Question 10.4

What's the best way to write a multi-statement macro?

The usual goal is to write a macro that can be invoked as if it were a statement consisting of a single function call. This means that the ``caller'' will be supplying the final semicolon, so the macro body should not. The macro body cannot therefore be a simple brace-enclosed compound statement, because syntax errors would result if it were invoked (apparently as a single statement, but with a resultant extra semicolon) as the if branch of an if/else statement with an explicit else clause.

The traditional solution, therefore, is to use

	#define MACRO(arg1, arg2) do {	\
		/* declarations */	\
		stmt1;			\
		stmt2;			\
		/* ... */		\
		} while(0)	/* (no trailing ; ) */

When the caller appends a semicolon, this expansion becomes a single statement regardless of context. (An optimizing compiler will remove any ``dead'' tests or branches on the constant condition 0, although lint may complain.)

If all of the statements in the intended macro are simple expressions, with no declarations or loops, another technique is to write a single, parenthesized expression using one or more comma operators. (For an example, see the first DEBUG() macro in question 10.26.) This technique also allows a value to be ``returned.''

References: H&S Sec. 3.3.2 p. 45
CT&P Sec. 6.3 pp. 82-3

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