Ark Khasin <ak*****@macroexpressions.comwrites:
John Bode wrote:
>On Jan 16, 8:57 am, Francois Grieu <fgr...@gmail.comwrote:
>>One of the C compiler that I use <OT>(Keil's CX51)</OTbarks at
#define a(b) b
int main(void){return a(
#if 0
#endif
0);}
More generally, this compiler seems confused by any preprocessing
directive in the middle of macro arguments, which comes handy e.g. to
conditionaly select one of several arguments passed to a macro.
Is it a compiler bug? Where does C89 define if this is valid or not?
It's not a bug; you cannot embed preprocessor commands in a macro.
All preprocessor directives are processed before any macro expansion
takes place, so any preprocessor directives embedded in a macro will
not be recognized as such, but will instead be translated as regular C
code.
The OP's code has no embedded directives; it's equivalent to
#define a(b) b
int main(void){return a(
0);}
Which is good C, IMHO. So I think it's a compiler bug.
I disagree. Are you saying that the OP's code is equivalent to the
above because of John Bode's: "All preprocessor directives are
processed before any macro expansion takes place"? If so, I think you
have been misled. In section 5.1.1.2 ("Translation phases") is says:
4. Preprocessing directives are executed, macro invocations are
expanded, and _Pragma unary operator expressions are executed. If a
character sequence that matches the syntax of a universal character
name is produced by token concatenation (6.10.3.3), the behavior is
undefined. A #include preprocessing directive causes the named
header or source file to be processed from phase 1 through phase 4,
recursively. All preprocessing directives are then deleted.
so all pre-processing happens "at once". I may have missed something
that does give the OP's code a portable meaning, but I need
persuading. Until then I stand by my previous quote that:
If there are sequences of preprocessing tokens within the list of
arguments that would otherwise act as preprocessing directives,
the behavior is undefined. [6.10.3]
--
Ben.
