The Structural Wrapper: do { ... } while (0)#

Standard macro design pattern. Forces the macro to behave like a single C statement, so it can’t break syntax when used inside an if/else without braces.

#define LOG_DEBUG(msg) do { \
    printk(KERN_DEBUG msg); \
    debug_count++; \
} while (0)

if (tmp)
    LOG_DEBUG("checking condition\n");
else
    do_something();

Two statements is where this trick stops being a nice-to-have and becomes necessary. Without the wrapper:

#define LOG_DEBUG(msg) printk(KERN_DEBUG msg); debug_count++;

expands the if branch into two separate statements, only the first one belongs to the if, the second runs unconditionally regardless of tmp. Wrap braces { } around it instead and you hit the classic dangling-; problem: the trailing ; after the macro call becomes an empty statement, and the else ends up with no matching if.

The do { } while (0) fixes both: it groups any number of statements into one block, and swallows the semicolon you put after the macro call. while (0) never loops, so the compiler optimizes it away, zero runtime cost.