How do I stringify macros that are the results of operations on macros?

Question

Here's a program that illustrates my problem:

#include <stdio.h>

#define NUMERATOR 8
#define DENOMINATOR 2
#define QUOTIENT (NUMERATOR / DENOMINATOR)

#define ZSTR(x) XSTR(#x)
#define YSTR(x) XSTR(x)
#define XSTR(x) STR(x)
#define STR(x) #x

int main()
{
    printf("QUOTIENT:       %d\n", QUOTIENT);
    printf("STR(QUOTIENT):  %s\n", STR(QUOTIENT));
    printf("XSTR(QUOTIENT): %s\n", XSTR(QUOTIENT));
    printf("YSTR(QUOTIENT): %s\n", YSTR(QUOTIENT));
    printf("ZSTR(QUOTIENT): %s\n", ZSTR(QUOTIENT));
    return 0;
}

And here's its output:

$ gcc -g -Wall -o stringify stringify.c && ./stringify 
QUOTIENT:       4
STR(QUOTIENT):  QUOTIENT
XSTR(QUOTIENT): (8 / 2)
YSTR(QUOTIENT): (8 / 2)
ZSTR(QUOTIENT): "QUOTIENT"

I would like to have a the string literal "4" passed to the compiler, but I'm losing hope. This is related to this question, but adds a level.

Answer 1

You can define macros that paste together their arguments and then define a (large) number of other macros that do the evaluation as kind of a table lookup:

#define DIV(X, Y)  DIV_(X, Y)
#define DIV_(X, Y) DIV_##X##_##Y
#define DIV_0_1  0
#define DIV_1_1  1
#define DIV_2_1  2
    :
#define DIV_8_2  4
    :

This is kind of tedious, but you can easily write a little program to generate a header file with the above stuff in it and run that as part of your build process. Then you just need

#define QUOTIENT  DIV(NUMERATOR, DENOMINATOR)

Note that his kind of thing only works for unsigned integers -- if you need negative numbers or floating point, it won't work

Answer 2

With some tricks you can implement basic arithmetic in a C99 conforming preprocessor. P99 implements arithmetic and logic for small decimal numbers. E.g

P99_IF_GT(1,0)(true)(false)
P99_ADD(3, 7)
P99_MUL(7, 2)
P99_DIV(7, 2)

would be preprocessed to something like

1
10
14
3

These macros can be processed further, stringified and everthing you like.

P99_STRINGIFY(P99_PASTE2(XXX_, P99_ADD(3, 7)))

leads to "XXX_10" as the result of preprocessing.

Answer 3

The best you can do is stringify the expansion of the macro which is done with your XSTR and YSTR examples. Although it may compile to 4 with optimizations all the preprocessor will be able see is (8 / 2)

Answer 4

Well, I'm a little reluctant to admit I know one way to make this work.

Attack it from the other direction. You want your compiler to see something like this.

#define QUOTIENT 4
#include <stdio.h>

int main(void)
{
  printf("QUOTIENT:       %d\n", QUOTIENT);
  return 0;
}

How do we do that without using a literal "4" in the definition of QUOTIENT, since the macro processor won't help us? By using an additional preprocessor. Write a source file, stringify.c.awk, like this.

/* stringify.c.awk --  Source file for stringify.c  
   (Put build instructions here.)  
 */
#define QUOTIENT NUMERATOR/DENOMINATOR
#include <stdio.h>

int main(void)
{
  printf("QUOTIENT:       %d\n", QUOTIENT);
  return 0;
}

Write the secondary preprocessor in awk. I deliberately used a really tight regular expression. I think it's the most likely regex to fail if there are changes to the source file, and I think that's usually what you want. (I usually want to discourage cosmetic changes to the #define.)

# stringify.awk -- calculate and substitute the value for #define QUOTIENT.
BEGIN {
  NUMERATOR = 8;
  DENOMINATOR = 2;
}
{
  if ($0~/^#define QUOTIENT NUMERATOR\/DENOMINATOR$/) {
    sub(/NUMERATOR\/DENOMINATOR/, NUMERATOR/DENOMINATOR);
  } 
  print $0;
}

Now you can build stringify.c from the stringify.c.awk file.

$ awk -f stringify.awk stringify.c.awk > stringify.c
$ gcc -Wall -o stringify stringify.c
$ ./stringify
QUOTIENT:       4

A makefile and generous comments takes a lot of the pain away.

(m4 won't help for more or less the same reasons the C preprocessor won't help.)