Compare two undefinded symbols in C

Question

I did not define both symboles SYMBOL1 and SYMBOL2, and I'm supprised when I see that the printf is called in the following code:

#include <stdio.h> 
int main()
{
#if (SYMBOL1==SYMBOL2)
    printf("Hello World");
#endif
    return 0;
}

Could you please explain why? any reference to the standard?

It's essentially equivalent to `#if (0 == 0)` which is true. — mediocrevegetable1, Oct 05 '21 at 08:39
Surely this is a duplicate. Do we have any canonical "how does `#if` work?" post? — Lundin, Oct 05 '21 at 08:48
@Lundin maybe you could dupe with [this](https://stackoverflow.com/questions/5085392/what-is-the-value-of-an-undefined-constant-used-in-if), but it's technically tagged [c++] (though the accepted answer cites both C and C++ standards). — mediocrevegetable1, Oct 05 '21 at 10:31

score 5 · Accepted Answer · answered Oct 05 '21 at 08:41

As per the ISO C standard (C11 6.10.1 Conditional inclusion):

After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers (including those lexically identical to keywords) are replaced with the pp-number 0, and then each preprocessing token is converted into a token. The resulting tokens compose the controlling constant expression which is evaluated ...

In other words, your expression becomes 0 == 0, which is obviously true. Hence the printf is included in the source stream.

The Average Google User · Answer 2 · 2021-10-05T08:45:00.050

0

The compiler will not catch that your MACRO is undefined and it will consider it a 0. Since 0 == 0 printf will be executed.

The C standard says that all undefined identifiers there are replaced with 0.

edited Oct 05 '21 at 08:45

answered Oct 05 '21 at 08:39

The Average Google User

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elcuco · Answer 3 · 2021-10-05T09:07:55.697

0

You should also check if both are defined, I think that if both (any) are undefined, you don't need to compile.

#if defined(SYMBOL1) && defined(SYMBOL1) && (SYMBOL1==SYMBOL2)
printf("Hello World");
#endif

(completely untested, but a rough idea).

edit: see also Preprocessor check if multiple defines are not defined

edited Oct 05 '21 at 09:07

answered Oct 05 '21 at 08:45

elcuco

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Compare two undefinded symbols in C

3 Answers3