GCC macro expansion arguments inside string

Question

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?

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
– Jimmy Cracked Corn, Commented Jul 13, 2011 at 23:09
possible duplicate of C Preprocessor, Stringify the result of a macro — Ciro Santilli OurBigBook.com
– Ciro Santilli OurBigBook.com, Commented May 31, 2015 at 23:38

Community · Accepted Answer · 2017-05-23 12:22:20Z

9

#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

edited May 23, 2017 at 12:22

CommunityBot

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answered Jul 13, 2011 at 23:16

Karoly Horvath

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2 Comments

Léo Over a year ago

Can you explain why simply S(PRE) does not work? (I've tested, I know it doesn't)

Karoly Horvath Over a year ago

sigh click on the link.

Adam Rosenfield · Accepted Answer · 2011-07-13 23:17:54Z

1

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");.

answered Jul 13, 2011 at 23:17

Adam Rosenfield

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4 Comments

Lightness Races in Orbit Over a year ago

String constants are concatenated by the preprocessor, I'd have thought?

Lightness Races in Orbit Over a year ago

C++11 2.14.5/13 says it's done in translation phase 6. That's me told.

Adam Rosenfield Over a year ago

@Lightness: If you want to be pedantic, then there's not really a separate "preprocessor" and "compiler" but rather a single "translator" with a number of phases of translation. C99 §5.1.1.2 puts string literal concatenation in phase 6 of 8 for C code, while C++03 §2.1 puts it in phase 6 of 9 for C++ code (I don't have a copy of C++11 handy). Both GCC and Clang do not concatenate string literals if you just run the preprocessor (the -E argument) without compiling.

Lightness Races in Orbit Over a year ago

@AdamRosenfield: C++11 2.2/4 is specifically explicit, in my view (it begins Preprocessing directives are executed and goes on to list the things that the preprocessor does, no more no less), to call phase 4 the "preprocessing phase".

BjoernD · Accepted Answer · 2011-07-13 23:14:38Z

0

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;
  }

edited Jul 13, 2011 at 23:14

answered Jul 13, 2011 at 23:01

BjoernD

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