Question

Consider the following C program (ignore the double side-effect issue):

#define max(a, b) (a>b?a:b)

int main(void){
    int max = max(5,6);
    return max;
}

The GCC preprocessor turns this into:

int main(void){
    int max = (5>6?5:6);
    return max;
}

Which is quite nice, since you don’t have to worry about unintentional collisions between max and max(). The GCC manual says:

A function-like macro is only expanded if its name appears with a pair of parentheses after it. If you write just the name, it is left alone

Is this standardized or just something done by convention?

Answer 1

Yes, the behavior here is well-defined.

Your macro max is a function-like macro (i.e., when you define it, its name is followed immediately by a left parenthesis and it takes arguments).

A use of max later in your code is only an invocation of that macro if the use of max is followed by a left parenthesis. So, these would not invoke the max macro:

int max;
max = 42;

But these would all invoke the max macro:

max(1, 2)
max (1, 2)
max
(
    1, 2
)
max()

(Note that the last line is ill-formed because the number of arguments does not match the number of parameters. This is still a macro invocation, though, and would cause a compilation error.)

This behavior is mandated by the C langauge standard. C99 §6.10.3/10 states that after a function-like macro has been defined,

Each subsequent instance of the function-like macro name followed by a ( as the next preprocessing token introduces the sequence of preprocessing tokens that is replaced by the replacement list in the definition (an invocation of the macro).