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The macro replacement process has never been described in any significant detail prior to ANSI C. This vagueness spawned a great many divergent implementations and any code that relied on anything fancier than manifest constant replacement and simple ?:-like macros was probably not truly portable. This tutorial cannot begin to uncover all the subtle and not so subtle differences between the old C macro replacement implementation and the ANSI C version. Fortunately, nearly all uses of macro replacement with the exception of token pasting and stringizing will produce exactly the same series of tokens as before. Furthermore, the ANSI C macro replacement algorithm can do things not possible in the old C version. For example:
#define name (causes any use of name to be replaced with an indirect reference through name. (The old C preprocessor would produce a large amount of parentheses and stars and eventually complain about macro recursion.)
The major change in the macro replacement approach taken by ANSI C is to require macro arguments (other than those that are operands of the macro substitution operators # and ## ) to be expanded recursively prior to their substitution in the replacement token list. However, this change seldom produces an actual difference in the resulting tokens.