Complex sentence: The getcontext function does not provide a return value to distinguish the cases, so the programmer must use an explicit flag variable, which must not be a register variable and must be declared as volatile to avoid constant propagation or other compiler optimisations.
Split into simpler sentences: The getcontext function does not provide a return value to distinguish the cases, so the programmer must use an explicit flag variable. This must be declared as volatile, and not a register variable, to avoid constant propagation or other compiler optimisations.