How to make a char string from a C macro's value?

Question

For example, how to avoid writing the 'func_name' twice?

#ifndef TEST_FUN
#  define TEST_FUN func_name
#  define TEST_FUN_NAME "func_name"
#endif

I'd like to follow the Single Point of Truth rule.

Version of C preprocessor:

$ cpp --version
cpp (GCC) 4.1.2 20070626 (Red Hat 4.1.2-14)

Answer 1

#define TEST_FUN_NAME #FUNC_NAME

see here

Answer 2

#include <stdio.h>

#define QUOTEME(x) #x

#ifndef TEST_FUN
#  define TEST_FUN func_name
#  define TEST_FUN_NAME QUOTEME(TEST_FUN)
#endif

int main(void)
{
 puts(TEST_FUN_NAME);
 return 0;
}

Reference: Wikipedia's C preprocessor page

Answer 3

@Shy: #FUNC_NAME doesn't work

#include <stdio.h>

#ifndef TEST_FUN
#  define TEST_FUN some_func
#  define TEST_FUN_NAME #TEST_FUN
#endif

void some_func(void)
{
  printf("some_func() called\n");
}

int main(void)
{
  TEST_FUN();
  printf("TEST_FUN_NAME=%s\n", TEST_FUN_NAME);
}

It produces compilation error:

make macro_sub && ./macro_sub
cc -std=c99 -Wall    macro_sub.c   -o macro_sub
macro_sub.c: In function ‘main’:
macro_sub.c:16: error: stray ‘#’ in program
macro_sub.c:16: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘void (*)(void)\
’
make: *** [macro_sub] Error 1

Answer 4

He who is Shy gave you the germ of an answer, but only the germ. The basic technique for converting a value into a string in the C pre-processor is indeed via the '#' operator, but a simple transliteration of the proposed solution gets a compilation error:

#define TEST_FUNC test_func
#define TEST_FUNC_NAME #TEST_FUNC

#include <stdio.h>
int main(void)
{
    puts(TEST_FUNC_NAME);
    return(0);
}

The syntax error is on the 'puts()' line - the problem is a 'stray #' in the source.

In section 6.10.3.2 of the C standard, 'The # operator', it says:

Each # preprocessing token in the replacement list for a function-like macro shall be followed by a parameter as the next preprocessing token in the replacement list.

The trouble is that you can convert macro arguments to strings -- but you can't convert random items that are not macro arguments.

So, to achieve the effect you are after, you most certainly have to do some extra work.

#define FUNCTION_NAME(name) #name
#define TEST_FUNC_NAME  FUNCTION_NAME(test_func)

#include <stdio.h>

int main(void)
{
    puts(TEST_FUNC_NAME);
    return(0);
}

I'm not completely clear on how you plan to use the macros, and how you plan to avoid repetition altogether. This slightly more elaborate example might be more informative. The use of a macro equivalent to STR_VALUE is an idiom that is necessary to get the desired result.

#define STR_VALUE(arg)      #arg
#define FUNCTION_NAME(name) STR_VALUE(name)

#define TEST_FUNC      test_func
#define TEST_FUNC_NAME FUNCTION_NAME(TEST_FUNC)

#include <stdio.h>

static void TEST_FUNC(void)
{
    printf("In function %s\n", TEST_FUNC_NAME);
}

int main(void)
{
    puts(TEST_FUNC_NAME);
    TEST_FUNC();
    return(0);
}

Answer 5

@Jason W: QUOTEME() doesn't work

It prints:

make quoteme &&  ./quoteme
cc -std=c99 -Wall    quoteme.c   -o quoteme
TEST_FUN

But it must print func_name instead of TEST_FUN

Answer 6

@Jonathan Leffler: Thank you. Your solution works.

A complete working example:

/** compile-time dispatch 

   $ gcc -Wall -DTEST_FUN=another_func macro_sub.c -o macro_sub && ./macro_sub
*/
#include <stdio.h>

#define QUOTE(name) #name
#define STR(macro) QUOTE(macro)

#ifndef TEST_FUN
#  define TEST_FUN some_func
#endif

#define TEST_FUN_NAME STR(TEST_FUN)

void some_func(void)
{
  printf("some_func() called\n");
}

void another_func(void)
{
  printf("do something else\n");
}

int main(void)
{
  TEST_FUN();
  printf("TEST_FUN_NAME=%s\n", TEST_FUN_NAME);
  return 0;
}

Example:

$ gcc -Wall -DTEST_FUN=another_func macro_sub.c -o macro_sub && ./macro_sub
do something else
TEST_FUN_NAME=another_func