Expand C preprocessor ternary macro for printing in pragma message? - macros

Question

Judging by this Print numeric value of a define that's based on other macros via pragma message? , what I want is likely not possible; then again, I'm not doing arithmetic here - so here is my example:
#include <stdio.h>
enum my_sizes { SIZE_BITS_8 = 0, SIZE_BITS_16 = 1, SIZE_BITS_32 = 2 };
#define XSTR(x) STR(x)
#define STR(x) #x
#define CHOSEN_SIZE_BITS SIZE_BITS_32
#define CHOSEN_SIZE_BYTES \
( CHOSEN_SIZE_BITS==SIZE_BITS_8 ? 1 \
: CHOSEN_SIZE_BITS==SIZE_BITS_16 ? 2 \
: CHOSEN_SIZE_BITS==SIZE_BITS_32 ? 4 \
: 0 )
#pragma message( "CHOSEN_SIZE_BITS " XSTR(CHOSEN_SIZE_BITS) " CHOSEN_SIZE_BYTES " XSTR(CHOSEN_SIZE_BYTES) )
int main() {
printf("Hello, world! bytes: %d!\r\n", CHOSEN_SIZE_BYTES);
return 0;
}
In https://replit.com/languages/c, this produces:
> clang-7 -pthread -lm -o main main.c
main.c:14:9: warning: CHOSEN_SIZE_BITS SIZE_BITS_32
CHOSEN_SIZE_BYTES ( SIZE_BITS_32==SIZE_BITS_8 ? 1 :
SIZE_BITS_32==SIZE_BITS_16 ? 2 :
SIZE_BITS_32==SIZE_BITS_32 ? 4 : 0 )
[-W#pragma-messages]
#pragma message( "CHOSEN_SIZE_BITS " XSTR(CHOSEN_SIZE_...
^
1 warning generated.
> ./main
Hello, world! bytes: 4!
>
What I wanted instead, was the printout of pragma message to be:
main.c:14:9: warning: CHOSEN_SIZE_BITS 2 CHOSEN_SIZE_BYTES 4
Note that XSTR would have expanded CHOSEN_SIZE_BITS to a number, in case it was defined via #define CHOSEN_SIZE_BITS 2, but it seemingly doesn't if an enum value is used in the define; and of course, the whole ternary expression is included in the pragma message printout - even if the C printout (via printf) confirms that the CHOSEN_SIZE_BYTES does indeed end up with the expected value 4.
So, is it somehow possible to get the printout that I want from the above macros - and if so, how?

Answer 1

To calculate something in preprocessor you have to calculate it in preprocessor. Ternary is evaluated at runtime. In preprocessor you have macro expansions and ## operator and then some more macro expansions and ## operator. Nothing more.
The following code:
#define SIZE_BITS_8 0
#define SIZE_BITS_16 1
#define SIZE_BITS_32 2
#define XSTR(x) STR(x)
#define STR(x) #x
#define BITS_TO_BYTE_0() 1
#define BITS_TO_BYTE_1() 2
#define BITS_TO_BYTE_2() 4
#define BITS_TO_BYTE_IN(SIZE_BITS) \
BITS_TO_BYTE_##SIZE_BITS()
#define BITS_TO_BYTE(SIZE_BITS) \
BITS_TO_BYTE_IN(SIZE_BITS)
#define CHOSEN_SIZE_BITS SIZE_BITS_32
#define CHOSEN_SIZE_BYTES BITS_TO_BYTE(CHOSEN_SIZE_BITS)
#pragma message( "CHOSEN_SIZE_BITS " XSTR(CHOSEN_SIZE_BITS) " CHOSEN_SIZE_BYTES " XSTR(CHOSEN_SIZE_BYTES) )
Outputs the following diagnostic on GCC:
<source>:22:9: note: '#pragma message: CHOSEN_SIZE_BITS 2 CHOSEN_SIZE_BYTES 4'
22 | #pragma message( "CHOSEN_SIZE_BITS " XSTR(CHOSEN_SIZE_BITS) " CHOSEN_SIZE_BYTES " XSTR(CHOSEN_SIZE_BYTES) )