Macro Replacement

The macro replacement process has never been described in detail prior to ANSI/ISO C. This vagueness spawned a great many divergent implementations. Any code that relied on anything fancier than manifest constant replacement and simple function-like macros was probably not truly portable. This manual cannot uncover all the differences between the old C macro replacement implementation and the ANSI/ISO C version. Nearly all uses of macro replacement with the exception of token pasting and stringizing produce exactly the same series of tokens as before. Furthermore, the ANSI/ISO C macro replacement algorithm can do things not possible in the old C version. For example,

#define name (*name) 

causes any use of name to be replaced with an indirect reference through name. The old C preprocessor would produce a huge number of parentheses and stars and eventually produce an error about macro recursion.

The major change in the macro replacement approach taken by ANSI/ISO 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.