上篇提到线程针对临界值操作时需要加锁,但是线程访问临界资源只通过锁来控制是不够的。 比如对一个数据进行操作,A线程需要读,B线程进行写。 A线程先访问临界资源,发现没有数据可以读,只能等待B线程先写,此时又占用了互斥锁,导致B线程无法得到锁,进行写操作。 此时就需要用到条件变量了,条件变量的目的就是控制线程的先后执行,保证临界资源的有效性。
下面依然是售票的一个场景,此时一个线程售票,一个线程退票。
#include <pthread.h>
#include <unistd.h>
#include <iostream>
using namespace std;
class ThreadInterface
{
public:
void CreateThread(void* (*func)(void *));
void WaitThread();
private:
pthread_t m_pTread;
};
void ThreadInterface::CreateThread(void* (*func)(void *))
{
pthread_create(&m_pTread, NULL, func, NULL);
}
void ThreadInterface::WaitThread()
{
pthread_join(m_pTread, NULL);
}
class MutexLockInterface
{
public:
void CreateMutexLock();
void GetMutexLock();
void ReleaseMutexLock();
void DestroyMutexLock();
pthread_mutex_t m_MutexLock;
};
void MutexLockInterface::CreateMutexLock()
{
int ret = pthread_mutex_init(&m_MutexLock, NULL);
if (0 != ret)
cout<<"init mutex error!";
}
void MutexLockInterface::GetMutexLock()
{
pthread_mutex_lock(&m_MutexLock);
}
void MutexLockInterface::ReleaseMutexLock()
{
pthread_mutex_unlock(&m_MutexLock);
}
void MutexLockInterface::DestroyMutexLock()
{
pthread_mutex_destroy(&m_MutexLock);
}
class CondInterface
{
public:
void CreateCond();
void WaitCond(pthread_mutex_t *mutex);
void WakeupCond();
void DestroyCond();
private:
pthread_cond_t m_Cond;
};
void CondInterface::CreateCond()
{
int ret = pthread_cond_init(&m_Cond, NULL);
if (0 != ret)
cout<<"init mutex error!";
}
void CondInterface::WaitCond(pthread_mutex_t *mutex)
{
pthread_cond_wait(&m_Cond, mutex);
}
void CondInterface::WakeupCond()
{
pthread_cond_broadcast(&m_Cond);
}
void CondInterface::DestroyCond()
{
pthread_cond_destroy(&m_Cond);
}
class Service
{
public:
static void* run(void *)
{
m_MutexLock.GetMutexLock();
cout<<"we have "<<m_Tickets<<"Tickets"<<endl;
sleep(1);
m_Tickets++;
//Cond.WakeupCond();
m_MutexLock.ReleaseMutexLock();
}
int SetData(int data){m_Tickets = data;};
int GetData(){return m_Tickets;};
static int m_Tickets;
static MutexLockInterface m_MutexLock;
static CondInterface Cond;
};
int Service::m_Tickets = 0;
MutexLockInterface Service::m_MutexLock;
CondInterface Service::Cond;
int main()
{
Service Srv;
ThreadInterface Thread;
Srv.m_MutexLock.CreateMutexLock();
Srv.Cond.CreateCond();
Thread.CreateThread(&Srv.run);
Srv.m_MutexLock.GetMutexLock();
if (0 == Srv.GetData())
{
//Srv.Cond.WaitCond(&Srv.m_MutexLock.m_MutexLock);
cout<<"wait!"<<endl;
}
cout<<"window1:we have "<<Srv.GetData()<<"Tickets"<<endl;
sleep(1);
Srv.SetData(Srv.GetData() - 1);
Srv.m_MutexLock.ReleaseMutexLock();
Thread.WaitThread();
cout<<Srv.GetData()<<endl;
return 0;
}
不使用条件变量执行结果如下:
线程1先执行,此时并没有票。此时应该先放弃锁,让线程2先执行,取消注释执行结果如下:
由此可以看出,条件变量让线程1暂时先放弃锁进入阻塞,等线程2执行完毕后,唤醒线程1。再进行正确操作。