using enum values in preprocessor directive

Question

I have the following code:

#include <stdio.h>

enum {A, B};

#define C A

int main() {
#if C == B
  printf("%d==%d\n", C, B);
#else
  printf("%d!=%d\n", C, B);
#endif
}

which, surprinsingly, gives the output:

0==1

Now, I understand that the code is wrong, because enum values are unknown to the preprocessor. What I don't understand is why no error is generated... A and B should be undefined at preprocessing time, how is that the preprocessor gives no error?

See anser for your question here: stackoverflow.com/questions/34677148/… — re_things
– re_things, Commented Aug 30, 2018 at 14:38
Possible duplicate of Why does the C preprocessor consider enum values as equal? — Evan Carroll
– Evan Carroll, Commented Sep 1, 2019 at 17:28

Some programmer dude · Accepted Answer · 2018-08-30 14:40:23Z

9

The preprocessor runs in a separate phase before the compiler proper handles the source. As such it doesn't know anything about symbols in the actual source code (like enumerations or variables).

For the preprocessor the symbol B is an unknown macro and when used in that context (#if C == B) it will be equal to zero. Since the symbol A is also not a macro, it will also evaluate to zero. All this lease to the comparison #if 0 == 0 which is indeed true.

See e.g. this phases of translation reference for more information about translation phases, and this preprocessor conditional reference for more information about that.

edited Aug 30, 2018 at 14:40

answered Aug 30, 2018 at 14:35

Some programmer dude

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5 Comments

StoryTeller - Unslander Monica Over a year ago

Equal to zero, or be an empty token sequence?

Some programmer dude Over a year ago

@StoryTeller I've read somewhere that in such a context it's equal to zero. I'll try to dig something up.

KamilCuk Over a year ago

Empty token sequence in #if comparision is equal to zero

StoryTeller - Unslander Monica Over a year ago

@Someprogrammerdude - Now I read it too :) port70.net/~nsz/c/c11/n1570.html#6.10.1p4

Some programmer dude Over a year ago

@StoryTeller From cppreference: "Any identifier, which is not literal, non defined using #define directive, evaluates to 0."

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