[c] How to suppress "unused parameter" warnings in C?

For instance:

Bool NullFunc(const struct timespec *when, const char *who)
{
   return TRUE;
}

In C++ I was able to put a /*...*/ comment around the parameters. But not in C of course, where it gives me the error error: parameter name omitted.

With gcc with the unused attribute:

int foo (__attribute__((unused)) int bar) {
    return 0;
}

In MSVC to suppress a particular warning it is enough to specify the it's number to compiler as /wd#. My CMakeLists.txt contains such the block:

If (MSVC)
    Set (CMAKE_EXE_LINKER_FLAGS "$ {CMAKE_EXE_LINKER_FLAGS} / NODEFAULTLIB: LIBCMT")
    Add_definitions (/W4 /wd4512 /wd4702 /wd4100 /wd4510 /wd4355 /wd4127)
    Add_definitions (/D_CRT_SECURE_NO_WARNINGS)
Elseif (CMAKE_COMPILER_IS_GNUCXX OR CMAKE_COMPILER_IS_GNUC)
    Add_definitions (-Wall -W -pedantic)
Else ()
    Message ("Unknown compiler")
Endif ()

Now I can not say what exactly /wd4512 /wd4702 /wd4100 /wd4510 /wd4355 /wd4127 mean, because I do not pay any attention to MSVC for three years, but they suppress superpedantic warnings that does not influence the result.

I got the same problem. I used a third-part library. When I compile this library, the compiler (gcc/clang) will complain about unused variables.

Like this

test.cpp:29:11: warning: variable 'magic' set but not used [-Wunused-but-set-variable] short magic[] = {

test.cpp:84:17: warning: unused variable 'before_write' [-Wunused-variable] int64_t before_write = Thread::currentTimeMillis();

So the solution is pretty clear. Adding -Wno-unused as gcc/clang CFLAG will suppress all "unused" warnings, even thought you have -Wall set.

In this way, you DO NOT NEED to change any code.

For the record, I like Job's answer above but I'm curious about a solution just using the variable name by itself in a "do-nothing" statement:

void foo(int x) {
    x; /* unused */
    ...
}

Sure, this has drawbacks; for instance, without the "unused" note it looks like a mistake rather than an intentional line of code.

The benefit is that no DEFINE is needed and it gets rid of the warning.

Are there any performance, optimization, or other differences?

Labelling the attribute is ideal way. MACRO leads to sometime confusion. and by using void(x),we are adding an overhead in processing.

If not using input argument, use

void foo(int __attribute__((unused))key)
{
}

If not using the variable defined inside the function

void foo(int key)
{
   int hash = 0;
   int bkt __attribute__((unused)) = 0;

   api_call(x, hash, bkt);
}

Now later using the hash variable for your logic but doesn’t need bkt. define bkt as unused, otherwise compiler says'bkt set bt not used".

NOTE: This is just to suppress the warning not for optimization.

A gcc/g++ specific way to suppress the unused parameter warning for a block of source code is to enclose it with the following pragma statements:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
<code with unused parameters here>
#pragma GCC diagnostic pop

You can use gcc/clang's unused attribute, however I use these macros in a header to avoid having gcc specific attributes all over the source, also having __attribute__ everywhere is a bit verbose/ugly.

#ifdef __GNUC__
#  define UNUSED(x) UNUSED_ ## x __attribute__((__unused__))
#else
#  define UNUSED(x) UNUSED_ ## x
#endif

#ifdef __GNUC__
#  define UNUSED_FUNCTION(x) __attribute__((__unused__)) UNUSED_ ## x
#else
#  define UNUSED_FUNCTION(x) UNUSED_ ## x
#endif

Then you can do...

void foo(int UNUSED(bar)) { ... }

I prefer this because you get an error if you try use bar in the code anywhere so you can't leave the attribute in by mistake.

and for functions...

static void UNUSED_FUNCTION(foo)(int bar) { ... }

Note 1):
As far as I know, MSVC doesn't have an equivalent to __attribute__((__unused__)).

Note 2):
The UNUSED macro won't work for arguments which contain parenthesis,
so if you have an argument like float (*coords)[3] you can't do,
float UNUSED((*coords)[3]) or float (*UNUSED(coords))[3], This is the only downside to the UNUSED macro I found so far, in these cases I fall back to (void)coords;

Seeing that this is marked as gcc you can use the command line switch Wno-unused-parameter.

For example:

gcc -Wno-unused-parameter test.c

Of course this effects the whole file (and maybe project depending where you set the switch) but you don't have to change any code.

I've seen this style being used:

if (when || who || format || data || len);

In gcc, you can label the parameter with the unused attribute.

This attribute, attached to a variable, means that the variable is meant to be possibly unused. GCC will not produce a warning for this variable.

In practice this is accomplished by putting __attribute__ ((unused)) just before the parameter. For example:

void foo(workerid_t workerId) { }

becomes

void foo(__attribute__((unused)) workerid_t workerId) { }