Compare two undefinded symbols in C-CodePudding

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?

CodePudding user response：

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.

CodePudding user response：

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.

CodePudding user response：

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).