Is it possible to do preprocessing evaluation with something different than a numeric constant using GreenHills?

Question

I am trying to do the following evaluation: In my header file (define value can change):

#define X ((void *) 0)

In my function:

uint8_t foo() {

   uint8_t value = 0;

#if ( X != 0 )
   value = 1;
#endif

   return value;
}

When I compile the code it throws the following errors:

#57: this operator is not allowed in a constant expression
#58: this operator is not allowed in a preprocessing expression

When I do operations to see if the DEFINE exists (or not) it works fine (#ifdef or #if defined(X)). But what I want is to be able to evaluate the value of X at compile time. Am I missing any flag or something I need to set in order to be able to make this work? I am using Green Hills compiler.

Please post the [Minimal, Complete, and Verifiable example](http://stackoverflow.com/help/mcve) that shows the problem. Standard C which will work with any C compiler. — Weather Vane, Jan 19 '18 at 20:51
Please make plain what `do something` means. Preprocessor statements do not have any direct action. — Weather Vane, Jan 19 '18 at 21:01
The edit is bizzarre. What is the [XY problem?](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem) — Weather Vane, Jan 19 '18 at 21:05
Is this the *exact* code you are trying? Or you have "simplified" it? — Eugene Sh., Jan 19 '18 at 21:14
Finally you show the real code, and that code is not valid, because `X` is not a numeric constant. The previous version of the question, where you had `#define X 0` is valid, and should work. — user3386109, Jan 19 '18 at 21:21
Was it that hard to show the actual code? You just wasted time of several people for nothing. — Eugene Sh., Jan 19 '18 at 21:27

melpomene · Answer 1 · 2018-01-19T23:16:33.567

1

You can't use casts in #if expressions¹. The preprocessor knows nothing about types. (And the only values supported by #if are simple integers, so pointers are right out.)

Just use a regular if statement:

if (X != 0) {
    value = 1;
}

The compiler will recognize that the condition is always true (or false, depending) at compile time.

¹ Reference: ISO 9899:1999, 6.10.1 Conditional inclusion:

The expression that controls conditional inclusion shall be an integer constant expression except that: it shall not contain a cast; [...]

edited Jan 19 '18 at 23:16

answered Jan 19 '18 at 21:25

melpomene

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In fact you *can* use casts in `#if` expressions, but not the particular one the OP tries to use. – John Bollinger Jan 19 '18 at 21:48
You can use any cast in a preprocessor expression as long as you understand it's compiled in context after the preprocessor step during compilation. – nicomp Jan 19 '18 at 21:57
@JohnBollinger My C99 says otherwise (in 6.10.1/1). – melpomene Jan 19 '18 at 23:16
@nicomp What do you mean by "preprocessor expression"? – melpomene Jan 19 '18 at 23:17
@melpomene, the current C standard is C2011. I quote its relevant provision (which happens to be numbered the same) in my own answer; it does not contain the prohibition against casts. – John Bollinger Jan 19 '18 at 23:24
@JohnBollinger Yeah, but C11 still converts unknown identifiers to `0`, so how can you even form a cast expression? – melpomene Jan 19 '18 at 23:25
You make an excellent point, and in practice, `gcc` agrees with you. It's not the final arbiter, of course, but it's a useful gauge. – John Bollinger Jan 19 '18 at 23:32

Is it possible to do preprocessing evaluation with something different than a numeric constant using GreenHills?

1 Answers1