sched_getaffinity() does not fail if the set is smaller than the
number of CPU. Try with an initial value of ncpus=1. So we cannot
start the heuristic with ncpus=16, because it would only return 16
even if the computer has more cpus.

Instead of this heuristic, why not simply alway using ncpus = CPU_SETSIZE?

I don't know if CPU_SETSIZE is part of the standard (POSIX?).

You may also use a constant size (CPU_SETSIZE) of the set used by
sched_setaffinity() to simplify the code.