Question

Possible Duplicates:
What’s the use of do while(0) when we define a macro?
Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?
C multi-line macro: do/while(0) vs scope block

I have seen a lot of usages like this, previously I though that the programmer wanted to break out of a block of code easily. Why do we need a do { … } while (0) loop here? Are we trying to tell the compiler something?

For instance in Linux kernel 2.6.25, include/asm-ia64/system.h

/*
 * - clearing psr.i is implicitly serialized (visible by next insn)
 * - setting psr.i requires data serialization
 * - we need a stop-bit before reading PSR because we sometimes
 *   write a floating-point register right before reading the PSR
 *   and that writes to PSR.mfl
 */
#define __local_irq_save(x)         \
do {                    \
    ia64_stop();                \
    (x) = ia64_getreg(_IA64_REG_PSR);   \
    ia64_stop();                \
    ia64_rsm(IA64_PSR_I);           \
} while (0)

Answer 1

It’s always used in macros so that a semicolon is required after a call, just like when calling a regular function.

In your example, you have to write

__local_irq_save(1);

while

__local_irq_save(1)

would result in an error about a missing semicolon. This would not happen if the do while was not there. If it was just about scoping, a simple curly brace pair would suffice.