cpp.info: Argument Prescan

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Argument Prescan

   Macro arguments are completely macro-expanded before they are
substituted into a macro body, unless they are stringified or pasted
with other tokens.  After substitution, the entire macro body, including
the substituted arguments, is scanned again for macros to be expanded.
The result is that the arguments are scanned _twice_ to expand macro
calls in them.
   Most of the time, this has no effect.  If the argument contained any
macro calls, they are expanded during the first scan.  The result
therefore contains no macro calls, so the second scan does not change
it.  If the argument were substituted as given, with no prescan, the
single remaining scan would find the same macro calls and produce the
same results.
   You might expect the double scan to change the results when a
self-referential macro is used in an argument of another macro (*note
Self-Referential Macros::): the self-referential macro would be
expanded once in the first scan, and a second time in the second scan.
However, this is not what happens.  The self-references that do not
expand in the first scan are marked so that they will not expand in the
second scan either.
   You might wonder, "Why mention the prescan, if it makes no
difference?  And why not skip it and make the preprocessor faster?"
The answer is that the prescan does make a difference in three special
cases:
   * Nested calls to a macro.
     We say that "nested" calls to a macro occur when a macro's argument
     contains a call to that very macro.  For example, if `f' is a macro
     that expects one argument, `f (f (1))' is a nested pair of calls to
     `f'.  The desired expansion is made by expanding `f (1)' and
     substituting that into the definition of `f'.  The prescan causes
     the expected result to happen.  Without the prescan, `f (1)' itself
     would be substituted as an argument, and the inner use of `f' would
     appear during the main scan as an indirect self-reference and
     would not be expanded.
   * Macros that call other macros that stringify or concatenate.
     If an argument is stringified or concatenated, the prescan does not
     occur.  If you _want_ to expand a macro, then stringify or
     concatenate its expansion, you can do that by causing one macro to
     call another macro that does the stringification or concatenation.
     For instance, if you have
          #define AFTERX(x) X_ ## x
          #define XAFTERX(x) AFTERX(x)
          #define TABLESIZE 1024
          #define BUFSIZE TABLESIZE
     then `AFTERX(BUFSIZE)' expands to `X_BUFSIZE', and
     `XAFTERX(BUFSIZE)' expands to `X_1024'.  (Not to `X_TABLESIZE'.
     Prescan always does a complete expansion.)
   * Macros used in arguments, whose expansions contain unshielded
     commas.
     This can cause a macro expanded on the second scan to be called
     with the wrong number of arguments.  Here is an example:
          #define foo  a,b
          #define bar(x) lose(x)
          #define lose(x) (1 + (x))
     We would like `bar(foo)' to turn into `(1 + (foo))', which would
     then turn into `(1 + (a,b))'.  Instead, `bar(foo)' expands into
     `lose(a,b)', and you get an error because `lose' requires a single
     argument.  In this case, the problem is easily solved by the same
     parentheses that ought to be used to prevent misnesting of
     arithmetic operations:

#define foo (a,b) or #define bar(x) lose((x))

     The extra pair of parentheses prevents the comma in `foo''s
     definition from being interpreted as an argument separator.