Linux信号量编程实例

作者：呆若木鸡

    本例示范Linux信号量的基本用法。该范例使用了两个线程分别对一个公用队列进行入队和出队操作，并用信号量进行控制，当队列空时出队操作可以被阻塞，当队列满时入队操作可以被阻塞。

主要用到的信号量函数有：
sem_init：初始化信号量sem_t，初始化的时候可以指定信号量的初始值，以及是否可以在多进程间共享。
sem_wait：一直阻塞等待直到信号量>0。
sem_timedwait：阻塞等待若干时间直到信号量>0。
sem_post：使信号量加1。
sem_destroy：释放信号量。和sem_init对应。
关于各函数的具体参数请用man查看。如man sem_init可查看该函数的帮助。

下面看具体的代码：

// --------------------------msgdequeue.h开始-------------------------------------
// 实现可控队列
#ifndef MSGDEQUEUE_H
#define  MSGDEQUEUE_H
#include  " tmutex.h "
#include  < iostream >
#include  < errno.h >
#include  < time.h >
#include  < semaphore.h >
#include  < deque >
using   namespace  std;

template < typename T,typename MUTEX_TYPE  =  ThreadMutex >
class  CMessageDequeue
... {
public:
        CMessageDequeue(size_t MaxSize) : m_MaxSize( MaxSize )
        ...{
                sem_init( &m_enques,0, m_MaxSize ); //入队信号量初始化为MaxSize，最多可容纳MaxSize各元素
                sem_init( &m_deques,0,0 ); //队列刚开始为空，出队信号量初始为0
        }

        ~CMessageDequeue()
        ...{
                sem_destroy(&m_enques);
                sem_destroy(&m_deques);
        }

        int sem_wait_i( sem_t *psem, int mswait )
        ...{//等待信号量变成>0，mswait为等待时间，若mswait<0则无穷等待，否则等待若干mswait毫秒。
                if( mswait < 0 )
                ...{
                        int rv = 0;                          
                        while( ((rv = sem_wait(psem) ) != 0 ) && (errno == EINTR
) );    //等待信号量，errno==EINTR屏蔽其他信号事件引起的等待中断
                        return rv;    
                }                                            
                else                                         
                ...{                                            
                        timespec ts;                         
                        clock_gettime(CLOCK_REALTIME, &ts );    //获取当前时间
                        ts.tv_sec += (mswait / 1000 );        //加上等待时间的秒数
                        ts.tv_nsec += ( mswait % 1000 ) * 1000; //加上等待时间纳秒数
                        int rv = 0;                          
                        while( ((rv=sem_timedwait( psem, &ts ))!=0) && (errno ==
EINTR) );   //等待信号量，errno==EINTR屏蔽其他信号事件引起的等待中断
                        return rv;   
                }                                            
                                                             
        }                                                    
        bool push_back( const T &item, int mswait = -1 )     
        ...{ //等待mswait毫秒直到将item插入队列，mswait为-1则一直等待                                                   
                if( -1 == sem_wait_i( &m_enques, mswait ))   
                ...{                                            
                        return false;                        
                }

                  //AUTO_GUARD：定界加锁，见Linux多线程及临界区编程例解的tmutex.h文件定义。                             
                AUTO_GUARD( g, MUTEX_TYPE, m_lock );
                try                                          
                ...{                                            
                        m_data.push_back( item );            
                        cout << "push " << item << endl;     
                        sem_post( &m_deques );               
                        return true;                         
                }                                            
                catch(...)                                   
                ...{                                            
                        return false;                        
                }                                            
        }        

      bool pop_front( T &item, bool bpop = true, int mswait = -1 )      
        ...{ //等待mswait毫秒直到从队列取出元素，mswait为-1则一直等待                                                     
                if( -1 == sem_wait_i( &m_deques, mswait ) )  
                ...{                                            
                        return false;                        
                }           
                 //AUTO_GUARD：定界加锁，见Linux多线程及临界区编程例解的tmutex.h文件定义。                   
                AUTO_GUARD( g, MUTEX_TYPE, m_lock );         
                try                                          
                ...{                                            
                        item = m_data.front();               
                        if( bpop )                           
                        ...{                                    
                                m_data.pop_front();          
                                cout << "pop " << item << endl;
                        }                                    
                                                             
                        sem_post( &m_enques );               
                        return true;                         
                }                                            
                catch(...)                                   
                ...{                                            
                        return false;                        
                }                                            
        }                                                    
        inline size_t size()                                 
        ...{                                                    
                return m_data.size();                        
        }     

private:                                                     
        MUTEX_TYPE m_lock;                                   
        deque<T> m_data;                                     
        size_t m_MaxSize;                                    
        sem_t m_enques;                                      
        sem_t m_deques;                                      
} ;                                                           
                                                             
#endif                          

// --------------------------msgdequeue.h结束-------------------------------------

// --------------------------test.cpp开始-------------------------------------
// 主程序文件

#include  " msgdequeue.h "
#include  < pthread.h >
#include  < iostream >
using   namespace  std;

CMessageDequeue < int >  qq( 5 );

void   * get_thread( void   * parg);
void   * put_thread( void   * parg);

void   * get_thread( void   * parg)
... {
        while(true)
        ...{
                int a = -1;
                if( !qq.pop_front( a,true, 1000 ) )
                ...{
                        cout << "pop failed. size=" << qq.size() << endl;
                }
        }
        return NULL;
}

void   * put_thread( void   * parg)
... {
        for(int i=1; i<=30; i++)
        ...{
                qq.push_back( i, -1 );
        }

        return NULL;                                         
}                                                             
                                                             
int  main()                                                   
... {                                                          
        pthread_t pget,pput;                                 
        pthread_create( &pget,NULL,get_thread,NULL);         
        pthread_create( &pput, NULL, put_thread,NULL);       
                                                             
        pthread_join( pget,NULL );                           
        pthread_join( pput,NULL );                           
                                                             
        return 0;                                            
}       

// --------------------------test.cpp结束-------------------------------------

    编译程序：g++ msgdequeue.h test.cpp -lpthread -lrt -o test
    -lpthread链接pthread库。-ltr链接clock_gettime函数相关库。

    编译后生成可执行文件test。输入./test执行程序。

    线程get_thread每隔1000毫秒从队列取元素，线程put_thread将30个元素依次入队。两个线程模拟两条入队和出队的流水线。因我们在 CMessageDequeue qq(5)处定义了队列最多可容纳5个元素，入队线程每入队到队列元素满5个后需阻塞等待出队线程将队列元素出队才能继续。测试时可调整队列可容纳最大元素个数来观察运行效果。