| 词条 | Spurious wakeup |
| 释义 |
Even after a condition variable appears to have been signaled from a waiting thread's point of view, the condition that was awaited may still be false. One of the reasons for this is a spurious wakeup; that is, a thread might be awoken from its waiting state even though no thread signaled the condition variable. For correctness it is necessary, then, to verify that the condition is indeed true after the thread has finished waiting. Because spurious wakeup can happen repeatedly, this is achieved by waiting inside a loop that terminates when the condition is true, for example: /* In any waiting thread: */ while(!buf->full) wait(&buf->cond, &buf->lock); /* In any other thread: */ if(buf->n >= buf->size){ buf->full = 1; signal(&buf->cond); } In this example it is known that another thread will set buf->full (the actual condition awaited) before signaling buf->cond (the means of synchronizing the two threads). The waiting thread will always verify the truth of the actual condition upon returning from According to David R. Butenhof's Programming with POSIX Threads {{ISBN|0-201-63392-2}}: "This means that when you wait on a condition variable, the wait may (occasionally) return when no thread specifically broadcast or signaled that condition variable. Spurious wakeups may sound strange, but on some multiprocessor systems, making condition wakeup completely predictable might substantially slow all condition variable operations. The race conditions that cause spurious wakeups should be considered rare." External links
| url=http://zthread.sourceforge.net/html/classZThread_1_1Condition.html | title=ZThreads: Condition Class Reference | author=Eric Crahen | quote=A Condition is not subject to spurious wakeup. | date=2003-07-16}}
| url=http://blog.vladimirprus.com/2005/07/spurious-wakeups.html | title= Spurious wakeups | author=Vladimir Prus | date=2005-07-06}}
| url=http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_timedwait.html | title=pthread_cond_timedwait, pthread_cond_wait - wait on a condition | website=Single UNIX Specification | publisher=The Open Group | date=2013}}
| url=https://msdn.microsoft.com/en-us/library/ms686301%28v=vs.85%29.aspx | title=SleepConditionVariableCS function | publisher=MSDN | date=2016-07-15}}
| url=https://blogs.msdn.microsoft.com/oldnewthing/20180201-00/?p=97946 | title=Spurious wake-ups in Win32 condition variables | author=Raymond Chen | date=2018-02-01}}
2 : C POSIX library|Threads (computing) |
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