接着上一篇，继续讨论如何利用pthread_cond_t来实现sem_t。目前的Linux内核都支持信号量sem_t，但也有一些老的OS，如AIX4，和早期的Solaris并不支持信号量，毕竟semaphore这个东东并没有包括在POSIX标准里。这种情况下有必要利用pthread_mutex_t + pthread_cond_t来模拟semaphore啦。（什么？pthread_mutex_t pthread_cond_t也不支持？ ！@#￥%！）
其实实现起来真的很简单，
class Semaphore {
protected:
pthread_mutex_t mutex;
pthread_cond_t cond;
unsigned int waiters;
public:
Semaphore();
virtual ~Semaphore() {}
int p();
int v();
}
Semaphore::Semaphore() {
mutex = PTHREAD_MUTEX_INITIALIZER;
cond = PTHREAD_COND_INITIALIZER;
waiters = 0;
}
int Semaphore::p() {
pthread_mutex_lock(&mutex);
while (waiters <= 0) {
pthread_cond_wait(&cond, &mutex);
}
waiters--;
pthread_mutex_unlock(&mutex);
}
int Semaphore::v() {
pthread_mutex_lock(&mutex);
waiters++;
if (waiters > 0) {
pthread_cond_signal(&cond);
}
pthread_mutex_unlock(&mutex);
}