26.5.2 Function-like Macros

You can also define macros whose use looks like a function call. These are called function-like macros. To define one, use the #define directive with a pair of parentheses immediately after the macro name. For example,

#define lang_init()  c_init()
lang_init()
     → c_init()

A function-like macro is expanded only when its name appears with a pair of parentheses after it. If you write just the name, without parentheses, 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. Whitespace and line breaks before or between the parentheses are ignored when the macro is called.

extern void foo(void);
#define foo() /* optimized inline version */
/* … */
  foo();
  funcptr = foo;

Here the call to foo() expands the macro, but the function pointer funcptr gets the address of the real function foo. If the macro were to be expanded there, 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 parentheses. Here is an example:

#define lang_init ()    c_init()
lang_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.

Any name can have at most one macro definition at a time. Thus, you can’t define the same name as an object-like macro and a function-like macro at once.