Книга: Programming with POSIX® Threads

2.2.3 Running and blocking

2.2.3 Running and blocking

Like us, threads usually can't stay awake their entire life. Most threads occasionally go to sleep. A thread can go to sleep because it needs a resource that is not available (it becomes "blocked") or because the system reassigned the processor on which it was running (it is "preempted"). A thread spends most of its active life in three states: ready, running, and blocked.

A thread is ready when it is first created, and whenever it is unblocked so that it is once again eligible to run. Ready threads are waiting for a processor. Also, when a running thread is preempted, for example, if it is timesliced (because it has run too long), the thread immediately becomes ready.

A thread becomes running when it was ready and a processor selects the thread for execution. Usually this means that some other thread has blocked, or has been preempted by a timeslice—the blocking (or preempted) thread saves its context and restores the context of the next ready thread to replace itself. On a multiprocessor, however, a previously unused processor may execute a readied thread without any other thread blocking.

A thread becomes blocked when it attempts to lock a mutex that is currently locked, when it waits on a condition variable, when it calls sigwait for a signal that is not currently pending, or when it attempts an I/O operation that cannot be immediately completed. A thread may also become blocked for other system operations, such as a page fault.

When a thread is unblocked after a wait for some event, it is made ready again. It may execute immediately, for example, if a processor is available. In lifecycle.c, the main thread blocks at line 23, in pthread_join, to wait for the thread it created to run. If the thread had not already run at this point, it would move from ready to running when main becomes blocked. As the thread runs to completion and returns, the main thread will be unblocked—returning to the ready state. When processor resources are available, either immediately or after the thread becomes terminated, main will again become running, and complete.

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