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    <p>Remember that a Mutex <strong>must</strong> first be acquired/taken <strong>then</strong> released/given. In addition, the task that acquires the mutex <strong>owns</strong> it. This prevents another task from releasing a mutex it doesn't own.</p> <p>With that being the case, it becomes clear that since an ISR cannot acquire a mutex (or any semaphore for that matter - it's a blocking operation), then it follows that it can't give the mutex.</p> <p>It is quite possible for an ISR to do give a Binary or Counting semaphore to signal a task that something happens. But mutexes are always a take/give pair.</p> <p>To clarify a point. In VxWorks, the ISR context is <strong>not</strong> the same as the context of a task!<br> The following scenario is invalid:</p> <pre> Task A ISR semTake(mutex) .... semGive(mutex) </pre> <p>Task A <strong>owns</strong> the mutex. When the ISR runs, it executes in a totaly different context. Most current processors have a separate ISR stack. Since Task A owns the mutex, how could the ISR give it up? In fact, what guarantee do you have that the ISR will fire while A has the mutex.<br> Even assuming you "could" give a mutex in an ISR, how would you handle the following scenario:</p> <pre> Task A Task B ISR semTake(mutex) ... &lt;context switch happens> &lt;B runs> semGive(mutex) </pre> <p>Task A gets switched out due to a call unrelated to the mutex, and Task B runs. The ISR now executes while B was running. Would it still be valid for the ISR to be given?</p> <p>Regardless of all this, the simple fact is that a mutex is always used in a pair of Get/Set. I fail to see a use case where you would have an isolated semGive.</p> <p>Is there a specific situation you have in mind that would require a semGive from an ISR context?</p>
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    1. POWhy a mutex cannot be released from an ISR
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    1. COI agree with you comment that any semaphore should not be acquired in an ISR , but in case of mutex cant the same task release it which had acquired it earlier . So the statement "Mutex cannot be released within ISR" sounds a misnomer
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    2. CONo i donot have specific situation, I was just going through Vxworks programmers guide which states the above, didnot realise that ISR runs in a different context.
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