Function-like Macros (The C Preprocessor)

### 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()
    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 */
    ...
      foo();
      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 parentheses.

    #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.