macros - Conditional C preprocessor directives -

in code had macro:

#define tps 1(or 0) int main() { ....    if(var)    { #ifdef tps #endif    } } 

but now, want merge if(var) macro acheive:

int var=1; #define tps   (if(var))  int main() {    int a, b, c;    a=1;b=2;c=3; #if tps    printf("a: %d\n", a);    printf("b: %d\n", b);    printf("c: %d\n", c); #endif    printf("++a: %d\n", ++a);    return 0;  } 

i.e. block of code inside macro conditionals should present if var=1 eg, var=1:

int main() {    int a, b, c;    a=1;b=2;c=3;    printf("a: %d\n", a);    printf("b: %d\n", b);    printf("c: %d\n", c);    printf("++a: %d\n", ++a);    return 0;  } 

and, var=0:

int main() {    int a, b, c;    a=1;b=2;c=3;    printf("++a: %d\n", ++a);    return 0;  } 

how can implement #define tps achieve this?

you cannot dreaming of.

preprocessing 1 of earliest phase of compiler (e.g. gcc). , tps looks want have compilation behavior depends on runtime variable var. conceptually, compiler first preprocessing source. can use gcc -c -e preprocessed textual form.

^{at compilation time, variable has name , compiler find location (but variable not have value during compilation). @ runtime, variable has location containing value. values don't exist @ compilation time, can't use them in preprocessing phase.}

however, preprocessing can conditionnal, like

#if wantprint    printf("a: %d\n", a); #endif 

and pass (or not) -dwantprint=1 flag compiler.

you code

int var; int main() {   int a, b, c;   a=1;b=2;c=3;   if (var) {     printf("a: %d\n", a);     printf("b: %d\n", b);     printf("c: %d\n", c);   };   printf("++a: %d\n", ++a);   return 0; } 

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btw, perhaps want dynamically load code @ runtime? on linux , posix systems can call dlopen(3) , dlsym. generate c code in (temporary) file, fork process compile shared object, , dlopen shared object, function pointer dlsym call it... see this answer.

fwiw, common lisp has powerful macro system , able "compile" @ runtime, , arbitrary computations @ "compile-time". actually, sbcl may generate machine code while running....

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addenda

perhaps want customize behavior of gcc compiler itself. might consider using melt (a domain specific language extend gcc). gcc don't enable customization of preprocessing yet (but of middle-end, working on internal gcc representations gimple)