Linux 线程同步的三种方法(互斥锁、条件变量、信号量)
互斥锁
1 #include <cstdio> 2 3 #include <cstdlib> 4 5 #include <unistd.h> 6 7 #include <pthread.h> 8 9 #include "iostream" 10 11 using namespace std; 12 13 pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; 14 15 int tmp; 16 17 void* thread(void *arg) 18 19 { 20 21 cout << "thread id is " << pthread_self() << endl; 22 23 pthread_mutex_lock(&mutex); 24 25 tmp = 12; 26 27 cout << "Now a is " << tmp << endl; 28 29 pthread_mutex_unlock(&mutex); 30 31 return NULL; 32 33 } 34 35 int main() 36 37 { 38 39 pthread_t id; 40 41 cout << "main thread id is " << pthread_self() << endl; 42 43 tmp = 3; 44 45 cout << "In main func tmp = " << tmp << endl; 46 47 if (!pthread_create(&id, NULL, thread, NULL)) 48 49 { 50 51 cout << "Create thread success!" << endl; 52 53 } 54 55 else 56 57 { 58 59 cout << "Create thread failed!" << endl; 60 61 } 62 63 pthread_join(id, NULL); 64 65 pthread_mutex_destroy(&mutex); 66 67 return 0; 68 69 } 70 71 //编译：g++ -o thread testthread.cpp -lpthread
条件变量
#include <stdio.h> #include <pthread.h> #include "stdlib.h" #include "unistd.h" pthread_mutex_t mutex; pthread_cond_t cond; void hander(void *arg) { free(arg); (void)pthread_mutex_unlock(&mutex); } void *thread1(void *arg) { pthread_cleanup_push(hander, &mutex); while(1) { printf("thread1 is running\n"); pthread_mutex_lock(&mutex); pthread_cond_wait(&cond, &mutex); printf("thread1 applied the condition\n"); pthread_mutex_unlock(&mutex); sleep(4); } pthread_cleanup_pop(0); } void *thread2(void *arg) { while(1) { printf("thread2 is running\n"); pthread_mutex_lock(&mutex); pthread_cond_wait(&cond, &mutex); printf("thread2 applied the condition\n"); pthread_mutex_unlock(&mutex); sleep(1); } } int main() { pthread_t thid1,thid2; printf("condition variable study!\n"); pthread_mutex_init(&mutex, NULL); pthread_cond_init(&cond, NULL); pthread_create(&thid1, NULL, thread1, NULL); pthread_create(&thid2, NULL, thread2, NULL); sleep(1); do { pthread_cond_signal(&cond); }while(1); sleep(20); pthread_exit(0); return 0; }
#include <pthread.h> #include <unistd.h> #include "stdio.h" #include "stdlib.h" static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER; static pthread_cond_t cond = PTHREAD_COND_INITIALIZER; struct node { int n_number; struct node *n_next; }*head = NULL; static void cleanup_handler(void *arg) { printf("Cleanup handler of second thread./n"); free(arg); (void)pthread_mutex_unlock(&mtx); } static void *thread_func(void *arg) { struct node *p = NULL; pthread_cleanup_push(cleanup_handler, p); while (1) { //这个mutex主要是用来保证pthread_cond_wait的并发性 pthread_mutex_lock(&mtx); while (head == NULL) { //这个while要特别说明一下，单个pthread_cond_wait功能很完善，为何 //这里要有一个while (head == NULL)呢？因为pthread_cond_wait里的线 //程可能会被意外唤醒，如果这个时候head != NULL，则不是我们想要的情况。 //这个时候，应该让线程继续进入pthread_cond_wait // pthread_cond_wait会先解除之前的pthread_mutex_lock锁定的mtx， //然后阻塞在等待对列里休眠，直到再次被唤醒(大多数情况下是等待的条件成立 //而被唤醒，唤醒后，该进程会先锁定先pthread_mutex_lock(&mtx);，再读取资源 //用这个流程是比较清楚的 pthread_cond_wait(&cond, &mtx); p = head; head = head->n_next; printf("Got %d from front of queue/n", p->n_number); free(p); } pthread_mutex_unlock(&mtx); //临界区数据操作完毕，释放互斥锁 } pthread_cleanup_pop(0); return 0; } int main(void) { pthread_t tid; int i; struct node *p; //子线程会一直等待资源，类似生产者和消费者，但是这里的消费者可以是多个消费者，而 //不仅仅支持普通的单个消费者，这个模型虽然简单，但是很强大 pthread_create(&tid, NULL, thread_func, NULL); sleep(1); for (i = 0; i < 10; i++) { p = (struct node*)malloc(sizeof(struct node)); p->n_number = i; pthread_mutex_lock(&mtx); //需要操作head这个临界资源，先加锁， p->n_next = head; head = p; pthread_cond_signal(&cond); pthread_mutex_unlock(&mtx); //解锁 sleep(1); } printf("thread 1 wanna end the line.So cancel thread 2./n"); //关于pthread_cancel，有一点额外的说明，它是从外部终止子线程，子线程会在最近的取消点，退出 //线程，而在我们的代码里，最近的取消点肯定就是pthread_cond_wait()了。 pthread_cancel(tid); pthread_join(tid, NULL); printf("All done -- exiting/n"); return 0; }
信号量
#include <stdlib.h> #include <stdio.h> #include <unistd.h> #include <pthread.h> #include <semaphore.h> #include <errno.h> #define return_if_fail(p) if((p) == 0){printf ("[%s]:func error!/n", __func__);return;} typedef struct _PrivInfo { sem_t s1; sem_t s2; time_t end_time; }PrivInfo; static void info_init (PrivInfo* thiz); static void info_destroy (PrivInfo* thiz); static void* pthread_func_1 (PrivInfo* thiz); static void* pthread_func_2 (PrivInfo* thiz); int main (int argc, char** argv) { pthread_t pt_1 = 0; pthread_t pt_2 = 0; int ret = 0; PrivInfo* thiz = NULL; thiz = (PrivInfo* )malloc (sizeof (PrivInfo)); if (thiz == NULL) { printf ("[%s]: Failed to malloc priv./n"); return -1; } info_init (thiz); ret = pthread_create (&pt_1, NULL, (void*)pthread_func_1, thiz); if (ret != 0) { perror ("pthread_1_create:"); } ret = pthread_create (&pt_2, NULL, (void*)pthread_func_2, thiz); if (ret != 0) { perror ("pthread_2_create:"); } pthread_join (pt_1, NULL); pthread_join (pt_2, NULL); info_destroy (thiz); return 0; } static void info_init (PrivInfo* thiz) { return_if_fail (thiz != NULL); thiz->end_time = time(NULL) + 10; sem_init (&thiz->s1, 0, 1); sem_init (&thiz->s2, 0, 0); return; } static void info_destroy (PrivInfo* thiz) { return_if_fail (thiz != NULL); sem_destroy (&thiz->s1); sem_destroy (&thiz->s2); free (thiz); thiz = NULL; return; } static void* pthread_func_1 (PrivInfo* thiz) { return_if_fail(thiz != NULL); while (time(NULL) < thiz->end_time) { sem_wait (&thiz->s2); printf ("pthread1: pthread1 get the lock./n"); sem_post (&thiz->s1); printf ("pthread1: pthread1 unlock/n"); sleep (1); } return; } static void* pthread_func_2 (PrivInfo* thiz) { return_if_fail (thiz != NULL); while (time (NULL) < thiz->end_time) { sem_wait (&thiz->s1); printf ("pthread2: pthread2 get the unlock./n"); sem_post (&thiz->s2); printf ("pthread2: pthread2 unlock./n"); sleep (1); } return; }
总结：
互斥锁是是访问共享变量的，防止多线程同时写出现脏数据。
信号量是用来线程同步的，可两线程双向互相通知，也可单向通知。
条件变量是信号量的一种封装，用于线程单向等待另一个线程的通知，也可先后多个线程等待同一个条件变量的唤醒。
参考资料：https://blog.csdn.net/zsf8701/article/details/7844316
