然而，如果调用线程已经存在一个事务，则将此事务挂起，并创建和使用一个新事务。
However, if there is already a transaction on the calling thread, that transaction is suspended, and a new transaction is created and used.
这个函数会挂起调用线程，直到这个信号量有一个非空计数为止。
This function suspends the calling thread until the semaphore has a non-zero count.
当拥有锁的线程离开临界区的时候，挂起的线程需要被重新唤醒，然后重新被调用，并交换上下文，回到CPU调度中。
When the thread that has the lock leaves the critical section, the suspended thread needs to be woken up. The thread will need to be re-scheduling and context switched back into the CPU.
LinuxPOSIX信号使用sem_wait来挂起发出调用的线程，直到信号拥有了非零的计数值。
Linux POSIX semaphores use sem_wait to suspend the calling thread until the semaphore has a non-zero count.
LinuxPOSIX信号使用sem_wait来挂起发出调用的线程，直到信号拥有了非零的计数值。
Linux POSIX semaphores use sem_wait to suspend the calling thread until the semaphore has a non-zero count.
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