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Editorial Team
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Asked: 2026-05-16T11:19:15+00:00

I’m trying to use preprocessor tricks to declare a magic variable. Something like this:

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I’m trying to use preprocessor tricks to declare a magic variable. Something like this:

DECLARE(x)

should expand to

int _DECLARED_VARIABLE_x_LINE_12

if the declaration was on line 12 of the input source. I was trying to use the ## token-pasting command and the __LINE__ macro, but I either get an uninterpreted “__LINE__” in there or the preprocessor seems to completely ignore my line. My current guess is:

 #define DECLARE(x) _DECLARED_VARIABLE_ ## x ## _LINE_ ## __LINE__
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1 Answer

  1. Editorial Team

    The normal trick in such cases is to use a second macro. However, that does not seem to work with GCC (4.5.1 on MacOS X 10.6.4), and a third level of macro was needed:

    #define DECLARE(x) _DECLARED_VARIABLE_ ## x ## _LINE_ ## __LINE__

#define DECLARE42(x, line) _DECLARED_VARIABLE_ ## x ## _LINE_ ## line
#define DECLARE41(x, line) DECLARE42(x, line)
#define DECLARE40(x) DECLARE41(x, __LINE__)

int DECLARE(y);
int DECLARE40(c) = 129;

    Output of ‘gcc -E’:

    # 1 "magicvars.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "magicvars.c"






int _DECLARED_VARIABLE_y_LINE___LINE__;
int _DECLARED_VARIABLE_c_LINE_8 = 129;

    I’m not sure I have a good explanation of why the third level of macro was needed.

    I am also curious to know how you are ever going to refer to these variables after you’ve created them.

    I went through a number of variations before managing to hit on the one that worked:

    #define DECLARE(x) _DECLARED_VARIABLE_ ## x ## _LINE_ ## __LINE__

#define DECLARE11(x, line) _DECLARED_VARIABLE_ ## x ## _LINE_ ## line
#define DECLARE10(x) DECLARE11(x, __LINE__)

#define DECLARE23(line) _LINE_ ## line
#define DECLARE22(x) _DECLARED_VARIABLE_ ## x
#define DECLARE21(x, line) DECLARE22(x) ## DECLARE23(line)
#define DECLARE20(x) DECLARE21(x, __LINE__)

#define DECLARE32(line) _LINE_ ## line
#define DECLARE31(x, line) _DECLARED_VARIABLE_ ## x ## DECLARE32(line)
#define DECLARE30(x) DECLARE31(x, __LINE__)

#define DECLARE42(x, line) _DECLARED_VARIABLE_ ## x ## _LINE_ ## line
#define DECLARE41(x, line) DECLARE42(x, line)
#define DECLARE40(x) DECLARE41(x, __LINE__)


int DECLARE(y);
int DECLARE10(z) = 12;
int DECLARE20(a) = 37;
int DECLARE30(b) = 91;
int DECLARE40(c) = 129;

    Have fun working out why the non-working ones didn’t work. They did, however, point me towards the working answer. (I note that the Sun C compiler produces essentially the same results as GCC on the same input.)

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