由用户空间委托内核空间进行维护；
有赋值、检查、操作、重置的语义；
线程依赖这些变量进行状态的维护和同步；
同步变量是可以线程间共享的变量；
同步变量的查询和操作通过系统调用执行；
可能会阻塞执行；
Synchronization
Computer Science
In computer science, synchronization refers to one of two distinct but related concepts: synchronization of processes, and synchronization of data. Process synchronization refers to the idea that multiple processes are to join up or handshake at a certain point, in order to reach an agreement or commit to a certain sequence of action.