关于互斥量和条件变量
条件变量一般用于阻塞线程以及给线程发信号，解除阻塞。而条件变量往往和互斥量一起使用，因为条件变量在阻塞的时候会有一个加锁解锁的过程。还记得以前写过一个生产者——消费者的实例，那时候对一些问题还不是很理解。下面是一个简单的练习实例，以及最后总结遇到的一些问题。
//wait后面要执行的语句必须在while循环之内
//wait后面必须要解锁
//最后一个函数执行的时候不要解锁
//查看pthread_mutex_destroy返回值
//关于内存泄露
#include "../common.h"
pthread_t pid1, pid2, pid3, pid4;
pthread_cond_t num;
pthread_mutex_t mutex1;
int count;
void **state;
void *func1(void *argc)
{
//pthread_detach(pthread_self());
printf("thread1 %lu create success!\n", pthread_self());
printf("1 wait for signal cmd!\n");
pthread_mutex_lock(&mutex1);
while(1)
{
while(count == 0)
{
printf("1count = %d\n", count);
printf("=============================\n");
pthread_cond_wait(&num, &mutex1);
printf("1 reserve signal, i will exit!\n");
printf("%lu exit!\n", pthread_self());
pthread_mutex_unlock(&mutex1);
pthread_exit((void*)&state);
}
}
}
void *func2(void *argc)
{
//pthread_detach(pthread_self());
printf("thread2 %lu create success!\n", pthread_self());
printf("2 wait for signal cmd!\n");
pthread_mutex_lock(&mutex1);
while(1)
{
while(count == 0)
{
printf("2count = %d\n", count);
printf("=============================\n");
pthread_cond_wait(&num, &mutex1);
printf("2 reserve signal, i will exit!\n");
printf("%lu exit!\n", pthread_self());
pthread_mutex_unlock(&mutex1);
pthread_exit((void*)&state);
}
}
}
void *func3(void *argc)
{
//pthread_detach(pthread_self());
printf("thread3 %lu create success!\n", pthread_self());
printf("3 wait for signal cmd!\n");
pthread_mutex_lock(&mutex1);
while(1)
{
while(count == 0)
{
printf("3count = %d\n", count);
printf("=============================\n");
pthread_cond_wait(&num, &mutex1);
printf("3 reserve signal, i will exit!\n");
printf("%lu exit!\n", pthread_self());
pthread_mutex_unlock(&mutex1);
pthread_exit((void*)&state);
}
}
}
void *func4(void *argc)
{
//pthread_detach(pthread_self());
printf("thread4 %lu create success!\n", pthread_self());
printf("4 wait for signal cmd!\n");
pthread_mutex_lock(&mutex1);
while(1)
{
while(count == 0)
{
printf("4count = %d\n", count);
printf("=============================\n");
pthread_cond_wait(&num, &mutex1);
printf("4 reserve signal, i will exit!\n");
printf("%lu exit!\n", pthread_self());
//pthread_mutex_unlock(&mutex1);
pthread_exit((void*)&state);
}
}
}
void add()
{
printf("alls go go go!\n");
if (count == 0)
{
count++;
pthread_cond_broadcast(&num);
pthread_mutex_unlock(&mutex1);
printf("send signal success!\n");
}
}
int main(int argc, char **argv)
{
void **state;
pthread_mutex_init(&mutex1, NULL);
pthread_cond_init(&num, NULL);
pthread_create(&pid1, NULL, func1, NULL);
sleep(1);
pthread_create(&pid2, NULL, func2, NULL);
sleep(1);
pthread_create(&pid3, NULL, func3, NULL);
sleep(1);
pthread_create(&pid4, NULL, func4, NULL);
sleep(1);
add();
#if 1
if (pthread_join(pid1, (void*)&state) != 0)
{
error("pthread_join1");
}
else
{
printf("pthread join 1 success!\n");
printf("1 = %d\n", state);
}
if (pthread_join(pid2, (void*)&state) != 0)
{
error("pthread_join2");
}
else
{
printf("pthread join 2 success!\n");
printf("2 = %d\n", state);
}
if (pthread_join(pid3, (void*)&state) != 0)
{
error("pthread_join3");
}
else
{
printf("pthread join 3 success!\n");
printf("3 = %d\n", state);
}
if (pthread_join(pid4, (void*)&state) != 0)
{
error("pthread_join4");
}
else
{
printf("pthread join 4 success!\n");
printf("4 = %d\n", state);
}
#endif
sleep(3);
if (pthread_mutex_destroy(&mutex1) != 0)
{
printf("%d\n", pthread_mutex_destroy(&mutex1)); //16
error("pthread_mutex_destroy");
}
if (pthread_cond_destroy(&num) != 0)
{
error("pthread_cond_destroy");
}
printf("main thread exit!\n");
printf("%d\n", EBUSY); //16
return 0;
}程序很好理解，就是阻塞四个线程，然后通过广播信号对他们解除阻塞，继续执行完程序。当然，也可以使用pthread_cond_signal单个通知线程。
下面是一个简单的输出：
thread1 77757296 create success!
1 wait for signal cmd!
1count = 0
=============================
thread2 92441456 create success!
2 wait for signal cmd!
2count = 0
=============================
thread3 102931312 create success!
3 wait for signal cmd!
3count = 0
=============================
thread4 113421168 create success!
4 wait for signal cmd!
4count = 0
=============================
alls go go go!
1 reserve signal, i will exit!
2 reserve signal, i will exit!
92441456 exit!
3 reserve signal, i will exit!
102931312 exit!
77757296 exit!
send signal success!
4 reserve signal, i will exit!
113421168 exit!
pthread join 1 success!
1 = 134521716
pthread join 2 success!
2 = 134521716
pthread join 3 success!
3 = 134521716
pthread join 4 success!
4 = 134521716
main thread exit!
16输出有点乱，因为广播解锁以后，所有阻塞的线程都会竞争锁资源，我们可以适当加延时以看到比较有秩序的输出。
最关键的问题在于wait之前的加锁，广播之后的解锁，以及执行完线程之后的解锁。
pthread_cond_wait之前要加锁，因为此函数会自动解锁资源，将其放于死循环里面的原因是有多个线程等待继续执行，因此我们要不断的判断条件，且在broadcast之后，要改变条件，不然wait会一直阻塞。
关于遇到的问题：
- wait解除阻塞后会自动再加锁，因此我们在后面还要对其解锁，不然程序永远只会执行一个线程，因为没有解锁我们的临界资源，其他的wait获取不到。
- wait之后要执行的语句需要放在while循环体里面，这样才会执行到
关于pthread_mutex_destroy的返回值，如果我们最后一个线程执行函数还在占用互斥量，则此函数的返回值为16，并且出错显示success，man手册查看了此函数的出错如下：
ERRORS The pthread_mutex_destroy() function may fail if: EBUSY The implementation has detected an attempt to destroy the object referenced by mutex while it is locked or referenced (for example, while being used in a pthread_cond_timedwait() or pthread_cond_wait()) by another thread. EINVAL The value specified by mutex is invalid. The pthread_mutex_init() function shall fail if: EAGAIN The system lacked the necessary resources (other than memory) to initialize another mutex. ENOMEM Insufficient memory exists to initialize the mutex. .................................可以看见当errno为EBUSY的时候，对应的说明是线程还在占用互斥量资源则会产生此错误，而打印EBUSY的值正好是16，所以按理说每一个线程wait以后的内容应该要给互斥量解锁，但是如果这里给最后一个线程，也就是func4解锁，那么最后的结果就是EBUSY，这个问题很奇怪，目前还未解决。
对于线程的内存泄漏，这个程序的问题很大， 有很多释放了但是还可以使用的资源，相当与free了指针，但是没有给此指针赋空值，这个问题目前也尚未解决，等以后查找到问题所在，再来看看。希望有看了文章的人，能给出意见。