条件变量
记得加锁和while判断
void cond_singal()
{
pthread_mutex_lock(&lock);
// 使自定义条件满足
pthread_cond_signal(&condition);
pthread_mutex_unlock(&lock);
}
void cond_wait()
{
pthread_mutex_lock(&lock);
while (/*判断自定义条件不满足*/) {
pthread_cond_wait(&condition, &lock);
}
pthread_mutex_unlock(&lock);
}
join实现
条件变量(condition variable)是一个显示队列，当条件不满足时，线程可以把自己加入到队列中，等待该条件
当另一个线程改变条件时，就可以唤醒一个或者多个等待的线程，让他们继续执行
#include <pthread/pthread.h>
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condition = PTHREAD_COND_INITIALIZER;
int done = 0;
void custom_thread_exist()
{
pthread_mutex_lock(&lock);
done = 1;
pthread_cond_signal(&condition);
pthread_mutex_unlock(&lock);
}
void custom_thread_join()
{
pthread_mutex_lock(&lock);
while (done == 0) {
pthread_cond_wait(&condition, &lock);
}
pthread_mutex_unlock(&lock);
}
void *childThread(void *args)
{
printf("child start\n");
custom_thread_exist();
printf("child end\n");
return NULL;
}
int testEntry()
{
printf("parent start\n");
pthread_t p;
pthread_create(&p, NULL, childThread, NULL);
custom_thread_join();
printf("parent end\n");
return 0;
}
pthread_cond_wait调用又一个参数，它是互斥量。它假定在wait调用的时候，这个互斥量是已上锁状态。
wait的职责是释放锁, 并让调用的线程休眠。当被唤醒时，它必须重新获取锁，在返回给调用者。
这样复杂的步骤也是为了避免在线程陷入休眠时，产生一些竞态条件。
done变量的必要性
void custom_thread_exist()
{
pthread_mutex_lock(&lock);
// done = 1;
pthread_cond_signal(&condition);
pthread_mutex_unlock(&lock);
}
void custom_thread_join()
{
pthread_mutex_lock(&lock);
// while (done == 0) {
pthread_cond_wait(&condition, &lock);
// }
pthread_mutex_unlock(&lock);
}
custom_thread_exist 和 custom_thread_join重的done变量还是有必要的，如果子线程先结束，主线程就会不加判断的盲目等待
有必要在wait/signal前后加锁么
void custom_thread_exist()
{
done = 1;
pthread_cond_signal(&condition);
}
void custom_thread_join()
{
while (done == 0) {
pthread_cond_wait(&condition, &lock);
}
}
假设父进程在执行join的时候，检查done==0，然后试图睡眠，在调用wait之前，被中断。子线程改变done=1，发出信号。父进程就会永久睡眠了
生产者消费者
#include <pthread/pthread.h>
#define kBufferSize 2
static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t empty = PTHREAD_COND_INITIALIZER;
static pthread_cond_t full = PTHREAD_COND_INITIALIZER;
static int buffer[kBufferSize];
static int count = 0; // 大小为1
static int use = 0;
static int fill = 0;
static void put(int value)
{
buffer[fill] = value;
fill = (fill + 1) % kBufferSize;
count++;
}
static int get()
{
int tmp = buffer[use];
use = (use + 1) % kBufferSize;
count--;
return tmp;
}
static void *producer(void *arg)
{
int loops = (int)arg;
for (int i = 0; i < loops; ++i) {
pthread_mutex_lock(&lock);
while (count == kBufferSize) {
pthread_cond_wait(&empty, &lock);
}
put(i);
printf("produce %d \n", i);
pthread_cond_signal(&full);
pthread_mutex_unlock(&lock);
}
return NULL;
}
static void *consumer(void *arg)
{
int loops = (int)arg;
for (int i = 0; i < loops; ++i) {
pthread_mutex_lock(&lock);
while (count == 0) {
pthread_cond_wait(&full, &lock);
}
int tmp = get();
printf("consume %d \n", tmp);
pthread_cond_signal(&empty);
pthread_mutex_unlock(&lock);
}
return NULL;
}
static int testEntry()
{
printf("parent start\n");
pthread_t p1, p2, c1, c2;
pthread_create(&p1, NULL, producer, (void *)10);
pthread_create(&p2, NULL, producer, (void *)10);
pthread_create(&c1, NULL, consumer, (void *)10);
pthread_create(&c2, NULL, consumer, (void *)10);
printf("parent end\n");
return 0;
}
C1需要分配100的内存，C2需要分配10的内存，当一个P1释放free(50)内存时pthread_cond_signal，可能C1会被唤醒, C1发现空间不够，继续睡觉，当pthread_cond_signal不确定会唤醒哪一个线程，可以使用pthread_cond_broadcast唤醒所有线程，代价是唤起了所有线程，性能可能会有消耗
生产者消费者中，由于生产者之间没有区别，消费者之间也没有区别，使用pthread_cond_signal足够了
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