How to close specified warning in C source code?

Question

For example, how can I get rid of "warning: unnamed struct/union that defines no instances" in the source file, not through compiler command-line options.

I want to define a C macro CONST_BUG_ON, which I use to check some const values at compile time.

#define CONST_BUG_ON(e)        struct  {int a:!(e);}

It gives the warning warning: unnamed struct/union that defines no instances, but in this case it is not a real problem.

Thanks Tom Tanner

#define CONST_BUG_ON_3(e, l) struct buggy##l {int a:!(e);}
#define CONST_BUG_ON_2(e, l) CONST_BUG_ON_3(e, l)
#define CONST_BUG_ON(e) CONST_BUG_ON_2(e, __LINE__)

That's good, but still have some problems: If file a's line 6 contain CONST_BUG_ON(e), and file a was inclued by file b, and line 6 of file b aslo contains CONST_BUG_ON(e), then gcc complains redefine error. Use__COUNTER__ instade of __LINE__ may perfect, but my old compiler does not support __COUNTER__.

Thanks Basile Starynkevitch

#define CONST_BUG_ON(e) do { \
   int tab[__builtin_constant_p(e)?1:-1] = {0}; \
   if (tab[0]) abort(); } while (0)

This is a C statement, can only be place in a function, I really want to use it outside the function.

Why is that so important to you? What is the real use case?? — Basile Starynkevitch
– Basile Starynkevitch, Commented Aug 27, 2012 at 14:09

Eric Postpischil · Accepted Answer · 2012-08-27 13:11:30Z

3

One way to resolve the compiler’s complaint is that you have an unnamed struct that defines no instances is to give it a name:

#define CONST_BUG_ON(e)        struct ForDebuggingOnly {int a:!(e);}

An alternate way to get the expression testing you want is to declare (but not define) an array that has an illegal size if e is true:

#define CONST_BUG_ON(e)    extern int ForDebuggingOnly[(e) ? -1 : 1]

answered Aug 27, 2012 at 13:11

Eric Postpischil

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2 Comments

yang wen Over a year ago

If use CONST_BUG_ON several times, this code do not works, gcc complains redefine error.

Eric Postpischil Over a year ago

@yangwen: Use the second method I showed, the one with extern int.

Tom Tanner · Accepted Answer · 2012-08-27 13:17:51Z

1

You could use macro magic to give yourself a unique ID by passing in the line number

#define CONST_BUG_ON_3(e, l) struct buggy##l {int a:!(e);}
#define CONST_BUG_ON_2(e, l) CONST_BUG_ON_3(e, l)
#define CONST_BUG_ON(e) CONST_BUG_ON_2(e, __LINE__)

That's fairly icky but it does give a unique name each time it is used (the 2nd level of indirection may be spurious, but this is what I have in some code that has stood the test of time).

answered Aug 27, 2012 at 13:17

Tom Tanner

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2 Comments

yang wen Over a year ago

That's good, but still have some problems: If file "a"'s line 6 contain CONST_BUG_ON(e), and file "a" was inclued by file "b", and line 6 of file "b" aslo contains CONST_BUG_ON(e), then gcc complains redefine error. Use __COUNTER__ instade of LINE may perfect, but my old compiler does not support __COUNTER__.

Tom Tanner Over a year ago

i think you might have other problems if you have source code set up like that.

Community · Accepted Answer · 2017-05-23 10:34:41Z

1

It looks like what you try is called a compile time assertion or compile time assert macro. There are various ways to do this, usually involving arrays with negative dimension when the assertion fails. Many projects call this macro CT_ASSERT() and there are a bunch of Stackoverflow questions relating to them.

edited May 23, 2017 at 10:34

CommunityBot

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answered Aug 27, 2012 at 13:25

Jens

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Comments

Basile Starynkevitch · Accepted Answer · 2012-08-27 14:07:28Z

1

Assuming a recent GCC compiler, you could use __builtin_constant_p to test for compile-time constants, perhaps with

  #define CONST_BUG_ON(e) do { \
       int tab[__builtin_constant_p(e)?1:-1] = {0}; \
       if (tab[0]) abort(); } while (0)

For your question about ignoring some warning, perhaps the GCC diagnostic pragmas could help.

If you want your CONST_BUG_ON to work only in declaration contexts, you might try

  #define CONST_BUG_ON(e) CONST_BUG_AT(e,__LINE__)
  #define CONST_BUG_AT(e,l) \
     extern int tab_##l[__builtin_constant_p(e)?0:-1];

At last you could even customize your GCC compiler (with your specific pragma) perhaps using MELT (a high-level domain specific language to extend GCC), but that will take you days of work.

edited Aug 27, 2012 at 14:07

answered Aug 27, 2012 at 12:47

Basile Starynkevitch

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