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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