The preprocessor in C is a very powerful tool which have at least one flaw: there is no type checking on macro arguments. Consider the following trivial (and completely useless) example:

#define negative(x) (((x) & 128) == 128) int main() { int8_t a = -1; assert(negative(a) == true); int8_t b = 1; assert(negative(b) == false); uint8_t c = 128; assert(negative(c) == false); } // Compiles silently with // gcc -std=c18 -Wall -Wextra -o main main.c // Output: // main: main.c:9: main: Assertion `negative(c) == false' failed.

Compiles silently and bugs silently, not good ! So here is a trick to add type checking on your macros' arguments: use _Generic !

#define negative(x) _Generic(x, \ int8_t: (((x) & 128) == 128)) int main() { int8_t a = -1; assert(negative(a) == true); int8_t b = 1; assert(negative(b) == false); } // Compile silently with // gcc -std=c18 -Wall -Wextra -o main main.c // Output: // -- nothing -- #define negative(x) _Generic(x, \ int8_t: (((x) & 128) == 128)) int main() { int8_t a = -1; assert(negative(a) == true); int8_t b = 1; assert(negative(b) == false); uint8_t c = 128; assert(negative(c) == false); } // Error at compilation: // main.c: In function ‘main’: // main.c:10:19: error: ‘_Generic’ selector of type ‘unsigned char’ is not // compatible with any association // 10 | assert(negative(c) == false); // | ^ // main.c:10:10: note: in expansion of macro ‘negative’ // 10 | assert(negative(c) == false); // | ^~~~~~~~

Et voila!