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);
-
#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);
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tmp = 12;
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cout << "Now a is " << tmp << endl;
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pthread_mutex_unlock(&mutex);
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return NULL;
-
}
-
int main()
-
{
-
pthread_t id;
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cout << "main thread id is " << pthread_self() << endl;
-
tmp = 3;
-
cout << "In main func tmp = " << tmp << endl;
-
if (!pthread_create(&id, NULL, thread, NULL))
-
{
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cout << "Create thread success!" << endl;
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}
-
else
-
{
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cout << "Create thread failed!" << endl;
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}
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pthread_join(id, NULL);
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pthread_mutex_destroy(&mutex);
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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); - **条件变量。pthread_cond_signal,pthread_cond_broadcast(**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);
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#include <stdio.h>
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#include <pthread.h>
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#include "stdlib.h"
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#include "unistd.h"
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pthread_mutex_t mutex;
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pthread_cond_t cond;
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void hander(void *arg)
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{
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free(arg);
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(void)pthread_mutex_unlock(&mutex);
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}
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void *thread1(void *arg)
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{
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pthread_cleanup_push(hander, &mutex);
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while(1)
-
{
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printf("thread1 is running\n");
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pthread_mutex_lock(&mutex);
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pthread_cond_wait(&cond, &mutex);
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printf("thread1 applied the condition\n");
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pthread_mutex_unlock(&mutex);
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sleep(4);
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}
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pthread_cleanup_pop(0);
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}
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void *thread2(void *arg)
-
{
-
while(1)
-
{
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printf("thread2 is running\n");
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pthread_mutex_lock(&mutex);
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pthread_cond_wait(&cond, &mutex);
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printf("thread2 applied the condition\n");
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pthread_mutex_unlock(&mutex);
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sleep(1);
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}
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}
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int main()
-
{
-
pthread_t thid1,thid2;
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printf("condition variable study!\n");
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pthread_mutex_init(&mutex, NULL);
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pthread_cond_init(&cond, NULL);
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pthread_create(&thid1, NULL, thread1, NULL);
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pthread_create(&thid2, NULL, thread2, NULL);
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sleep(1);
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do
-
{
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pthread_cond_signal(&cond);
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}while(1);
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sleep(20);
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pthread_exit(0);
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return 0;
-
}
-
#include <pthread.h>
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#include <unistd.h>
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#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;
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struct node *n_next;
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}*head = NULL;
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static void cleanup_handler(void *arg)
-
{
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printf("Cleanup handler of second thread./n");
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free(arg);
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(void)pthread_mutex_unlock(&mtx);
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}
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static void *thread_func(void *arg)
-
{
-
struct node *p = NULL;
-
pthread_cleanup_push(cleanup_handler, p);
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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);
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free(p);
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}
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pthread_mutex_unlock(&mtx); //临界区数据操作完毕,释放互斥锁
-
}
-
pthread_cleanup_pop(0);
-
return 0;
-
}
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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");
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//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);
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
#include <errno.h>
#define INTERLOCKEDEXCHANGE(x,y) xchg(x,y)
#define INTERLOCKEDEXCHANGEADD(v,p) __sync_fetch_and_add(&v, p) //v的值原子添加P的大小
#define INTERLOCKEDINCREMENT(t) __sync_fetch_and_add(&t, 1) //原子+1
#define INTERLOCKEDDECREMENT(t) __sync_fetch_and_sub(&t, 1) //原子-1
int nRuning = 0;
int bUpdate = 0;
#define return_if_fail_void(p) if((p) == 0){printf ("[%s]:func error!/n", __func__);return;}
#define return_if_fail_point(p) if((p) == 0){printf ("[%s]:func error!/n", __func__);return p;}
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(void* thiz);
static void* pthread_func_2(void* thiz);
static void* pthread_func_3(void* thiz);
static void* pthread_func_4(void* thiz);
int main(int argc, char** argv)
{
pthread_t pt_1 = 0;
pthread_t pt_2 = 0;
pthread_t pt_3 = 0;
pthread_t pt_4 = 0;
int ret = 0;
PrivInfo* thiz = NULL;
thiz = (PrivInfo*)malloc(sizeof(PrivInfo));
if (thiz == NULL)
{
printf ("[%s]: Failed to malloc priv.\n", __func__);
return -1;
}
info_init(thiz);
ret = pthread_create(&pt_1, NULL, pthread_func_1, (void*)thiz);
if (ret != 0)
{
perror ("pthread_1_create:");
}
ret = pthread_create(&pt_2, NULL, pthread_func_2, (void*)thiz);
if (ret != 0)
{
perror ("pthread_2_create:");
}
ret = pthread_create(&pt_3, NULL, pthread_func_3, (void*)thiz);
if (ret != 0)
{
perror ("pthread_3_create:");
}
ret = pthread_create(&pt_4, NULL, pthread_func_4, (void*)thiz);
if (ret != 0)
{
perror ("pthread_4_create:");
}
pthread_join(pt_1, NULL);
pthread_join(pt_2, NULL);
pthread_join(pt_3, NULL);
pthread_join(pt_4, NULL);
info_destroy(thiz);
return 0;
}
static void info_init(PrivInfo* thiz)
{
if (thiz == NULL)
{
printf ("[%s]:func error!\n", __func__);
}
thiz->end_time = time(NULL) + 60;
sem_init(&thiz->s1, 0, 4);
sem_init(&thiz->s2, 0, 1);
}
static void info_destroy(PrivInfo* thiz)
{
if (thiz == NULL)
{
printf ("[%s]:func error!\n", __func__);
}
sem_destroy(&thiz->s1);
sem_destroy(&thiz->s2);
free(thiz);
thiz = NULL;
}
static void* pthread_func_1(void* thiz)
{
static int snCount = 1;
if (thiz == NULL)
{
printf ("[%s]:func error!\n", __func__);
}
PrivInfo* pi = (PrivInfo*)thiz;
while (time(NULL) < pi->end_time)
{
if (bUpdate > 0)
{
//printf("pthread1:start bUpdate=[%d]\n", bUpdate);
goto reload;
}
else
{
INTERLOCKEDINCREMENT(nRuning); //+1
printf("pthread1:start nRuning=[%d]\n", nRuning);
INTERLOCKEDINCREMENT(snCount);
if (snCount%3 == 0)
{
INTERLOCKEDINCREMENT(bUpdate);
printf("pthread1:收到/reload all bUpdate=[%d]\n", bUpdate);
}
//sem_wait(&pi->s1);
sleep(3);
//sem_post(&pi->s1);
INTERLOCKEDDECREMENT(nRuning); //-1
printf("pthread------1:end------ nRuning=[%d]\n", nRuning);
}
reload:
{
sem_wait(&pi->s2);
//printf("pthread1: pthread1 lock\n");
if (nRuning == 0 && bUpdate > 0)
{
sleep(3);
INTERLOCKEDDECREMENT(bUpdate);
printf("pthread1:执行/reload all nRuning=[%d]\n", nRuning);
}
//printf("pthread1: pthread1 unlock\n");
sem_post(&pi->s2);
}
}
return thiz;
}
static void* pthread_func_2(void* thiz)
{
static int snCount = 1;
if (thiz == NULL)
{
printf ("[%s]:func error!/n", __func__);
}
PrivInfo* pi = (PrivInfo*)thiz;
while (time(NULL) < pi->end_time)
{
if (bUpdate > 0)
{
//printf("pthread1:start bUpdate=[%d]\n", bUpdate);
goto reload;
}
else
{
INTERLOCKEDINCREMENT(nRuning); //+1
printf("pthread2:start nRuning=[%d]\n", nRuning);
INTERLOCKEDINCREMENT(snCount);
//if (snCount%3 == 0)
//{
// INTERLOCKEDINCREMENT(bUpdate);
// printf("pthread2:收到/reload all bUpdate=[%d]\n", bUpdate);
//}
//sem_wait(&pi->s1);
sleep(3);
//sem_post(&pi->s1);
INTERLOCKEDDECREMENT(nRuning); //-1
printf("pthread------2:end------ nRuning=[%d]\n", nRuning);
}
reload:
{
sem_wait(&pi->s2);
//printf("pthread2: pthread2 lock\n");
if (nRuning == 0 && bUpdate > 0)
{
sleep(3);
INTERLOCKEDDECREMENT(bUpdate);
printf("pthread2:执行/reload all nRuning=[%d]\n", nRuning);
}
//printf("pthread2: pthread2 unlock\n");
sem_post(&pi->s2);
}
}
return thiz;
}
static void* pthread_func_3(void* thiz)
{
static int snCount = 1;
if (thiz == NULL)
{
printf ("[%s]:func error!\n", __func__);
}
PrivInfo* pi = (PrivInfo*)thiz;
while (time(NULL) < pi->end_time)
{
if (bUpdate > 0)
{
//printf("pthread1:start bUpdate=[%d]\n", bUpdate);
goto reload;
}
else
{
INTERLOCKEDINCREMENT(nRuning); //+1
printf("pthread3:start nRuning=[%d]\n", nRuning);
INTERLOCKEDINCREMENT(snCount);
// if (snCount%3 == 0)
// {
// INTERLOCKEDINCREMENT(bUpdate);
// printf("pthread3:收到/reload all bUpdate=[%d]\n", bUpdate);
// }
//sem_wait(&pi->s1);
sleep(3);
//sem_post(&pi->s1);
INTERLOCKEDDECREMENT(nRuning); //-1
printf("pthread------3:end------ nRuning=[%d]\n", nRuning);
}
reload:
{
sem_wait(&pi->s2);
//printf("pthread3: pthread3 lock\n");
if (nRuning == 0 && bUpdate > 0)
{
sleep(3);
INTERLOCKEDDECREMENT(bUpdate);
printf("pthread3:执行/reload all nRuning=[%d]\n", nRuning);
}
//printf("pthread3: pthread1 unlock\n");
sem_post(&pi->s2);
}
}
return thiz;
}
static void* pthread_func_4(void* thiz)
{
static int snCount = 1;
if (thiz == NULL)
{
printf ("[%s]:func error!/n", __func__);
}
PrivInfo* pi = (PrivInfo*)thiz;
while (time(NULL) < pi->end_time)
{
if (bUpdate > 0)
{
//printf("pthread1:start bUpdate=[%d]\n", bUpdate);
goto reload;
}
else
{
INTERLOCKEDINCREMENT(nRuning); //+1
printf("pthread4:start nRuning=[%d]\n", nRuning);
INTERLOCKEDINCREMENT(snCount);
// if (snCount%3 == 0)
// {
// INTERLOCKEDINCREMENT(bUpdate);
// printf("pthread4:收到/reload all bUpdate=[%d]\n", bUpdate);
// }
//sem_wait(&pi->s1);
sleep(3);
//sem_post(&pi->s1);
INTERLOCKEDDECREMENT(nRuning); //-1
printf("pthread------4:end------ nRuning=[%d]\n", nRuning);
}
reload:
{
sem_wait(&pi->s2);
//printf("pthread4: pthread4 lock\n");
if (nRuning == 0 && bUpdate > 0)
{
sleep(3);
INTERLOCKEDDECREMENT(bUpdate);
printf("pthread4:执行/reload all nRuning=[%d]\n", nRuning);
}
//printf("pthread4: pthread4 unlock\n");
sem_post(&pi->s2);
}
}
return thiz;
}
/*
int sem_wait(sem_t*sem);
intsem_trywait(sem_t *sem);
intsem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
struct timespec
*/