Assuming you weregoing to use Figure11.16 as part of a larger program, describe whereyou might use pthread_testcancel(3) to enable the threads to abort processing early (because some part of this larger program will have a reason to cancel threads and continue with other processing).

Question

What will be an ideal response?

Answer 1

Calling pthread_testcancel inside complong every time it is called would be excessive, because this function will be called many times by the sort library function. However, we can maintain a counter in thread-local memory,increment the counter inside complong,and use the counter to limit the number of times we call pthread_testcancel, to every 1000 calls (for example). Note that it would be a bad idea to call pthread_getspecific(3) inside complong every time it is called, because it is likely just as expensive as pthread_testcancel.Wecan’t change the signature of the complong function, because it is dictated by the sorting library.Toavoid the overhead of calculating which counter to increment, we need a different approach. A better idea is to use a different function for each thread, and hardcode the functions to use different counters. This avoids the runtime overhead at the cost of a sightly larger program. Wecould maintain an array of functions and counters, and pass the index of the array to each thread in a structure, which also contains the starting index of the number range to sort. This approach has the added bene?t that the counters can be used without the need for any synchronization, because each thread uses a different counter.