【回眸】Linux 内核 (十六) 之多线程编程下

前言

前面介绍了互斥锁，本篇博文介绍死锁及其他多线程遇到的情况。

什么情况会造成死锁

死锁指的是两个或两个以上的运算单元（进程、线程或协程），互相持有对方所需的资源，导致它们都无法向前推进，从而导致永久阻塞的问题就是死锁。

线程条件控制实现线程同步

初始化线程条件控制

int pthread_cond_init(pthread_cond_t * cond, const pthread_condattr_t * attr);

也可以使用静态初始化：

pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

互斥锁也可以静态初始化


pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

阻塞条件线程

int pthread_cond_wait(pthread_cond_t* cond, pthread_mutex_t* mutex);
int pthread_cond_timedwait(pthread_cond_t* cond,pthread_mutex_t* mutex, const struct timespec* abstime);

备注：abstime 参数指的是绝对时间

pthread_cond_wait() 函数可以永久阻塞线程，直到条件变量成立的那一刻；pthread_cond_timedwait() 函数只能在 abstime 参数指定的时间内阻塞线程，超出时限后，该函数将重新对互斥锁执行“加锁”操作，并解除对线程的阻塞，函数的返回值为 ETIMEDOUT。

（参考文章）

解除条件线程阻塞

int pthread_cond_signal(pthread_cond_t* cond);
int pthread_cond_broadcast(pthread_cond_t* cond);

pthread_cond_signal() 函数至少解除一个线程的“被阻塞”状态，如果等待队列中包含多个线程，优先解除哪个线程将由操作系统的线程调度程序决定；

pthread_cond_broadcast() 函数可以解除等待队列中所有线程的“被阻塞”状态。

有可能解除阻塞后还是无法执行，可能存在的原因是互斥锁锁定了，等待互斥锁释放资源后即可继续执行。

销毁条件线程

int pthread_cond_destroy(pthread_cond_t *cond);

多线程编程

thread9.c（死锁 2个锁产生冲突）

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>pthread_mutex_t mutex;
pthread_mutex_t mutex2;
pthread_attr_t attr;
pthread_attr_t attr2;
int g_data = 0;
void *func1(void *arg){
pthread_mutex_lock(&mutex);
sleep(1);
pthread_mutex_lock(&mutex2);
while(1){
printf("t1:%d\n",g_data++);
sleep(1);
if (g_data == 4){
pthread_mutex_unlock(&mutex);
//printf("t1 Quit.=============================\n");
pthread_exit(NULL);
exit(0);
}
}}
void *func2(void *arg){printf("t2:%ld thread is created.\n",(unsigned long)pthread_self());
printf("t2:param is %d\n",*((int *)arg));
while(1){
pthread_mutex_lock(&mutex2);
sleep(1);
pthread_mutex_lock(&mutex);
printf("t2:%d\n",g_data);
g_data++;
pthread_mutex_unlock(&mutex);
sleep(1); 
}
}
int main(){
int ret;
int param =100;
pthread_t t1;
pthread_t t2;
pthread_mutex_init(&mutex,NULL);
pthread_mutex_init(&mutex2,NULL);
ret = pthread_create(&t1,NULL,func1,(void *)&param);
if(ret == 0){
printf("main:Create t1 sucess.\n");
}
ret = pthread_create(&t2,NULL,func2,(void *)&param);
if(ret == 0){
printf("main:Create t2 sucess.\n");
}
printf("mainID:%ld .\n",(unsigned long)pthread_self());
printf("main:g_data = %d.\n",g_data);
pthread_join(t1,NULL);//等待
pthread_join(t2,NULL);//等待
pthread_attr_destroy(&attr);
pthread_attr_destroy(&attr2);
return 0;
}

thread10.c（条件线程）

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>pthread_mutex_t mutex;
pthread_cond_t cond;
int g_data = 0;
void *func1(void *arg){
pthread_mutex_lock(&mutex);//保证t1优先
while(1){
pthread_cond_wait(&cond,&mutex);//等待
printf("t1:%d\n",g_data++);
printf("t1 Run.=============================\n");
sleep(1);
g_data = 0;
}
}void *func2(void *arg){printf("t2:%ld thread is created.\n",(unsigned long)pthread_self());
printf("t2:param is %d\n",*((int *)arg));
while(1){
pthread_mutex_lock(&mutex);
printf("t2:%d\n",g_data);
g_data++;
if(g_data == 3)
{
pthread_cond_signal(&cond);//触发
} 
pthread_mutex_unlock(&mutex);
sleep(1);
}
}
int main(){
int ret;
int param =100;
pthread_t t1;
pthread_t t2;
pthread_mutex_init(&mutex,NULL);
pthread_cond_init(&cond,NULL);
ret = pthread_create(&t1,NULL,func1,(void *)&param);
if(ret == 0){
printf("main:Create t1 sucess.\n");
}
ret = pthread_create(&t2,NULL,func2,(void *)&param);
if(ret == 0){
printf("main:Create t2 sucess.\n");
}
printf("mainID:%ld .\n",(unsigned long)pthread_self());
printf("main:g_data = %d.\n",g_data);
pthread_join(t1,NULL);//等待
pthread_join(t2,NULL);//等待
pthread_attr_destroy(&mutex);
pthread_cond_destroy(&cond);
return 0;
}