the best way to solve "variable 'xxx' was declared but never referenced"

Question

-Pre-condition:

I know it has many ways to ignore this warning, but I do need fix it but NOT just add some flags for compiler to ignore it, because I can do this makefile CFLAG modification on my local, but the compile/build policy can NOT been changed due to company quality control reason. This question I just want to discus who to fix it in a good way but NOT ignore them, many thanks!

Meanwhile, not ONLY some variables are never referenced, but also it has warning:

function 'xxx' was declared but never referenced

the similar problem in head file, it has some static functions, which ONLY use in one c file but many others c files include this head file.

Furthermore, this code run on a dedicate target, which has critical memory consumption, that means I need take care of each bit both in ROM and RAM.

-------Problem shows in below-----------

I currently build some codes, which provide by vendor and it contains lots of warning, because I want to have a warning free build, therefore, I put -Werror for gcc and --diag_error=[error_ref_number] for armcc.

After above makefile modification, the most warning I currently meet is

variable 'xxx' was declared but never referenced

It caused by following coding style:

in veryBIG.h, it defined some variables like (just for example but the vendor's code has exactly the same way):

static const int a = 10;
static const int b = 20;
static const int c = 30;
...
static const int z = xx;

however, many c files include this BIG head file, but ONLY one of these c file use one of above variables, it like a.c will use variable a, b.c use variable b, etc,.

I have two ways to fix:

1. move these kind of variables into it dedicate c file, and it means head file become useless

2. separate head files, means I create a.h, b.h, ..., z.h, and each dedicate c file include ONLY one of above head file

However, both of these two ways have limitation for my work,

Way 1:

I'm NOT sure if vendor further update will change this head file to what or have some update values, because vendor do NOT want to fix this compile warnings, that means if I follow way 1, I will manually update these variables if it has been changed by vendor (ONLY in head file, and I must synchronize them to c file) and this way makes my further merge work become complicated

Way 2:

It also makes my further merge work become NOT easy, because I do NOT use the vendor way and I also need modify the system makefile to adapt these head files into INC PATH if they are NOT locate in same folder (which the PATH already define for that veryBIG.h)

If there any idea to overcome this problem?

UPDATE

I can temporary use Wno-xxx for gcc and delete some [error_ref_number] in flag --diag_error (like CFLAG += --diag_error=550,223,188,177,....,, and I delete 177 for pass this warning in armcc) and makes my compile go on and first fix others warnings, but I do need fix ALL warning.

Answer 1

AFAIK, the warning variable 'xxx' was declared but never referenced has NO RISK at all for modern devices. The only threat is that the variable may take up a little memory space when the program is loaded, even the smallest modern machines won't suffer OOM with just that little memory waste. BTW modern compilers will optimize that for you, why won't they since the compiler is already warning you about it. You can just ignore the warning.

If you find the warning annoying, just add the -Wno-unused-variable flag during compilation.

Edit: As Alex says in the comment, I'm wrong. But you can see, that even that little risk I mentioned is fake. So just relax.

Almost all open source projects I know will have unreferenced variables, and I don't see any one making much effort to eliminate that.

Edit Again: Alright, I see that you need absolute warning free. Well the only solution I can think of that requires you less hours of work is to use #define macros.

You make blocks of #ifdef/#else/#endif in your header file, and add #defines in your C files. But then you still have to use lots of time figuring out the dependencies.

Your header file would look something like this:

#ifdef __A__
static const int var_used_in_A;
#endif
#ifdef __B__
static const int var_used_in_B;
#endif
#ifdef __EXTRA__
static const int var_used_in_both;
#endif

and your A.c file would look something like this:

#define __A__
#define __EXTRA__
#include "BigHeader.h"
.....

That how the glext.h works for OpenGL. This is better than breaking into lots of smaller header files, as you can see the dependencies clearly with the MACROS, while lots of files will require you to make good documentation. And Macros are more flexible than files.

It would be nice if you can negotiate with the VENDOR you mentioned for them to always tell you the difference they made, and you can just add those to the correct place. If you can't just keep histories of the original VeryBIG.h and use a diff tool to examine the changes of the new one.

Answer 2

If you care about memory usage to the point where every bit matters, then you probably should break up that header file into many small pieces, because there is no actual guarantee that the compiler won't emit every single one of those static variables and functions over and over in every single .c file even though only a few of them are used. (The as-if rule says the compiler may delete all the unused statics, but nothing says it has to.)

In your shoes I would probably write some sort of script which would automatically do the break-up; then I wouldn't have to do it all over again when new releases happened.

Answer 3

Can you write?

static_assert(x == x);

or

assert(x == x);

These are removed at compile time.