I want to use macro parameter like this:
#define D(cond,...) do{ \
#if cond \
#define YYY 1 \
#else \
#define YYY 0 \
} while(0)
Is it possible?
UPD
Maybe when sources will be preprocessed twice: gcc -E source.c | gcc -xc - next will work:
#define D(cond,...) #define YYY cond&DEBUG
#if YYY
#define D(...) printf( __VA_ARGS__ )
#else
#define D(...)
#endif
This is not possible. Read about the GNU cpp preprocessor and the C11 standard (i.e. n1570), and check here. The C preprocessor is (conceptually at least) run before the rest of the compiler (which gets the preprocessed form of your translation unit). BTW, for a file foo.c you could use gcc -C -E foo.c > foo.i (using GCC) to get inside foo.i its preprocessed form, and you can inspect that foo.i -since it is a textual file- with a pager or an editor.
However, a .c file can be generated (generating C code is a common practice, at least since the 1980s; for example with yacc, bison, rpcgen, swig, ....; many large software projects use specialized generators of C or C++ code...). You might consider using some other tool, perhaps the GPP preprocessor (or GNU m4) or some other program or script, to generate your C file (from something else). Look also into autoconf (it might have goals similar to yours).
You may want to configure your build automation tool for such purpose, e.g. edit your Makefile for GNU make.
No, this is not possible.
During translation, all preprocessing directives (#define, #include, etc.) are executed before any macro expansion occurs, so if a macro expands into a preprocessing directive, it won't be interpreted as such - it will be interpreted as (invalid) source code.
As pointed out by others this is not possible but there is a work around:
int YYY;
/* global scope variables are sometimes considered bad practice... */
#define D(cond,...) do{ \
if (cond) { \
YYY = 1; \
} \
else { \
YYY = 0; \
} \
} while(0)
Use an optimizing flag (ex: gcc/clang -O3) and it will replace the dead code as if it was a macro. Obviously you may want to change the type of YYY but you seem to use it like a boolean.
No, because C 2011 [N1570] 6.10.3.4 3 says, about macro replacement, “The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one,…”
No, you cannot. The C preprocessor cannot know what is going to occur during runtime.
The preprocessor goes through the program before it is even compiled and replaces every macro defined with its assigned value.
