18muduo_base库源码分析（九）

1.线程特定数据

(1)在单线程程序中，我们经常要用到"全局变量"以实现多个函数间共享数据。

(2)在多线程环境下，由于数据空间是共享的，因此全局变量也为所有线程所共有。

(3)但有时应用程序设计中有必要提供线程私有的全局变量，仅在某个线程中有效，但却可以跨多个函数访问。

(4)POSIX线程库通过维护一定的数据结构来解决这个问题，这个些数据称为（Thread-specific Data，或 TSD）。

(5)线程特定数据也称为线程本地存储TLS（Thread-local storage）

(6)对于POD类型的线程本地存储，可以用__thread关键字

2.ThreadLocal<T>类图

3.代码

ThreadLocal.h

// Use of this source code is governed by a BSD-style license// that can be found in the License file.//// Author: Shuo Chen (chenshuo at chenshuo dot com)#ifndef MUDUO_BASE_THREADLOCAL_H#define MUDUO_BASE_THREADLOCAL_H#include <boost/noncopyable.hpp>#include <pthread.h>namespace muduo{template<typename T>class ThreadLocal : boost::noncopyable{ public:  ThreadLocal()  {    pthread_key_create(&pkey_, &ThreadLocal::destructor);  }  ~ThreadLocal()  {    pthread_key_delete(pkey_);  }  T& value()  {    T* perThreadValue = static_cast<T*>(pthread_getspecific(pkey_));    if (!perThreadValue) {      T* newObj = new T();      pthread_setspecific(pkey_, newObj);      perThreadValue = newObj;    }    return *perThreadValue;  } private:  static void destructor(void *x)  {    T* obj = static_cast<T*>(x);    typedef char T_must_be_complete_type[sizeof(T) == 0 ? -1 : 1];    delete obj;  } private:  pthread_key_t pkey_;};}#endif

ThreadLocal_test.cc

#include <muduo/base/ThreadLocal.h>#include <muduo/base/CurrentThread.h>#include <muduo/base/Thread.h>#include <boost/noncopyable.hpp>#include <stdio.h>class Test : boost::noncopyable{ public:  Test()  {    printf("tid=%d, constructing %p\n", muduo::CurrentThread::tid(), this);  }  ~Test()  {    printf("tid=%d, destructing %p %s\n", muduo::CurrentThread::tid(), this, name_.c_str());  }  const std::string& name() const { return name_; }  void setName(const std::string& n) { name_ = n; } private:  std::string name_;};muduo::ThreadLocal<Test> testObj1;muduo::ThreadLocal<Test> testObj2;void print(){  printf("tid=%d, obj1 %p name=%s\n",         muduo::CurrentThread::tid(), &testObj1.value(),         testObj1.value().name().c_str());  printf("tid=%d, obj2 %p name=%s\n",         muduo::CurrentThread::tid(), &testObj2.value(),         testObj2.value().name().c_str());}void threadFunc(){  print();  testObj1.value().setName("changed 1");  testObj2.value().setName("changed 42");  print();}int main(){  testObj1.value().setName("main one");  print();  muduo::Thread t1(threadFunc);  t1.start();  t1.join();  testObj2.value().setName("main two");  print();  pthread_exit(0);

运行结果

SingletonThreadLocal_test.cc

#include <muduo/base/Singleton.h>#include <muduo/base/CurrentThread.h>#include <muduo/base/ThreadLocal.h>#include <muduo/base/Thread.h>#include <boost/bind.hpp>#include <boost/noncopyable.hpp>#include <stdio.h>class Test : boost::noncopyable{ public:  Test()  {    printf("tid=%d, constructing %p\n", muduo::CurrentThread::tid(), this);  }  ~Test()  {    printf("tid=%d, destructing %p %s\n", muduo::CurrentThread::tid(), this, name_.c_str());  }  const std::string& name() const { return name_; }  void setName(const std::string& n) { name_ = n; } private:  std::string name_;};#define STL muduo::Singleton<muduo::ThreadLocal<Test> >::instance().value()void print(){  printf("tid=%d, %p name=%s\n",         muduo::CurrentThread::tid(),         &STL,         STL.name().c_str());}void threadFunc(const char* changeTo){  print();  STL.setName(changeTo);  sleep(1);  print();}int main(){  STL.setName("main one");  muduo::Thread t1(boost::bind(threadFunc, "thread1"));  muduo::Thread t2(boost::bind(threadFunc, "thread2"));  t1.start();  t2.start();  t1.join();  print();  t2.join();  pthread_exit(0);}

运行结果：Threadlocal<Test>类型是单例，muduo::Singletion<muduo::Threadlocal<Test>>::instance，每一个线程中只会有一个Test类的对象（因为是单例，不像第一个例子一个线程中会存在两个Test类对象，不是每个线程只能有一个特定属性Threadlocal），但是每一个线程都会有Test类对象