If a macro ceases to be useful, it may be undefined 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.
However, if an identifier which is currently a macro is redefined, then the new definition must be effectively the same as the old one. Two macro definitions are effectively the same if:
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)
If a macro is redefined with a definition that is not effectively the same as the old one, the preprocessor issues a warning and changes the macro to use the new definition. If the new definition is effectively the same, the redefinition is silently ignored. This allows, for instance, two different headers to define a common macro. The preprocessor will only complain if the definitions do not match.