線程同步的方法有哪些？在linux下，系統(tǒng)提供了很多種方式來實現(xiàn)線程同步，其中最常用的便是互斥鎖、條件變量和信號量這三種方式，可能還有很多伙伴對于這三種方法都不熟悉，下面就給大家詳細(xì)介紹下。

Linux下實現(xiàn)線程同步的三種方法：

一、互斥鎖（mutex）

通過鎖機(jī)制實現(xiàn)線程間的同步。

1、初始化鎖。在Linux下，線程的互斥量數(shù)據(jù)類型是pthread_mutex_t。在使用前，要對它進(jìn)行初始化。

靜態(tài)分配：pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER；

動態(tài)分配：int pthread_mutex_init（pthread_mutex_t *mutex， const pthread_mutex_attr_t *mutexattr）；

2、加鎖。對共享資源的訪問，要對互斥量進(jìn)行加鎖，如果互斥量已經(jīng)上了鎖，調(diào)用線程會阻塞，直到互斥量被解鎖。

int pthread_mutex_lock（pthread_mutex *mutex）；

int pthread_mutex_trylock（pthread_mutex_t *mutex）；

3、解鎖。在完成了對共享資源的訪問后，要對互斥量進(jìn)行解鎖。

int pthread_mutex_unlock（pthread_mutex_t *mutex）；

4、銷毀鎖。鎖在是使用完成后，需要進(jìn)行銷毀以釋放資源。

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//編譯：g++ -o thread testthread.cpp -lpthread復(fù)制代碼#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;}//編譯：g++ -o thread testthread.cpp -lpthread

二、條件變量（cond）

與互斥鎖不同，條件變量是用來等待而不是用來上鎖的。條件變量用來自動阻塞一個線程，直到某特殊情況發(fā)生為止。通常條件變量和互斥鎖同時使用。條件變量分為兩部分： 條件和變量。條件本身是由互斥量保護(hù)的。線程在改變條件狀態(tài)前先要鎖住互斥量。條件變量使我們可以睡眠等待某種條件出現(xiàn)。條件變量是利用線程間共享的全局變量進(jìn)行同步的一種機(jī)制，主要包括兩個動作：一個線程等待“條件變量的條件成立”而掛起；另一個線程使“條件成立”（給出條件成立信號）。條件的檢測是在互斥鎖的保護(hù)下進(jìn)行的。如果一個條件為假，一個線程自動阻塞，并釋放等待狀態(tài)改變的互斥鎖。如果另一個線程改變了條件，它發(fā)信號給關(guān)聯(lián)的條件變量，喚醒一個或多個等待它的線程，重新獲得互斥鎖，重新評價條件。如果兩進(jìn)程共享可讀寫的內(nèi)存，條件變量可以被用來實現(xiàn)這兩進(jìn)程間的線程同步。

1、初始化條件變量。

靜態(tài)態(tài)初始化，pthread_cond_t cond = PTHREAD_COND_INITIALIER；

動態(tài)初始化，int pthread_cond_init（pthread_cond_t *cond， pthread_condattr_t *cond_attr）；

2、等待條件成立。釋放鎖，同時阻塞等待條件變量為真才行。timewait（）設(shè)置等待時間，仍未signal，返回ETIMEOUT（加鎖保證只有一個線程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、激活條件變量。pthread_cond_signal，pthread_cond_broadcast（激活所有等待線程）

int pthread_cond_signal（pthread_cond_t *cond）；

int pthread_cond_broadcast（pthread_cond_t *cond）; //解除所有線程的阻塞

5、清除條件變量。無線程等待，否則返回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}復(fù)制代碼[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//這個mutex主要是用來保證pthread_cond_wait的并發(fā)性25pthread_mutex_lock(&mtx);26while (head == NULL)27{28//這個while要特別說明一下，單個pthread_cond_wait功能很完善，為何29//這里要有一個while (head == NULL)呢？因為pthread_cond_wait里的線30//程可能會被意外喚醒，如果這個時候head != NULL，則不是我們想要的情況。31//這個時候，應(yīng)該讓線程繼續(xù)進(jìn)入pthread_cond_wait32// pthread_cond_wait會先解除之前的pthread_mutex_lock鎖定的mtx，33//然后阻塞在等待對列里休眠，直到再次被喚醒(大多數(shù)情況下是等待的條件成立34//而被喚醒，喚醒后，該進(jìn)程會先鎖定先pthread_mutex_lock(&mtx);，再讀取資源35//用這個流程是比較清楚的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); //臨界區(qū)數(shù)據(jù)操作完畢，釋放互斥鎖43}44pthread_cleanup_pop(0);45return 0;46}47int main(void)48{49pthread_t tid;50int i;51struct node *p;52//子線程會一直等待資源，類似生產(chǎn)者和消費者，但是這里的消費者可以是多個消費者，而53//不僅僅支持普通的單個消費者，這個模型雖然簡單，但是很強(qiáng)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這個臨界資源，先加鎖，61p->n_next = head;62head = p;63pthread_cond_signal(&cond);64pthread_mutex_unlock(&mtx); //解鎖65sleep(1);66}67printf("thread 1 wanna end the line.So cancel thread 2./n");68//關(guān)于pthread_cancel，有一點額外的說明，它是從外部終止子線程，子線程會在最近的取消點，退出69//線程，而在我們的代碼里，最近的取消點肯定就是pthread_cond_wait()了。70pthread_cancel(tid);71pthread_join(tid, NULL);72printf("All done -- exiting/n");73return 0;74}復(fù)制代碼#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){//這個mutex主要是用來保證pthread_cond_wait的并發(fā)性pthread_mutex_lock(&mtx);while (head == NULL){//這個while要特別說明一下，單個pthread_cond_wait功能很完善，為何//這里要有一個while (head == NULL)呢？因為pthread_cond_wait里的線//程可能會被意外喚醒，如果這個時候head != NULL，則不是我們想要的情況。//這個時候，應(yīng)該讓線程繼續(xù)進(jìn)入pthread_cond_wait// pthread_cond_wait會先解除之前的pthread_mutex_lock鎖定的mtx，//然后阻塞在等待對列里休眠，直到再次被喚醒(大多數(shù)情況下是等待的條件成立//而被喚醒，喚醒后，該進(jìn)程會先鎖定先pthread_mutex_lock(&mtx);，再讀取資源//用這個流程是比較清楚的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); //臨界區(qū)數(shù)據(jù)操作完畢，釋放互斥鎖}pthread_cleanup_pop(0);return 0;}int main(void){pthread_t tid;int i;struct node *p;//子線程會一直等待資源，類似生產(chǎn)者和消費者，但是這里的消費者可以是多個消費者，而//不僅僅支持普通的單個消費者，這個模型雖然簡單，但是很強(qiáng)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這個臨界資源，先加鎖，p->n_next = head;head = p;pthread_cond_signal(&cond);pthread_mutex_unlock(&mtx); //解鎖sleep(1);}printf("thread 1 wanna end the line.So cancel thread 2./n");//關(guān)于pthread_cancel，有一點額外的說明，它是從外部終止子線程，子線程會在最近的取消點，退出//線程，而在我們的代碼里，最近的取消點肯定就是pthread_cond_wait()了。pthread_cancel(tid);pthread_join(tid, NULL);printf("All done -- exiting/n");return 0;}

三、信號量（sem）

如同進(jìn)程一樣，線程也可以通過信號量來實現(xiàn)通信，雖然是輕量級的。信號量函數(shù)的名字都以“sem_”打頭。線程使用的基本信號量函數(shù)有四個。

1、信號量初始化。

int sem_init （sem_t *sem ， int pshared， unsigned int value）；

這是對由sem指定的信號量進(jìn)行初始化，設(shè)置好它的共享選項（linux 只支持為0，即表示它是當(dāng)前進(jìn)程的局部信號量），然后給它一個初始值VALUE。

2、等待信號量。給信號量減1，然后等待直到信號量的值大于0。

int sem_wait（sem_t *sem）；

3、釋放信號量。信號量值加1。并通知其他等待線程。

int sem_post（sem_t *sem）；

4、銷毀信號量。我們用完信號量后都它進(jìn)行清理。歸還占有的一切資源。

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}復(fù)制代碼#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下實現(xiàn)線程同步常用的三種方法，大家都知道，線程的最大的亮點便是資源共享性，而資源共享中的線程同步問題卻是一大難點，希望小編的歸納能夠?qū)Υ蠹矣兴鶐椭?/p>