Portable UNUSED parameter macro used on function signature for C and C++

Question

I'm interested in creating a macro for eliminating the unused variable warning.

This question describes a way to suppress the unused parameter warning by writing a macro inside the function code:

Universally compiler independent way of implementing an UNUSED macro in C/C++

But I'm interested in a macro that can be used in the function signature:

void callback(int UNUSED(some_useless_stuff)) {}

This is what I dug out using Google (source)

#ifdef UNUSED
#elif defined(__GNUC__)
# define UNUSED(x) UNUSED_ ## x __attribute__((unused))
#elif defined(__LCLINT__)
# define UNUSED(x) /*@unused@*/ x
#elif defined(__cplusplus)
# define UNUSED(x)
#else
# define UNUSED(x) x
#endif

Can this be further expanded for other compilers?

Edit: For those who can't understand how tagging works: I want a solution for both C and C++. That is why this question is tagged both C and C++ and that is why a C++ only solution is not acceptable.

There's an easier way, in C++ you can just omit the parameter name. — john, Aug 17 '11 at 10:08
@john Yes in C++ you can. In C you can't. I have modified the macro. — Šimon Tóth, Aug 17 '11 at 10:08
if you aren't interested in the C++ answer don't tag your question C++ — jk., Aug 17 '11 at 10:12
warnings about unused variables are optional features, so also ways to disable that warning will be optional, and as such there are surely some compilers where this is not possible — PlasmaHH, Aug 17 '11 at 10:14
Why make it a parameter macro? Pretty much everything post GCC 2.1 supports the standard `__attribute__` tagging too. — Steve-o, Aug 17 '11 at 10:15
@jk I'm not interested in answer that will work only in C++. — Šimon Tóth, Aug 17 '11 at 10:15
@Steve-o If you have a non-parametric version that works on a broad range of compilers, feel free to post it as an answer. — Šimon Tóth, Aug 17 '11 at 10:16
I don't have MS VC++ at hand, but I think `#define UNUSED(x) __pragma(warning(suppress:4100)) x` might work. — user786653, Aug 17 '11 at 13:12
The code you posted looks pretty good, can you explain further how it falls short or which particular compiler it doesn't work for? — Mark Ransom, Aug 17 '11 at 16:41
@Mark Hmm, works in Intel and Clang. I still need to test MSVC. — Šimon Tóth, Aug 17 '11 at 17:16
I know from personal experience that MSVC will follow the __cplusplus path and work properly if you're compiling C++. Not sure about using it as a C compiler. — Mark Ransom, Aug 17 '11 at 17:21

Matt Clarkson · Answer 1 · 2015-03-04T14:35:56.673

35

The way I do it is like this:

#define UNUSED(x) (void)(x)
void foo(const int i) {
    UNUSED(i);
}

I've not had a problem with that in Visual Studio, Intel, gcc and clang.

The other option is to just comment out the parameter:

void foo(const int /*i*/) {
  // When we need to use `i` we can just uncomment it.
}

edited Mar 04 '15 at 14:35

answered Sep 13 '13 at 12:35

Matt Clarkson

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The first answer solves a completely different problem, the second solution works only for C++. – Šimon Tóth May 23 '16 at 14:51
1

Casting to void works on unused variables too, unused functions even `(void)printf`. And leaking implementation details by tagging parameters in function signatures tastes funny to begin with. – diapir Aug 16 '16 at 09:36

doc_ds · Answer 2 · 2013-11-10T17:05:49.727

12

Just one small thing, better using __attribute__((__unused__)) as __attribute__((unused)), because unused could be somewhere defined as macro, personally I had a few issues with this situation.

But the trick I'm using is, which I found more readable is:

#define UNUSED(x) (void)x;

It works however only for the variables, and arguments of the methods, but not for the function itself.

edited Nov 10 '13 at 17:05

answered Nov 10 '13 at 16:59

doc_ds

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There are compilers that treat "(void)x;" as still unused. Diab for example. – sandos Apr 07 '16 at 11:15

score 10 · Accepted Answer · answered Mar 07 '12 at 13:36

10

After testing and following the comments, the original version mentioned in the question turned out to be good enough.

Using: #define UNUSED(x) __pragma(warning(suppress:4100)) x (mentioned in comments), might be necessary for compiling C on MSVC, but that's such a weird combination, that I didn't include it in the end.

answered Mar 07 '12 at 13:36

Šimon Tóth

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This is what i was missing. I'm actually using `#define UNUSED(x) __pragma(warning(suppress: 4100 4101)) x` to be able to decorate both function arguments and local variables unused (`__attribute__((unused))` works the same way in g++). Super cool! – the swine Jan 31 '14 at 13:33
1

This didnt work for me with MSVC2013, Still got `C4100` warnings all over. – ideasman42 Feb 17 '14 at 21:51
1

Out of curiosity, why did you choose this over @MattClarkson's answer? This is certainly less portable compared to simple `(void)x`. – vgru Mar 22 '17 at 13:21
I know why I go in this direction: I have unused parameter in a `constexpr` function which (in VS2015) cannot have multiple statements. – CygnusX1 May 06 '17 at 21:32

score 6 · Answer 4 · answered Aug 17 '11 at 10:27

6

Across many compilers I have used the following, excluding support for lint.

#if (__GNUC__ > 2) || (__GNUC__ == 2 && __GNUC_MINOR__ > 4)
#       define PGM_GNUC_UNUSED         __attribute__((__unused__))
#else
#       define PGM_GNUC_UNUSED
#endif

Tested compilers: GCC, Clang, EKOPath, Intel C Compiler / Composer XE, MinGW32 on Cygwin / Linux / MSYS, MinGW-w64 on Cygwin / Linux, Sun ONE Studio / Oracle Solaris Studio, Visual Studio 2008 / 2010.

Example usage:

pgm_tsc_init (
        PGM_GNUC_UNUSED pgm_error_t**   error
        )
{
...
}

PGM is the standard prefix for this C based project. GNUC is the convention from GLib for this attribute.

I think one compile warns about __attribute__ in certain circumstances but certainly no error.

answered Aug 17 '11 at 10:27

Steve-o

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Are you sure? I don't think that MSVC actually defines `__GNUC__` macro, that would be very weird. – Šimon Tóth Aug 17 '11 at 10:32
2

@Let_Me_Be: if it doesn't, then in the context of a `#if` condition, it's as if `__GNUC__` were defined equal to 0. So you'll get the `#else` branch, which Steve-o has tested to work on Visual Studio 2008 and 2010. Not necessarily at the same warning level you're using. – Steve Jessop Aug 17 '11 at 10:37
@Let_Me_Be that is why Cygwin & Mingw use it but MSVC does not, the end result is a working system from Debian 4 / RHEL 4 and newer. – Steve-o Aug 17 '11 at 10:37
2

@Steve-o Well, then this solution is a step backward, since it will only work on GCC. It won't suppress the warning on any other compiler. – Šimon Tóth Aug 17 '11 at 11:07
According to the docs at , the first GCC version that accepts `__attribute__((unused))` is 2.7, not 2.5. – Alex Shpilkin Jan 13 '21 at 18:35

Portable UNUSED parameter macro used on function signature for C and C++

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