Get define value in preprocessor time in c

Question

I want to create a define using macro expansion. The crated define shall hold the value of an other define. :)

Example:

#define ONE            1
#define TWO            2
#define MACRO(x, y)    ...
...
#define MACRO_1_2      3

My question is: how can I resolve / use the value of a define in macro expansion?

I have tried to use

#define TEST_MACRO_V2(x, y)      TEST_MACRO_V2_"x"_"y"

, but I get the following error:

main.c:18:34: error: ‘TEST_MACRO_V2_’ undeclared (first use in this function) #define TEST_MACRO_V2(x, y) TEST_MACRO_V2_"x"_"y"

My test code:

#include <stdio.h>

#define ONE   1
#define TWO   2

#define TEST_MACRO_V1(x, y)      TEST_MACRO_V1_##x##_##y
#define TEST_MACRO_V1_ONE_TWO    3

//#define TEST_MACRO_V2(x, y)    TEST_MACRO_V2_##('x')##_##('y')  -> this version does not work
#define TEST_MACRO_V2(x, y)      TEST_MACRO_V2_"x"_"y"
#define TEST_MACRO_V2_1_2        4

int main()
{
    printf("Test Macro V1: %d\n", TEST_MACRO_V1(ONE, TWO));
    printf("Test Macro V2: %d\n", TEST_MACRO_V2(ONE, TWO));

    return 0;
}

Note: TEST_MACRO_V1 works fine. I need TEST_MACRO_V2.

:) The solution shall be independent from the name of macro argument. It means, the user can use other define name, but this defines have always the same value, like 0,1,2 … So, in the “end define” I need the values of the defines used in the macro call. — Vili
– Vili, Commented Feb 9, 2020 at 20:52
It's not clear what you are asking. Please post exactly what the expected expansion of TEST_MACRO_V2(ONE, TWO) should be, or what the expected output is of the program you posted — M.M
– M.M, Commented Feb 9, 2020 at 21:18
Also explain what is the point of MACRO_1_2 , TEST_MACRO_V1_ONE_TWO and TEST_MACRO_V2_1_2 ? — M.M
– M.M, Commented Feb 9, 2020 at 21:19

David K · Accepted Answer · 2020-02-09 21:14:24Z

1

I tried this on ideone.com:

#include <stdio.h>

#define ONE   1
#define TWO   2

#define TEST_MACRO_HELPER(x, y) TEST_MACRO_##x##_##y
#define TEST_MACRO(x, y)        TEST_MACRO_HELPER(x, y)
#define TEST_MACRO_1_2          4

int main()
{
    printf("Test Macro: %d\n", TEST_MACRO(ONE, TWO));
    return 0;
}

Output:

Test Macro: 4

answered Feb 9, 2020 at 21:14

David K

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