macros

[j0mbolar]

given:

#define FOO(x) ((x) + (x))

I recall a thread awhile back saying that
the standard wasn't clear as to what happens if FOO()
were invoked with no arguments. But others were saying
it was clear. Does it lead to undefined behavior or not?

 

[Jens.Toerring]

given:

#define FOO(x) ((x) + (x))

I recall a thread awhile back saying that
the standard wasn't clear as to what happens if FOO()
were invoked with no arguments. But others were saying
it was clear. Does it lead to undefined behavior or not?

Since a macro is just doing a text replacement I would expect
the compiler to complain when you write

printf("%d\n", FOO());

and it thus gets, after the preprocessor did its job,

printf("%d\n", (() + ()));

You won't get that far that this could lead to undefined
behavior.
Regards, Jens

 

[Keith Thompson]

given:

#define FOO(x) ((x) + (x))

I recall a thread awhile back saying that
the standard wasn't clear as to what happens if FOO()
were invoked with no arguments. But others were saying
it was clear. Does it lead to undefined behavior or not?

A very quick summary: C99 allows empty macro arguments. The intent of
the standard, stated in a footnote and in the rationale, is that if
FOO is defined to expect one argument, FOO() is actually an invocation
with a single empty argument, not an invocation with no arguments.

We had a very long discussion about whether that intent is adequately
expressed in the normative wording of the standard.

This was discussed at great length. Do a groups.google.com search for
subject "Empty macro arguments" in comp.std.c. Please don't try to
re-kindle the discussion unless you have something new to add.

 

[Douglas A. Gwyn]

Since a macro is just doing a text replacement I would expect
...

Don't try to intuit the requirements; read them.

For C89 the usage would violate a constraint;
for C99 the usage would be taken as providing
an empty argument. The macro expansion then
produces a syntax error.

The behavior is not just undefined, it requires
a diagnostic.