Linux下线程同步的几种方法

Linux下线程同步的几种方法,第1张

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)

[csharp] view plain copy

#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)

[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 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

}

[cpp] view plain copy

#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 = 0i <10i++)

{

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)

[cpp] view plain copy

#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

}

看我下面的代码, 父进程是消费者,子进程是生产者。

REPEATS 决定总共生产的次数 (可以自己修改)

CONSUMER_SPEED 决定消费的速度 (越大越慢,可以自己修改)

PRODUCER_SPEED 决定生产的速度 (越大越慢,可以自己修改)

我的例子里,生产者生产一个随机数。另外消费速度比生产速度慢,所以可以看到输出中,+++ (生产者) 开头的出现的比--- (消费者)多,当生产者结束后,就只有 --- 打印了。

对这个程序由什么问题,可以baidu hi我。在linux/unix下用 gcc 编译。

#include <stdio.h>

#include <unistd.h>

#include <time.h>

#include <string.h>

#include <stdlib.h>

#include <sys/sem.h>

#include <sys/shm.h>

#include <sys/stat.h>

#define REPEATS (10) /* count of production/consumption */

#define MAX_BUFFER_SIZE (8)

typedef struct

{

int bottom

int top

int data[MAX_BUFFER_SIZE]

} STRUCT_BUFFER

STRUCT_BUFFER * pBuffer = NULL

/* Define speed of consumer/producer, change them as u like */

#define PRODUCER_SPEED (1) /* 1/sec */

#define CONSUMER_SPEED (2) /* 1/2sec */

int sem_consume/* consumer sem */

int sem_produce/* producer sem */

int shm_buffer /* shared buffer */

#define FLAG (IPC_CREAT | S_IRWXU)

/* Init semphores &shared buffer */

void init()

{

union semun {

int val

struct semid_ds *buf

unsigned short *array

} arg

shm_buffer = shmget(0x1111, sizeof(STRUCT_BUFFER), FLAG)

pBuffer = shmat(shm_buffer, 0, 0)

memset(pBuffer, 0, sizeof(STRUCT_BUFFER))

sem_consume = semget(0x2222, 1, FLAG)

arg.val = 0

if (semctl(sem_consume, 0, SETVAL, arg) <0)

{

perror("Consumer")

exit(1)

}

sem_produce = semget(0x3333, 1, FLAG)

arg.val = MAX_BUFFER_SIZE

if (semctl(sem_produce, 0, SETVAL, arg) <0)

{

perror("Producer")

exit(1)

}

}

/* destroy semphores &shared buffer */

void deinit()

{

shmctl(shm_buffer, IPC_RMID, NULL)

semctl(sem_consume, 0, IPC_RMID)

semctl(sem_produce, 0, IPC_RMID)

}

int main()

{

int pid, i

struct sembuf sbuf

init()

printf("Start fork...\n")

pid = fork()

if (pid >0)

{

/* parent process, consumer */

for (i = 0i <REPEATSi++)

{

/* Try decrementing 1 from consumer */

sbuf.sem_num=0

sbuf.sem_op=-1

sbuf.sem_flg=0

semop(sem_consume, &sbuf, 1)

/* OK */

printf("Consumer get %6d\n", pBuffer->data[pBuffer->bottom])

pBuffer->bottom = (pBuffer->bottom+1)%MAX_BUFFER_SIZE

/* Try incrementing 1 to producer */

sbuf.sem_op = 1

semop(sem_produce, &sbuf, 1)

sleep(CONSUMER_SPEED)

}

wait(0)

shmdt(pBuffer)

}

else if (pid == 0)

{

srand(time(NULL))

/* child process, producer */

for (i = 0i <REPEATSi++)

{

/* Try decrementing 1 from producer */

sbuf.sem_num=0

sbuf.sem_op=-1

sbuf.sem_flg=0

semop(sem_produce, &sbuf, 1)

/* OK */

pBuffer->data[pBuffer->top] = (rand()%1000)*1000 + i + 1

printf("Producer put %6d\n", pBuffer->data[pBuffer->top])

pBuffer->top = (pBuffer->top+1)%MAX_BUFFER_SIZE

/* Try incrementing 1 to consumer */

sbuf.sem_op = 1

semop(sem_consume, &sbuf, 1)

sleep(PRODUCER_SPEED)

}

shmdt(pBuffer)

exit(0)

}

deinit()

return 0

}

read会不会被中断或者挂起?

会,这可要看底层驱动的实现

如果要加临界区,应该用什么函数?

用信号量,

sem_init():初始化

sem_wait():临界区前调用

临界区

sem_post()临界区后调用

其实用法大体上和加锁没有多大的区别


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