey;
  62  * if (!tlsKey.init()) {
  63  *   // deal with the error
  64  * }
  65  *
  66  * // Set the TLS value
  67  * tlsKey.set(123);
  68  *
  69  * // Get the TLS value
  70  * int value = tlsKey.get();
  71  */
  72 template<typename T>
  73 class ThreadLocal
  74 {
  75 #if defined(XP_WIN)
  76   typedef unsigned long key_t;
  77 #else
  78   typedef pthread_key_t key_t;
  79 #endif
  80
  81   union Helper
  82   {
  83     void* mPtr;
  84     T mValue;
  85   };
  86
  87 public:
  88   MOZ_WARN_UNUSED_RESULT inline bool init();
  89
  90   inline T get() const;
  91
  92   inline void set(const T aValue);
  93
  94   bool initialized() const { return mInited; }
  95
  96 private:
  97   key_t mKey;
  98   bool mInited;
  99 };
 100
 101 template<typename T>
 102 inline bool
 103 ThreadLocal<T>::init()
 104 {
 105   static_assert(sizeof(T) <= sizeof(void*),
 106                 "mozilla::ThreadLocal can't be used for types larger than "
 107                 "a pointer");
 108   MOZ_ASSERT(!initialized());
 109 #ifdef XP_WIN
 110   mKey = TlsAlloc();
 111   mInited = mKey != 0xFFFFFFFFUL; // TLS_OUT_OF_INDEXES
 112 #else
 113   mInited = !pthread_key_create(&mKey, nullptr);
 114 #endif
 115   return mInited;
 116 }
 117
 118 template<typename T>
 119 inline T
 120 ThreadLocal<T>::get() const
 121 {
 122   MOZ_ASSERT(initialized());
 123   Helper h;
 124 #ifdef XP_WIN
 125   h.mPtr = TlsGetValue(mKey);
 126 #else
 127   h.mPtr = pthread_getspecific(mKey);
 128 #endif
 129   return h.mValue;
 130 }
 131
 132 template<typename T>
 133 inline void
 134 ThreadLocal<T>::set(const T aValue)
 135 {
 136   MOZ_ASSERT(initialized());
 137   Helper h;
 138   h.mValue = aValue;
 139 #ifdef XP_WIN
 140   bool succeeded = TlsSetValue(mKey, h.mPtr);
 141 #else
 142   bool succeeded = !pthread_setspecific(mKey, h.mPtr);
 143 #endif
 144   if (!succeeded) {
 145     MOZ_CRASH();
 146   }
 147 }
 148
 149 } // namespace mozilla
 150
 151 #endif /* mozilla_ThreadLocal_h */