Question 11.18

What does the message ``warning: macro replacement within a string literal'' mean?

Some pre-ANSI compilers/preprocessors interpreted macro definitions like

	#define TRACE(var, fmt) printf("TRACE: var = fmt\n", var)

such that invocations like

	TRACE(i, %d);

were expanded as

	printf("TRACE: i = %d\n", i);

In other words, macro parameters were expanded even inside string literals and character constants.

Macro expansion is not defined in this way by K&R or by Standard C. When you do want to turn macro arguments into strings, you can use the new # preprocessing operator, along with string literal concatenation (another new ANSI feature):

	#define TRACE(var, fmt) \
		printf("TRACE: " #var " = " #fmt "\n", var)