linux线程之互斥锁与条件变量

四 互斥锁

POISX互斥锁用pthread_mutex_t描述，主要用来保护临界资源(同一个时刻只能有同一个人访问)

A.定义互斥锁

pthread_mutex_t lock;

B.初始化互斥锁
//动态初始化互斥锁，锁不使用的时候，需要调用pthread_mutex_destroy销毁互斥锁
int pthread_mutex_init(pthread_mutex_t *restrict mutex,
  const pthread_mutexattr_t *restrict attr);

//静态初始化互斥锁
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

//销毁互斥锁
int pthread_mutex_destroy(pthread_mutex_t *mutex);

C.获得互斥锁
int pthread_mutex_lock(pthread_mutex_t *mutex);
特点:线程没有获得互斥锁则阻塞

D.释放互斥锁
int pthread_mutex_unlock(pthread_mutex_t *mutex);

五 条件变量

POISX 条件变量描述pthread_cond_t

A.初始化条件变量
int pthread_cond_destroy(pthread_cond_t *cond);
int pthread_cond_init(pthread_cond_t *restrict cond,
  const pthread_condattr_t *restrict attr);
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

B.等待条件变量
int pthread_cond_wait(pthread_cond_t *restrict cond,
  pthread_mutex_t *restrict mutex);

内部实现:
int pthread_cond_wait(....)
{
   1.判断条件是否满足
  不满足，释放互斥锁，阻塞调用者
   2.条件满足
   先获得互斥锁，然后返回
}

条件满足:有其他线程调用pthread_cond_signal或pthread_cond_broadcast时，唤醒等待的线程，条件满足

一般使用:

pthread_mutex_lock(&lock);
pthread_cond_wait(&cond,&lock);

.....

pthread_mutex_unlock(&lock);

练习:(使用互斥锁与条件变量)
1.定义全局buf
2.两个线程，一个线程从键盘向buf输入数据,另一个线程输出buf数据
3.输入"quit",两个线程结束

#include
#include
#include
#include
#include
#include
#include
#include

//全局buffer
char buf[1024];

//定义互斥锁,静态初始化
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

void *read_thread(void *arg)
{
 while(1)
 {
  pthread_mutex_lock(&lock);
  //等待被唤醒
  pthread_cond_wait(&cond,&lock);

  printf("Read %s from buffer.\n",buf);
  if(strncmp(buf,"quit",4) == 0)
   break;
  
  pthread_mutex_unlock(&lock);
 }

 pthread_exit(NULL);
}

void *write_thread(void *arg)
{
 while(1)
 {
  //让读线程先等待
  usleep(500);
  
  pthread_mutex_lock(&lock);
  
  printf(">");
  fgets(buf,sizeof(buf),stdin);
  buf[strlen(buf) - 1] = '\0';
 
  pthread_mutex_unlock(&lock);
  
  //唤醒等待的线程
  //有效:必须有对应的线程已经处于等待
  pthread_cond_signal(&cond);

  if(strncmp(buf,"quit",4) == 0)
   break;
 }

 pthread_exit(NULL);
}

//./a.out
int main(int argc, const char *argv[])
{
 int ret;
 int fd;
 pthread_t tid[2];
 
 ret = pthread_create(&tid[0],NULL,read_thread,NULL);
 if(ret != 0){
  perror("Fail to pthread_create");
  exit(EXIT_FAILURE);
 }

 ret = pthread_create(&tid[1],NULL,write_thread,NULL);
 if(ret != 0){
  perror("Fail to pthread_create");
  exit(EXIT_FAILURE);
 }

 pthread_join(tid[0],NULL);
 pthread_join(tid[1],NULL);

 exit(EXIT_SUCCESS);
}