線程同步的方法有哪些?在linux下,系統提供瞭很多種方式來實現線程同步,其中最常用的便是互斥鎖、條件變量和信號量這三種方式,可能還有很多夥伴對於這三種方法都不熟悉,下面就給大傢詳細介紹下。
Linux下實現線程同步的三種方法:
一、互斥鎖(mutex)
通過鎖機制實現線程間的同步。
1、初始化鎖。在Linux下,線程的互斥量數據類型是pthread_mutex_t。在使用前,要對它進行初始化。
靜態分配:pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
動態分配: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,要對互斥量進行加鎖,如果互斥量已經上瞭鎖,調用線程會阻塞,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。在完成瞭對共享資源的訪問後,要對互斥量進行解鎖。
int pthread_mutex_unlock(pthread_mutex_t *mutex);
4、銷毀鎖。鎖在是使用完成後,需要進行銷毀以釋放資源。
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}
- 28else
- 29{
- 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
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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的。線程在改變條件狀態前先要鎖住互斥量。條件變量使我們可以睡眠等待某種條件出現。條件變量是利用線程間共享的全局變量進行同步的一種機制,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下進行的。如果一個條件為假,一個線程自動阻塞,並釋放等待狀態改變的互斥鎖。如果另一個線程改變瞭條件,它發信號給關聯的條件變量,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。如果兩進程共享可讀寫的內存,條件變量可以被用來實現這兩進程間的線程同步。
1、初始化條件變量。
靜態態初始化,pthread_cond_t cond = PTHREAD_COND_INITIALIER;
動態初始化,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()設置等待時間,仍未signal,返回ETIMEOUT(加鎖保證隻有一個線程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 copy
- 02#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 running\n");
- 19pthread_mutex_lock(&mutex);
- 20pthread_cond_wait(&cond, &mutex);
- 21printf("thread1 applied the condition\n");
- 22pthread_mutex_unlock(&mutex);
- 23sleep(4);
- 24}
- 25pthread_cleanup_pop(0);
- 26}
- 27void *thread2(void *arg)
- 28{
- 29while(1)
- 30{
- 31printf("thread2 is running\n");
- 32pthread_mutex_lock(&mutex);
- 33pthread_cond_wait(&cond, &mutex);
- 34printf("thread2 applied the condition\n");
- 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);
- 48do
- 49{
- 50pthread_cond_signal(&cond);
- 51}while(1);
- 52sleep(20);
- 53pthread_exit(0);
- 54return 0;
- 55}
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- 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 node
- 08{
- 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的並發性
- 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//這裡要有一個while (head == NULL)呢?因為pthread_cond_wait裡的線
- 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,應該讓線程繼續進入pthread_cond_wait
- 32// pthread_cond_wait會先解除之前的pthread_mutex_lock鎖定的mtx,
- 33//然後阻塞在等待對列裡休眠,直到再次被喚醒(大多數情況下是等待的條件成立
- 34//而被喚醒,喚醒後,該進程會先鎖定先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); //臨界區數據操作完畢,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,類似生產者和消費者,但是這裡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//關於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,而在我們的代碼裡,最近的取消點肯定就是pthread_cond_wait()瞭。
- 70pthread_cancel(tid);
- 71pthread_join(tid, NULL);
- 72printf("All done — exiting/n");
- 73return 0;
- 74}
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三、信號量(sem)
如同進程一樣,線程也可以通過信號量來實現通信,雖然是輕量級的。信號量函數的名字都以“sem_”打頭。線程使用的基本信號量函數有四個。
1、信號量初始化。
int sem_init (sem_t *sem , int pshared, unsigned int value);
這是對由sem指定的信號量進行初始化,設置好它的共享選項(linux 隻支持為0,即表示它是當前進程的局部信號量),然後給它一個初始值VALUE。
2、等待信號量。給信號量減1,然後等待直到信號量的值大於0。
int sem_wait(sem_t *sem);
3、釋放信號量。信號量值加1。並通知其他等待線程。
int sem_post(sem_t *sem);
4、銷毀信號量。我們用完信號量後都它進行清理。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 _PrivInfo
- 09{
- 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}
以上便是Linux下實現線程同步常用的三種方法,大傢都知道,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對大傢有所幫助,想瞭解更多內容可以對GuideAH進行關註!
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