26.5.8 Undefining and Redefining Macros

You can undefine a macro with the #undef directive. #undef takes a single argument, the name of the macro to undefine. You use the bare macro name, even if the macro is function-like. It is an error if anything appears on the line after the macro name. #undef has no effect if the name is not a macro.

#define FOO 4
x = FOO;        → x = 4;
#undef FOO
x = FOO;        → x = FOO;

Once a macro has been undefined, that identifier may be redefined as a macro by a subsequent #define directive. The new definition need not have any resemblance to the old definition.

You can define a macro again without first undefining it only if the new definition is effectively the same as the old one. Two macro definitions are effectively the same if:

• Both are the same type of macro (object- or function-like).
• All the tokens of the replacement list are the same.
• If there are any parameters, they are the same.
• Whitespace appears in the same places in both. It need not be exactly the same amount of whitespace, though. Remember that comments count as whitespace.

These definitions are effectively the same:

#define FOUR (2 + 2)
#define FOUR         (2    +    2)
#define FOUR (2 /* two */ + 2)

but these are not:

#define FOUR (2 + 2)
#define FOUR ( 2+2 )
#define FOUR (2 * 2)
#define FOUR(score,and,seven,years,ago) (2 + 2)

This allows two different header files to define a common macro.

You can redefine an existing macro with #define, but redefining an existing macro name with a different definition results in a warning.