Return Values
sem_trywait() returns zero after completing successfully. Any other returned value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.
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EINVAL
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sem points to an illegal address.
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EINTR
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A signal interrupted this function.
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EAGAIN
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The semaphore was already locked, so it cannot be immediately locked by the sem_trywait() operation.