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  * mozilla::ThreadLocal<int> tlsKey;
  59  * if (!tlsKey.init()) {
  60  *   // deal with the error
  61  * }
  62  *
  63  * // Set the TLS value
  64  * tlsKey.set(123);
  65  *
  66  * // Get the TLS value
  67  * int value = tlsKey.get();
  68  */
  69 template<typename T>
  70 class ThreadLocal
  71 {
  72 #if defined(XP_WIN)
  73     typedef unsigned long key_t;
  74 #else
  75     typedef pthread_key_t key_t;
  76 #endif
  77
  78     union Helper {
  79       void* ptr;
  80       T value;
  81     };
  82
  83   public:
  84     MOZ_WARN_UNUSED_RESULT inline bool init();
  85
  86     inline T get() const;
  87
  88     inline void set(const T value);
  89
  90     bool initialized() const {
  91       return inited;
  92     }
  93
  94   private:
  95     key_t key;
  96     bool inited;
  97 };
  98
  99 template<typename T>
 100 inline bool
 101 ThreadLocal<T>::init()
 102 {
 103   static_assert(sizeof(T) <= sizeof(void*),
 104                 "mozilla::ThreadLocal can't be used for types larger than "
 105                 "a pointer");
 106   MOZ_ASSERT(!initialized());
 107 #ifdef XP_WIN
 108   key = TlsAlloc();
 109   inited = key != 0xFFFFFFFFUL; // TLS_OUT_OF_INDEXES
 110 #else
 111   inited = !pthread_key_create(&key, nullptr);
 112 #endif
 113   return inited;
 114 }
 115
 116 template<typename T>
 117 inline T
 118 ThreadLocal<T>::get() const
 119 {
 120   MOZ_ASSERT(initialized());
 121   Helper h;
 122 #ifdef XP_WIN
 123   h.ptr = TlsGetValue(key);
 124 #else
 125   h.ptr = pthread_getspecific(key);
 126 #endif
 127   return h.value;
 128 }
 129
 130 template<typename T>
 131 inline void
 132 ThreadLocal<T>::set(const T value)
 133 {
 134   MOZ_ASSERT(initialized());
 135   Helper h;
 136   h.value = value;
 137   bool succeeded;
 138 #ifdef XP_WIN
 139   succeeded = TlsSetValue(key, h.ptr);
 140 #else
 141   succeeded = !pthread_setspecific(key, h.ptr);
 142 #endif
 143   if (!succeeded)
 144     MOZ_CRASH();
 145 }
 146
 147 } // namespace mozilla
 148
 149 #endif /* mozilla_ThreadLocal_h */