IEEE Standards Interpretations for IEEE Std 1003.1c™-1995 IEEE Standard for Information Technology--Portable Operating System Interface (POSIX(R)) - System Application Program Interface (API) Amendment 2: Threads Extension (C Language)
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Interpretation Request #33
Topic: Thread Cancellation, Async-Cancel Safety Relevant Clauses: 18.1.4
pthread_cancel is specified to be async-cancel safe, along with the pthread_setcancelstate and pthread_setcanceltype functions. I see no reasonable justification for allowing cancellation while async cancellation is enabled, nor does the rationale attempt to give any justification.
I believe that pthread_cancel should be removed from the list of async-cancel safe functions. Async cancellation is an unusual "aberration", and I see no reason to allow use of any POSIX standard functions with async cancellation type except the functions required to disable cancellation.
The standard is clear that pthread_cancel shall be async-cancel safe, and conforming implementations must conform to this.
Rationale for Interpretation
Requiring pthread_cancel() to be async-cancel safe potentially makes it easier for helper threads running with asynchronous cancellation enabled to themselves cancel other threads. This imposes no undue burden on implementations since, if necessary, the pthread_cancel() implementation can always internally set the cancellation state to defer cancellation while pthread_cancel() is being executed and then restore the cancellation state after the cancel request has been executed.