线程局部存储空间 pthread_key_t、__thread 即 ThreadLocal

https://www.jianshu.com/p/495ea7ce649b?utm_source=oschina-app

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__thread 修饰的变量

• __thread是GCC内置的线程局部存储设施,__thread变量每一个线程有一份独立实体，各个线程的值互不干扰。可以用来修饰那些带有全局性且值可能变，但是各线程独立不干扰的变量；
• 只能修饰POD类型(类似整型指针的标量)，不能修饰class类型，因为无法自动调用构造函数和析构函数;
• 可以用于修饰全局变量，函数内的静态变量，不能修饰函数的局部变量或者class的普通成员变量;
• 且__thread变量值只能初始化为编译器常量。

#include <pthread.h>
#include <cstdio>
#include <cstdlib>
#include <assert.h>
#include <stdint.h>

__thread uint64_t pkey = 0;

void run2( )
{
    FILE* fp = NULL;
    cout<<"thread1 run2  thread = "<<pkey<<endl;//有值了 
    
    if( !pkey )
    {
        char fName[128] = "";
        sprintf( fName, "thread%lu.log", static_cast<unsigned long>( pthread_self() ) );
        fp   = fopen( fName, "w" );
        pkey = reinterpret_cast<uint64_t>( fp ); 

    }else fp = reinterpret_cast<FILE*>( pkey );

    fprintf( fp, "hello __thread 2
" );
    return ;
}

void* run1( void* arg )
{
    FILE* fp = NULL;
    cout<<"thread1 run1  thread = "<<pkey<<endl;// 0
    
    if( !pkey )
    {
        char fName[128] = "";
        sprintf( fName, "thread%lu.log", static_cast<unsigned long>( pthread_self() ) );
        fp   = fopen( fName, "w" );
        pkey = reinterpret_cast<uint64_t>( fp ); 

    }else fp = reinterpret_cast<FILE*>( pkey );

    fprintf( fp, "hello __thread 1
" );

    run2();

    return NULL;
}

int main(int argc, char const *argv[])
{
    char fName[128] = "";
    sprintf( fName, "thread%lu.log", static_cast<unsigned long>( pthread_self() ) );
    FILE* fp = fopen( fName, "w" );
    cout<<"main  thread = "<<pkey<<endl;//0
    
    pkey = reinterpret_cast<uint64_t>( fp );
    fprintf( fp, "hello __thread
" );
    cout<<"main  thread  end = "<<pkey<<endl;///其他值

    pthread_t threads[2];
    pthread_create( &threads[0], NULL, run1, NULL );//开启线程
    pthread_create( &threads[1], NULL, run1, NULL );
    pthread_join( threads[0], NULL );
    pthread_join( threads[1], NULL );
    return 0;
}


main  thread = 0
main  thread  end = 11430944
thread1 run1  thread = 0
thread1 run2  thread = 140098611972288
thread1 run1  thread = 0
thread1 run2  thread = 140098611976960

一共创建了 3个文件  分别对应三个线程

pthread_key_t

pthread_key_t 优于 __thread 从下面几个方面来说：

• 依赖 linux 环境的 libpthread， 而非 gcc 编译器可移植性增强
• 如上所示，可以认为对每个 pthread_key, 库内部提供了一个 __thread void* 接受 pthread_setspecific 设置的指针，从而可以指向 class 类型
• pthread_key_t 可以作为函数的局部变量，也可以作为局部变量

#include <pthread.h> 
    // pthread_key_t, pthread_setspecific, pthread_getspecific, pthread_self
    // pthread_key_create, pthread_key_delete, pthread_create, pthread_join
#include <iostream>
#include <cstdio>
#include <cstdlib>

using namespace std;
 
static pthread_key_t pkt;
// 1, callback function to destroy resource associated with key
// 2, the in_param is pthread_getspecific()
// 3, gettid()是内核给线程（轻量级进程）分配的进程id，全局（所有进程中）唯一
// 4, pthread_self()是在用户态实现的，获取的id实际上是主线程分配给子线程的线程描述符的地址而已，只是在当前进程空间中是唯一的。
void destroy( void *arg )
{
    printf("exit at thread %d, fclose file 
", static_cast<int>( pthread_self() ) );
    if( arg ) fclose( reinterpret_cast<FILE*>(arg) );
}
// 5, pthread_getspecific() Return current value of the thread-specific data slot identified by KEY.
void writeLog( const char* log )
{
    FILE* logHandle = reinterpret_cast<FILE*>( pthread_getspecific( pkt) );
    fprintf( logHandle, "%s
", log );
}
// 6, pthread_setspecific Store POINTER in the thread-specific data slot identified by KEY 
void* work( void* arg)
{
    FILE* logHandle = NULL;
    char fileName[128] = "";
    sprintf( fileName, "Thread%d.log", static_cast<int>(pthread_self()) );
    logHandle = fopen( fileName, "w");
    pthread_setspecific( pkt, reinterpret_cast<void*>( logHandle ) );
    writeLog( "Thread starting." );
}
// 7, pthread_key_create( &pkt, destroy ) Create a key value identifying a location in the thread-specific　　　　　　//identifying 识别
//    data area. Each thread maintains a distinct thread-specific data area.
//    the destroy callback function will called with the key is dectroyed
// 8, pthread_key_delete( ) detroy the key use callback function clear the resource
int main(int argc, char const *argv[])
{
    pthread_key_create( &pkt, destroy );
    pthread_t pids[2] = {0};
    pthread_create( &pids[0], NULL, work, NULL );
    pthread_create( &pids[1], NULL, work, NULL );
    pthread_join( pids[0], NULL );
    pthread_join( pids[1], NULL );
    pthread_key_delete( pkt );
    printf("stop
");
    return 0;
}

　　

ThreadLocal

对 pthread_key_t 进行了 RAII 的封装，使用更加安全。

#include <pthread.h>
#include <boost/noncopyable.hpp>    // noncopyable
#include <boost/checked_delete.hpp> // check_delete
#include <cstdio>
#include <cstdlib>
#include <string>
#include <stdexcept>

template<typename T>
class ThreadLocal : public boost::noncopyable
{
    public:
    typedef ThreadLocal<T>* pThreadLocal;
    ThreadLocal()
    { pthread_key_create( &pkey_, &ThreadLocal::destroy ); }

    ~ThreadLocal()
    { pthread_key_delete( pkey_ ); }

    T& value()
    {
        T* pvalue = reinterpret_cast<T*>( pthread_getspecific( pkey_ ) );
        if( !pvalue )
        {
            T* obj = new T();
            pthread_setspecific( pkey_, reinterpret_cast<void*>( obj ) );
            pvalue = obj;
        }
        return *pvalue;
    }

    private:
    static void destroy( void* arg )
    { 
        T* obj = reinterpret_cast<T*>( arg );
        boost::checked_delete( obj );
    }

    pthread_key_t pkey_;
};

class Logger
{
    public:
    Logger()
    {
        char fName[128] = "";
        sprintf(  fName, "log_%lu.log", static_cast<unsigned long>( pthread_self() ) );
        fp = fopen( fName, "w" );
        if( !fp ) throw std::runtime_error( std::string("can not create ") + fName );
    }

    ~Logger() { fclose( fp ); }

    void log( const std::string& s ) { fprintf( fp, "%s
", s.c_str() ); }

    private:
    FILE* fp;
};

void* run( void* arg )
{
    auto ptllogger  = reinterpret_cast< ThreadLocal<Logger>::pThreadLocal>( arg);
    Logger& plogger = ptllogger->value();
    plogger.log( "Hello thread local" );
}

int main()
{
    ThreadLocal<Logger>::pThreadLocal p = new ThreadLocal<Logger>;
    Logger& plogger = p->value();
    plogger.log( "Hello thread local" );

    pthread_t threads[2] = {0};
    pthread_create( &threads[0], NULL, run, reinterpret_cast<void*>( p ) );
    pthread_create( &threads[1], NULL, run, reinterpret_cast<void*>( p ) );
    pthread_join( threads[0], NULL );
    pthread_join( threads[1], NULL );
    delete p;
}

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