Odd Macro Problem

[kid joe]

Hello all

Long time no posting! I've mostly been using Java these couple years, but
now I'm working on a C project again. The group seems a lot quieter than
it used to be and lots of familiar names no longer appear in the recent
post lists - quite a change!

Anyway, I have a macro that can be used like:

E(errno);

but for reasons of brevity I frequently use the form:

E(errno = EINVAL);

But I would like to be able to have two definitions such that it evaluates
to an active form like:

printf("%d\n", errno);
OR
printf("%d\n", errno = EINVAL);

and an inactive form that does nothing. The problem is, if I define the
macro as simply:

#ifdef INACTIVATE
#define E(val)
#else
#define E(val) printf("%d\n", (val))
#endif

then the inactive form will not emit the 'errno = EINVAL' expression
and set the value. If I define the inactive macro as:

#define E(val) (val)

that can emit a useless expression like:

errno;

Is there a way to write this macro to solve this problem and still
satisfy all of the other requirements?

--


( )
( )
G O O D ( ) M O R N I N G ! !
( )
) )
( ( /\
(_) / \ /\
________[_]________ /\/ \/ \
/\ /\ ______ \ / /\/\ /\/\
/ \ //_\ \ /\ \ /\/\/ \/ \
/\ / /\/\ //___\ \__/ \ \/
/ \ /\/ \//_____\ \ |[]| \
/\/\/\/ //_______\ \|__| \
/ \ /XXXXXXXXXX\ \
\ /_I_II I__I_\__________________\
I_I| I__I_____[]_|_[]_____I
I_II I__I_____[]_|_[]_____I
I II__I I XXXXXXX I
~~~~~" "~~~~~~~~~~~~~~~~~~~~~~~~

 

[Eric Sosman]

Anyway, I have a macro that can be used like:

E(errno);

but for reasons of brevity I frequently use the form:

E(errno = EINVAL);

But I would like to be able to have two definitions such that it evaluates
to an active form like:

printf("%d\n", errno);
OR
printf("%d\n", errno = EINVAL);

and an inactive form that does nothing. The problem is, if I define the
macro as simply:

#ifdef INACTIVATE
#define E(val)
#else
#define E(val) printf("%d\n", (val))
#endif

then the inactive form will not emit the 'errno = EINVAL' expression
and set the value. If I define the inactive macro as:

#define E(val) (val)

that can emit a useless expression like:

errno;

Is there a way to write this macro to solve this problem and still
satisfy all of the other requirements?

Live with the useless expression: it does no harm, aside
from (perhaps) eliciting a warning message from the compiler.

A better approach, I think, is to step back and ask yourself
why you want one macro to do two rather different things: be both
a debugging aid (I guess) and an integral part of the program logic.
You may get it to work, but the "do one thing well" principle seems
to suggest you shouldn't try unless there's a compelling reason.

"It's a floor wax *and* a dessert topping!" -- SNL

"Simplify, simplify!" -- HDT

 

[Shao Miller]

#ifdef INACTIVATE
#define E(val)
#else
#define E(val) printf("%d\n", (val))
#endif

What about:

#ifdef INACTIVE
# define E(val) ((void)(val))
#else
# define E(val) (printf("%d\n", (val)))
#endif

?

 

[Harald van Dijk]

What about:

   #ifdef INACTIVE
   #  define E(val) ((void)(val))
   #else
   #  define E(val) (printf("%d\n", (val)))
   #endif

?

Your solution, while valid, was posted more than three months after
the previous post in the thread, which already suggested the same
thing.