how does do{} while(0) work in macro?

Question

Though this topic has been discussed many times in this forum and all other forums, still I have doubts. Please help.

How does the do{} while(0) in macr

Answer 1

IIRC the use of the do-while in macros is to make them look more like a normal function invocation; there are some subtle syntax issues around unbraced if statements and things like that. Without the do-while the macro might look like a normal function invocation but would work differently.

I would guess that in this case those macros are being used so certain function calls compile away to nothing; it looks like that might be what you get if CONFIG_PREEMPT wasn't set, so certain parts of the kernel that are only necessary for preempt simply vanish without it. So those loops do not disable preempt or reschedule anything; there'll be another definition (probably a real function) elsewhere in the kernel source.