Do undefined constants evaluate to 0 in #if conditions?

Undefined Constants and #if Conditions

When using the C or C preprocessor, it's essential to understand the behavior of undefined constants within #if conditions.

Question:

Can undefined constants be relied upon to have a value of 0 for evaluation in #if conditions?

Answer:

Yes, undefined constants can be relied upon for this purpose.

Explanation:

According to the C99 standard (§6.10.1 ¶3), and similarly in the C standard (§16.1 ¶4):

• After macro expansions and unary defined operations, all remaining identifiers (including undefined constants) are replaced with the pp-number 0.

Therefore, it's safe to assume that undefined constants will evaluate to 0 in #if conditions, which ensures consistent preprocessor behavior.

Note:

In C , the keywords true and false have special meanings during preprocessing and are not replaced by 0.

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