<P> A process transitions to a blocked state when it can not carry on without an external change in state or event occurring . For example , a process may block on a call to an I / O device such as a printer , if the printer is not available . Processes also commonly block when they require user input , or require access to a critical section which must be executed atomically . Such critical sections are protected using a synchronization object such as a semaphore or mutex . </P>
when does a process enters a blocked state
when it can not carry on without an external change in state or event occurring