线程同步的方法有哪些？在linux下，系统提供了很多种方式来实现线程同步，其中最常用的便是互斥锁、条件变量和信号量这三种方式，可能还有很多伙伴对于这三种方法都不熟悉，下面就给大家详细介绍下。
Linux下实现线程同步的三种方法：
一、互斥锁（mutex）
通过锁机制实现线程间的同步。
1、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）；
2、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）；
3、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）；
4、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）；
01#include <cstdio>02#include <cstdlib>03#include <unistd.h>04#include <pthread.h>05#include "iostream"06using namespace std;07pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;08int tmp;09void* thread(void *arg)10{11cout << "thread id is " << pthread_self() << endl;12pthread_mutex_lock(&mutex);13tmp = 12;14cout << "Now a is " << tmp << endl;15pthread_mutex_unlock(&mutex);16return NULL;17}18int main()19{20pthread_t id;21cout << "main thread id is " << pthread_self() << endl;22tmp = 3;23cout << "In main func tmp = " << tmp << endl;24if (!pthread_create(&id, NULL, thread, NULL))25{26cout << "Create thread success!" << endl;27}28else29{30cout << "Create thread failed!" << endl;31}32pthread_join(id, NULL);33pthread_mutex_destroy(&mutex);34return 0;35}36//编译：g++ -o thread testthread.cpp -lpthread复制代码#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）
与互斥锁不同，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。
1、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）；
2、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）；
4、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
5、清除条件变量。无线程等待，否则返回EBUSY
int pthread_cond_destroy（pthread_cond_t *cond）；
01[cpp] view plain copy02#include <stdio.h>03#include <pthread.h>04#include "stdlib.h"05#include "unistd.h"06pthread_mutex_t mutex;07pthread_cond_t cond;08void hander(void *arg)09{10free(arg);11(void)pthread_mutex_unlock(&mutex);12}13void *thread1(void *arg)14{15pthread_cleanup_push(hander, &mutex);16while(1)17{18printf("thread1 is runningn");19pthread_mutex_lock(&mutex);20pthread_cond_wait(&cond, &mutex);21printf("thread1 applied the conditionn");22pthread_mutex_unlock(&mutex);23sleep(4);24}25pthread_cleanup_pop(0);26}27void *thread2(void *arg)28{29while(1)30{31printf("thread2 is runningn");32pthread_mutex_lock(&mutex);33pthread_cond_wait(&cond, &mutex);34printf("thread2 applied the conditionn");35pthread_mutex_unlock(&mutex);36sleep(1);37}38}39int main()40{41pthread_t thid1,thid2;42printf("condition variable study!n");43pthread_mutex_init(&mutex, NULL);44pthread_cond_init(&cond, NULL);45pthread_create(&thid1, NULL, thread1, NULL);46pthread_create(&thid2, NULL, thread2, NULL);47sleep(1);48do49{50pthread_cond_signal(&cond);51}while(1);52sleep(20);53pthread_exit(0);54return 0;55}复制代码[cpp] view plain copy#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 runningn");pthread_mutex_lock(&mutex);pthread_cond_wait(&cond, &mutex);printf("thread1 applied the conditionn");pthread_mutex_unlock(&mutex);sleep(4);}pthread_cleanup_pop(0);}void *thread2(void *arg){while(1){printf("thread2 is runningn");pthread_mutex_lock(&mutex);pthread_cond_wait(&cond, &mutex);printf("thread2 applied the conditionn");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;}01#include <pthread.h>02#include <unistd.h>03#include "stdio.h"04#include "stdlib.h"05static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;06static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;07struct node08{09int n_number;10struct node *n_next;11}*head = NULL;12static void cleanup_handler(void *arg)13{14printf("Cleanup handler of second thread./n");15free(arg);16(void)pthread_mutex_unlock(&mtx);17}18static void *thread_func(void *arg)19{20struct node *p = NULL;21pthread_cleanup_push(cleanup_handler, p);22while (1)23{24//这个mutex主要是用来保证pthread_cond_wait的并发性25pthread_mutex_lock(&mtx);26while (head == NULL)27{28//这个while要特别说明一下，单个pthread_cond_wait功能很完善，为何29//这里要有一个while (head == NULL)呢？因为pthread_cond_wait里的线30//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。31//这个时候，应该让线程继续进入pthread_cond_wait32// pthread_cond_wait会先解除之前的pthread_mutex_lock锁定的mtx，33//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34//而被唤醒，唤醒后，该进程会先锁定先pthread_mutex_lock(&mtx);，再读取资源35//用这个流程是比较清楚的36pthread_cond_wait(&cond, &mtx);37p = head;38head = head->n_next;39printf("Got %d from front of queue/n", p->n_number);40free(p);41}42pthread_mutex_unlock(&mtx); //临界区数据操作完毕，释放互斥锁43}44pthread_cleanup_pop(0);45return 0;46}47int main(void)48{49pthread_t tid;50int i;51struct node *p;52//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53//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54pthread_create(&tid, NULL, thread_func, NULL);55sleep(1);56for (i = 0; i < 10; i++)57{58p = (struct node*)malloc(sizeof(struct node));59p->n_number = i;60pthread_mutex_lock(&mtx); //需要操作head这个临界资源，先加锁，61p->n_next = head;62head = p;63pthread_cond_signal(&cond);64pthread_mutex_unlock(&mtx); //解锁65sleep(1);66}67printf("thread 1 wanna end the line.So cancel thread 2./n");68//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69//线程，而在我们的代码里，最近的取消点肯定就是pthread_cond_wait()了。70pthread_cancel(tid);71pthread_join(tid, NULL);72printf("All done — exiting/n");73return 0;74}复制代码#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。
1、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。
2、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）；
3、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）；
4、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）；
01#include <stdlib.h>02#include <stdio.h>03#include <unistd.h>04#include <pthread.h>05#include <semaphore.h>06#include <errno.h>07#define return_if_fail(p) if((p) == 0){printf ("[%s]:func error!/n", __func__);return;}08typedef struct _PrivInfo09{10sem_t s1;11sem_t s2;12time_t end_time;13}PrivInfo;14static void info_init (PrivInfo* thiz);15static void info_destroy (PrivInfo* thiz);16static void* pthread_func_1 (PrivInfo* thiz);17static void* pthread_func_2 (PrivInfo* thiz);18int main (int argc, char** argv)19{20pthread_t pt_1 = 0;21pthread_t pt_2 = 0;22int ret = 0;23PrivInfo* thiz = NULL;24thiz = (PrivInfo* )malloc (sizeof (PrivInfo));25if (thiz == NULL)26{27printf ("[%s]: Failed to malloc priv./n");28return -1;29}30info_init (thiz);31ret = pthread_create (&pt_1, NULL, (void*)pthread_func_1, thiz);32if (ret != 0)33{34perror ("pthread_1_create:");35}36ret = pthread_create (&pt_2, NULL, (void*)pthread_func_2, thiz);37if (ret != 0)38{39perror ("pthread_2_create:");40}41pthread_join (pt_1, NULL);42pthread_join (pt_2, NULL);43info_destroy (thiz);44return 0;45}46static void info_init (PrivInfo* thiz)47{48return_if_fail (thiz != NULL);49thiz->end_time = time(NULL) + 10;50sem_init (&thiz->s1, 0, 1);51sem_init (&thiz->s2, 0, 0);52return;53}54static void info_destroy (PrivInfo* thiz)55{56return_if_fail (thiz != NULL);57sem_destroy (&thiz->s1);58sem_destroy (&thiz->s2);59free (thiz);60thiz = NULL;61return;62}63static void* pthread_func_1 (PrivInfo* thiz)64{65return_if_fail(thiz != NULL);66while (time(NULL) < thiz->end_time)67{68sem_wait (&thiz->s2);69printf ("pthread1: pthread1 get the lock./n");70sem_post (&thiz->s1);71printf ("pthread1: pthread1 unlock/n");72sleep (1);73}74return;75}76static void* pthread_func_2 (PrivInfo* thiz)77{78return_if_fail (thiz != NULL);79while (time (NULL) < thiz->end_time)80{81sem_wait (&thiz->s1);82printf ("pthread2: pthread2 get the unlock./n");83sem_post (&thiz->s2);84printf ("pthread2: pthread2 unlock./n");85sleep (1);86}87return;88}复制代码#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;}
以上便是Linux下实现线程同步常用的三种方法，大家都知道，线程的最大的亮点便是资源共享性，而资源共享中的线程同步问题却是一大难点，希望小编的归纳能够对大家有所帮助！
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