/* The state of a semaphore is represented by a long int encoding either the semaphore count if >= 0 and no thread is waiting on it, or the head of the list of threads waiting for the semaphore. To distinguish the two cases, we encode the semaphore count N as 2N+1, so that it has the lowest bit set.
A sequence of sem_wait operations on a semaphore initialized to N result in the following successive states: 2N+1, 2N-1, ..., 3, 1, &first_waiting_thread, &second_waiting_thread, ... */
int sem_init(sem_t *sem, int pshared, unsigned int value) { if (value > SEM_VALUE_MAX) { errno = EINVAL; return -1; } // 还没实现 if (pshared) { errno = ENOSYS; return -1; } // 记录资源数 sem->sem_status = ((long)value << 1) + 1; return 0; }
int sem_wait(sem_t * sem) { long oldstatus, newstatus; volatile pthread_t self = thread_self(); pthread_t * th;
while (1) { do { // oldstatus可能是线程或者资源数 oldstatus = sem->sem_status; // 大于1说明有资源，等于1说着0说明没有资源或没有资源并且有线程阻塞 if ((oldstatus & 1) && (oldstatus != 1)) newstatus = oldstatus - 2; else { // 没有可用资源，需要阻塞 newstatus = (long) self; // 保存这时候的资源数或者上一个被阻塞的线程 self->p_nextwaiting = (pthread_t) oldstatus; } } // sem_status可能指向资源数或者被阻塞的线程链表。赋值成功则返回1，否则返回0 while (! compare_and_swap(sem, oldstatus, newstatus)); // self是按偶数地址对齐的，低位为1说明是还有可用资源 if (newstatus & 1) /* We got the semaphore. */ return 0; /* Wait for sem_post or cancellation */ // 等待被restart或者cancel信号唤醒 suspend_with_cancellation(self); /* This is a cancellation point */ // 判断是否被取消了，即是被cancel信号唤醒的，不是的话重新判断是否有资源，即回到上面的while(1) if (self->p_canceled && self->p_cancelstate == PTHREAD_CANCEL_ENABLE) { /* Remove ourselves from the waiting list if we're still on it */ /* First check if we're at the head of the list. */ do { // 得到被阻塞的第一个线程 oldstatus = sem->sem_status; // 相等说明当前线程是最后一个被阻塞的线程 if (oldstatus != (long) self) break; // 得到该线程被阻塞时的资源数或下一个被阻塞的线程 newstatus = (long) self->p_nextwaiting; } // sem_status指向资源数或者下一个被阻塞的线程 while (! compare_and_swap(sem, oldstatus, newstatus)); /* Now, check if we're somewhere in the list. There's a race condition with sem_post here, but it does not matter: the net result is that at the time pthread_exit is called, self is no longer reachable from sem->sem_status. */ // 可能是break或者while为true，不是当前线程并且不是资源数，即oldstatus指向一个其他线程 if (oldstatus != (long) self && (oldstatus & 1) == 0) { th = &(((pthread_t) oldstatus)->p_nextwaiting); // 不是资源数，即是线程，从等待的线程链表中删除self线程，因为他即将退出 while (*th != (pthread_t) 1 && *th != NULL) { if (*th == self) { *th = self->p_nextwaiting; break; } th = &((*th)->p_nextwaiting); } } // 当前线程退出 pthread_exit(PTHREAD_CANCELED); } } } // 非阻塞获取信号量 int sem_trywait(sem_t * sem) { long oldstatus, newstatus;
