3.2. Function-like Macros
You can also define macros whose use looks like a function call. These
are called function-like macros. To define a function-like macro,
you use the same #define directive, but you put a pair of
parentheses immediately after the macro name. For example,
#define lang_init() c_init()
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.
This can be useful when you have a function and a macro of the same
name, and you wish to use the function sometimes.
extern void foo(void);
#define foo() /* optimized inline version */
funcptr = foo;
Here the call to foo() will use the macro, but the function
pointer will get the address of the real function. If the macro were to
be expanded, it would cause a syntax error.
If you put spaces between the macro name and the parentheses in the
macro definition, that does not define a function-like macro, it defines
an object-like macro whose expansion happens to begin with a pair of
#define lang_init () c_init()
==> () c_init()()
The first two pairs of parentheses in this expansion come from the
macro. The third is the pair that was originally after the macro
invocation. Since lang_init is an object-like macro, it does not
consume those parentheses.