GCC macro expansion arguments inside string

6
votes

I have a situation like this

#define PRE 0xF1

#define SR0 0B0000
#define SR1 0B0001
#define SR2 0B0010
#define SR3 0B0011

#define VIOTA(A0)  asm(".byte PRE, A0")

int main()
{
  VIOTA(SR1);
  return 0;
}

I have a top-level macro that expands out however the expansion contains more macros. These aren't being expanded and causing some problems.

The behaviour that I desire is that the end expansion is

asm(".byte 0xF1, 0B0000")

Here the inner macros have been expanded. I'm really not sure what I'm doing wrong at all. Any advice?

3
cgccmacros
An additional bit of information. Yes, the quotations are the problem here but it's important that the two existing double quotes end up in the final expanded line. Perhaps there's some way to escape the quotes? - Jimmy Cracked Corn
possible duplicate of C Preprocessor, Stringify the result of a macro - Ciro Santilli 新疆再教育营六四事件法轮功郝海东

3 Answers

9
votes
#define S(x) #x
#define SX(x) S(x)

#define VIOTA(A0) asm(".byte " SX(PRE) ", " SX(A0))

See details here: C Preprocessor, Stringify the result of a macro

1
votes

Use the stringizing operator # to convert tokens into strings. However, since the stringizing operator can only be applied to macro parameters, you need to add some extra layers of macros:

#define PRE 0xF1

#define SR0 0B0000
#define SR1 0B0001
#define SR2 0B0010
#define SR3 0B0011

#define VIOTA(A0) VIOTA_HELPER1(PRE, A0)
#define VIOTA_HELPER1(PRE, A0) VIOTA_HELPER2(PRE, A0)
#define VIOTA_HELPER2(PRE, A0) asm(".byte" #PRE ", " #A0)

int main(void)
{
  VIOTA(SR1);
  return 0;
}

After preprocessing, this expands to this:

int main(void)
{
  asm(".byte " "0xF1" ", " "0B0001");
  return 0;
}

String constants get concatenated at compile time, so this is equivalent to asm(".byte 0xF1, 0B0001");.

0
votes

You are making PRE part of the string that is passed to asm(). Macros in strings are not expanded.

This seems to work for me:

  #define PRE 0xF1

  #define SR0 0B0000
  #define SR1 0B0001
  #define SR2 0B0010
  #define SR3 0B0011

  #define str(s) #s
  #define VIOTA(PRE, A0)  asm( ".byte " str(PRE) ", " str(A0) )

  int main()
  {    
       VIOTA(PRE, SR1);
         return 0;
  }