30 Chapter 3. Macros
If, within a macro invocation, that macro is redefined, then the new definition takes effect in time for argument pre-expansion, but the original definition is still used for argument replacement. Here is a pathological example:
#define f(x) x x f (1
#undef f
#define f 2 f) which expands to
1 2 1 2 with the semantics described above.
3.10. Macro Pitfalls
In this section we describe some special rules that apply to macros and macro expansion, and point out certain cases in which the rules have counter-intuitive consequences that you must watch out for.
3.10.1. Misnesting
When a macro is called with arguments, the arguments are substituted into the macro body and the result is checked, together with the rest of the input file, for more macro calls. It is possible to piece together a macro call coming partially from the macro body and partially from the arguments. For example,
#define twice(x) (2*(x))
#define call_with_1(x) x(1) call_with_1 (twice)
==> twice(1)
==> (2*(1))
Macro definitions do not have to have balanced parentheses. By writing an unbalanced open parenthesis in a macro body, it is possible to create a macro call that begins inside the macro body but ends outside of it. For example,
#define strange(file) fprintf (file, "%s %d",
...
strange(stderr) p, 35)
==> fprintf (stderr, "%s %d", p, 35)
The ability to piece together a macro call can be useful, but the use of unbalanced open parentheses in a macro body is just confusing, and should be avoided.
