Odd Macro Problem

[Michael B Allen]

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?

Thanks,
Mike

[Eric Sosman]

Re: Odd Macro Problem

Michael B Allen wrote:[color=blue]
> 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?[/color]

No, because "all the other requirements" leaves too much
to the imagination ... Nonetheless, I'll don my Carnak The
Magnificent turban, dust off the Magic Eight-Ball, oil the
Ouija board, cast the yarrow stalks, burn a chicken feather,
and intone:

YE MAY BE WELL-PLEASED BY ONE FROM YE OLDE LISTE:

#define E(val) (val)
/* "Useless expression," pfui. Damn the torpedoes! */

#define E(val) ((void)(val))
/* Still useless, but perhaps less noisy. */

#define E(val) report_error(va l)
/* Let the function decide what to do, if anything. */

AN NONE OF THESE PLEASE YE, ABANDON THY LOVE OF "BREVITY"
AND ESCHEW YON SIDE-EFFECTS IN YR MACRO ARGUMENTS. TREMBLE,
MORTAL, FOR THOU HAST BEEN WARNED!

(Alarums and excursions. Blinding lightning and
cataclysmic thunder. When sight returns, the Oracle has
vanished, leaving behind only a whiff of brimstone and one
small demon buzzing around in hope of finding Cyrano.)

--
Eric Sosman
esosman@acm-dot-org.invalid