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Petr Skocik
Petr Skocik

Reputation: 60097

Silencing -Wtype-limits and -Wbool-compare in code, without pragmas

When compiled with gcc -Wall -Wextra, the following code

boolcmp.c:

#include <stdio.h>
int main(void)
{
    #define TEST(X) do{if((X)>=0) puts("no minus");}while(0)
    TEST(1);
    unsigned u = 0; (void)u; 
    _Bool b = 0; (void)b;
    TEST(u); //-Wtype-limits
    TEST(b); //-Wbool-compare

}

generates -Wtype-limits (with -Wextra) and -Wbool-comapare (with -Wall) warnings as marked.

boolcmp.c: In function ‘main’:
boolcmp.c:4:27: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
  #define TEST(X) do{if((X)>=0) puts("positive");}while(0)
                           ^
boolcmp.c:8:2: note: in expansion of macro ‘TEST’
  TEST(u); //-Wtype-limits
  ^~~~
boolcmp.c:4:27: warning: comparison of constant ‘0’ with boolean expression is always true [-Wbool-compare]
  #define TEST(X) do{if((X)>=0) puts("positive");}while(0)
                           ^
boolcmp.c:9:2: note: in expansion of macro ‘TEST’
  TEST(b); //-Wbool-compare
  ^~~~

Can these warnings be silenced from within the code without pragmas?

Upvotes: 1

Views: 287

Answers (1)

Petr Skocik
Petr Skocik

Reputation: 60097

My _Generic-based non-complaining replacement for an X>=0 test:

#define MY_nominus_eh(X)  \
_Generic((X)+0LL,llong:my_spos_,ullong:my_upos_,default:my_fpos_)(X)
static inline _Bool my_upos_(ullong X) { (void)X; return 1; }
static inline _Bool my_spos_(llong X) { return X>=0; }
static inline _Bool my_fpos_(ldouble X) { return X>=0; }
//assumes: typedef long long llong; 
//typedef unsigned long long ullong;
//typedef long double ldouble;

Thanks to Christian Gibbons for the suggestion.

Later I found out I actually wanted the macro to expand to an integer constant expression (usable in _Static_assert's) so I had to come up with something different.

The trick I used was to use _Generic to replace the suspiciously typed variable with an integer constant, which, while also leading to a tautological comparison, did not trigger gcc's warning.

#include <stdio.h>
int main(void)
{
    #define TEST(X) do{if( _Generic((X),_Bool:0,default:_Generic(+(X),unsigned:0,\
                    unsigned long:0,unsigned long long:0,default:X)) >=0) puts("no minus"); else puts("minus"); }while(0)
    TEST(1);
    unsigned u = 0; (void)u; 
    _Bool b = 0; (void)b;
    TEST(u); //-Wtype-limits
    TEST(b); //-Wbool-compare
    TEST(-1);

}

Upvotes: 2

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