Approaches of Mutual Exclusion
•Software Approaches
•Hardware Support
•Semaphores
•Monitors
•Message Passing
Semaphores
•Special variable called a semaphore is used for signaling. •If a process is waiting for a signal, it is blocked until that signal is sent.
•Wait and Signal operations cannot be interrupted. •Queue is used to hold processes waiting on the semaphore.
Semaphores字符常量池是什么意思
•Semaphore is a variable that has an integer value.
–May be initialized to a nonnegative number .
–Wait and Signal are primitives (atomic, cannot be interrupted and each routine can be treated as an indivisible step).
Semaphores
•Wait operation decrements the semaphore value  - wait(s)：s－1
- wait操作：申请资源且可能阻塞自己(s＜0)
•Signal operation increments semaphore value
-  signal(s)：s＋1
- signal操作：释放资源并唤醒阻塞进程(s≤0)
Types of Semaphores
•General Semaphore (通用信号量）
–通用信号量是记录型，其中一个域为整型，另一个域为队列，其元素为等待该信号量的阻塞进程（FIFO）
•Binary Semaphore (二进制信号量）

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