Why is the macro giving me the warning

Q

Why is the macro
#define TRACE(n) printf("TRACE: %d\n", n)
giving me the warning ``macro replacement within a string literal''? It seems to be expanding
TRACE(count);
as
printf("TRACE: %d\count", count);

✍: Guest

A

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. (This interpretation may even have been an accident of early implementations, but it can prove useful for macros like this.)
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)

2016-01-25, 1804👍, 0💬