6.5.4 Macro Replacement

The macro replacement process has never been described in detail prior to 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 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 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 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.