problems using defines inside macros

[236Rko23]

hi there, i'm doing something in C where i needs to modify an initializer at compile time.

#define _PCN 3
struct SOMETHING_t {
   uint8_t variableA;
   float   variableB[_PCN];
   uint8_t variableC[_PCN];
   uint8_t variableD;
}

and the declaration with the initializer

struct SOMETHING_t stuff = {\
    8,\
    {1.0,1.0,1.0},\
    {3,3,3},\
    2\
};

the problem is that i want it to change by changing _PCN so i made some macros.

#define	InitxN(a) _InitxN(a,_PCN)
#define _InitxN(a,b) Initx ## b(a)

#define Initx1(a) a
#define Initx2(a) {a,a}
#define Initx3(a) {a,a,a} 

so that it would like this

struct SOMETHING_t stuff = {\
    8,\
    InitxN(1.0),\
    InitxN(3),\
    2\
};

but i can't figure how to do the macro so that it expands _PCN before passing it to the next one, it ends up as Initx_PCN(a) instead of Initx3(a). I've read the gnu gcc page but cant find something about this. There's about macro inside macro but not about define inside macro...

any idea?

[236Rko23]

anyone?

[cppabuser]

I do not think a single preprocessor pass will accomplish this. Try a multi pass by just issuing a preprocess command before you do your normal compile and link. You can do this on gcc with "gcc -E mycode.c" . Then do a normal build.

[236Rko23]

@cppbuser
it wasn't neccessary to do multipass, but that command sure helped me. I used it to see the actual output from the preprocesor rather than just the blablabalerrorhere

also found a little help here in case someone is also interested
http://stackoverflow.com/questions/1597007/creating-c-macro-with-and-line-token-concatenation-with-positioning-macr

the solution was to add some extra levels to the macro to give it a chance to expand

#define Initx1(a) a
#define Initx2(a) {a,a}
#define Initx3(a) {a,a,a}
#define Initx4(a) {a,a,a,a}

#define __InitxN(a,b) Initx ## a (b)

#define	_InitxN(a,b) __InitxN(a,b)

#define	InitxN(a) _InitxN(_PCN,a)  

thanks for the help

[cppabuser]

right .. I just did this simple example. Basically you just need to use "that" to call "this". Without "that", "this" will produce CSOME and fail.

#include <stdio.h>

#define CAT 1234
#define SOME AT
#define this(a, b) a ## b
#define that(a, b) this(a,b)


int main() {

printf("%d\n",that(C, SOME));

return 0;
}

[cppabuser]

also if you do want to multi pass, (things that will horrify your CS professor) you can do something like this. It fails on 1 pass, compile, and link, but with 2 pass, compile, and link it works fine. This opens up a lot of interesting possibilities if you start using conditionals. Perhaps you can do a lot of cpp stuff with ordinary c ...

#define FIRST #define SECOND 9999
#define BLUE 52
#define REDGREEN BLUE
#define RED 51
#define COLOR(a,b) a##b

int main() {

FIRST

printf("%d\n",COLOR(RED,GREEN));

return 0;
}