本文介绍了Linux下三种常用的线程同步方法：互斥锁、条件变量和信号量。详细讲解了每种同步机制的工作原理、使用步骤及示例代码。
Linux 线程同步的三种方法
线程的最大特点是资源的共享性，但资源共享中的同步问题是多线程编程的难点。linux下提供了多种方式来处理线程同步，最常用的是互斥锁、条件变量和信号量。
一、互斥锁(mutex)
通过锁机制实现线程间的同步。
- 初始化锁。在Linux下，线程的互斥量数据类型是pthread_mutex_t。在使用前,要对它进行初始化。
静态分配：pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
动态分配：int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutex_attr_t *mutexattr); - 加锁。对共享资源的访问，要对互斥量进行加锁，如果互斥量已经上了锁，调用线程会阻塞，直到互斥量被解锁。
int pthread_mutex_lock(pthread_mutex *mutex);
int pthread_mutex_trylock(pthread_mutex_t *mutex); - 解锁。在完成了对共享资源的访问后，要对互斥量进行解锁。
int pthread_mutex_unlock(pthread_mutex_t *mutex); - 销毁锁。锁在是使用完成后，需要进行销毁以释放资源。
int pthread_mutex_destroy(pthread_mutex *mutex);
- #include <cstdio>
- #include <cstdlib>
- #include <unistd.h>
- #include <pthread.h>
- #include "iostream"
- using namespace std;
- pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
- int tmp;
- void* thread(void *arg)
- {
- cout << "thread id is " << pthread_self() << endl;
- pthread_mutex_lock(&mutex);
- tmp = 12;
- cout << "Now a is " << tmp << endl;
- pthread_mutex_unlock(&mutex);
- return NULL;
- }
- int main()
- {
- pthread_t id;
- cout << "main thread id is " << pthread_self() << endl;
- tmp = 3;
- cout << "In main func tmp = " << tmp << endl;
- if (!pthread_create(&id, NULL, thread, NULL))
- {
- cout << "Create thread success!" << endl;
- }
- else
- {
- cout << "Create thread failed!" << endl;
- }
- pthread_join(id, NULL);
- pthread_mutex_destroy(&mutex);
- return 0;
- }
- //编译：g++ -o thread testthread.cpp -lpthread
#include <cstdio>
#include <cstdlib>
#include <unistd.h>
#include <pthread.h>
#include "iostream"
using namespace std;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int tmp;
void* thread(void *arg)
{
cout << "thread id is " << pthread_self() << endl;
pthread_mutex_lock(&mutex);
tmp = 12;
cout << "Now a is " << tmp << endl;
pthread_mutex_unlock(&mutex);
return NULL;
}
int main()
{
pthread_t id;
cout << "main thread id is " << pthread_self() << endl;
tmp = 3;
cout << "In main func tmp = " << tmp << endl;
if (!pthread_create(&id, NULL, thread, NULL))
{
cout << "Create thread success!" << endl;
}
else
{
cout << "Create thread failed!" << endl;
}
pthread_join(id, NULL);
pthread_mutex_destroy(&mutex);
return 0;
}
//编译：g++ -o thread testthread.cpp -lpthread
二、条件变量(cond)
互斥锁不同，条件变量是用来等待而不是用来上锁的。条件变量用来自动阻塞一个线程，直到某特殊情况发生为止。通常条件变量和互斥锁同时使用。条件变量分为两部分: 条件和变量。条件本身是由互斥量保护的。线程在改变条件状态前先要锁住互斥量。条件变量使我们可以睡眠等待某种条件出现。条件变量是利用线程间共享的全局变量进行同步的一种机制，主要包括两个动作：一个线程等待"条件变量的条件成立"而挂起；另一个线程使"条件成立"（给出条件成立信号）。条件的检测是在互斥锁的保护下进行的。如果一个条件为假，一个线程自动阻塞，并释放等待状态改变的互斥锁。如果另一个线程改变了条件，它发信号给关联的条件变量，唤醒一个或多个等待它的线程，重新获得互斥锁，重新评价条件。如果两进程共享可读写的内存，条件变量可以被用来实现这两进程间的线程同步。
- 初始化条件变量。
静态态初始化，pthread_cond_t cond = PTHREAD_COND_INITIALIER;
动态初始化，int pthread_cond_init(pthread_cond_t *cond, pthread_condattr_t *cond_attr); - 等待条件成立。释放锁,同时阻塞等待条件变量为真才行。timewait()设置等待时间,仍未signal,返回ETIMEOUT(加锁保证只有一个线程wait)
int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex);
int pthread_cond_timewait(pthread_cond_t *cond,pthread_mutex *mutex,const timespec *abstime); - 激活条件变量。pthread_cond_signal,pthread_cond_broadcast（激活所有等待线程）
int pthread_cond_signal(pthread_cond_t *cond);
int pthread_cond_broadcast(pthread_cond_t *cond); //解除所有线程的阻塞 - 清除条件变量。无线程等待,否则返回EBUSY
int pthread_cond_destroy(pthread_cond_t *cond);
- #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 <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;
- }
#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;
}三、信号量(sem)
如同进程一样，线程也可以通过信号量来实现通信，虽然是轻量级的。信号量函数的名字都以"sem_"打头。线程使用的基本信号量函数有四个。
- 信号量初始化。
int sem_init (sem_t *sem , int pshared, unsigned int value);
这是对由sem指定的信号量进行初始化，设置好它的共享选项(linux 只支持为0，即表示它是当前进程的局部信号量)，然后给它一个初始值VALUE。 - 等待信号量。给信号量减1，然后等待直到信号量的值大于0。
int sem_wait(sem_t *sem); - 释放信号量。信号量值加1。并通知其他等待线程。
int sem_post(sem_t *sem); - 销毁信号量。我们用完信号量后都它进行清理。归还占有的一切资源。
int sem_destroy(sem_t *sem);
- #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;
- }
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