Do Undefined Constants Evaluate to 0 in #if Conditions in C and C ?

Undefined Constants and #if Condition Evaluation in C and C

In preprocessor logic, macros and constants often play crucial roles. One common scenario arises when dealing with undefined constants in #if conditions. Here, we'll explore whether relying on a predictable behavior from these undefined constants is valid.

Problem:

Developers often assume that undefined constants take on a value of 0 when used in #if evaluations. Is this assumption reliable, or should we expect undefined behavior from undefined constants?

Answer:

Yes, you can rely on this assumption. The C99 standard clearly specifies in §6.10.1 ¶3:

After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers are replaced with the pp-number 0

Similarly, the C standard (§16.1 ¶4) dictates:

After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers and keywords, except for true and false, are replaced with the pp-number 0

In other words, after macro expansions and other preprocessing steps, any undefined identifier or constant that appears in an #if condition is automatically replaced with the value 0. This behavior is consistent across C and C .

This predictable behavior allows developers to conveniently use undefined constants as flags or placeholders in #if conditions without risking undefined outcomes or errors. When the undefined constant is not defined before the preprocessing phase, the compiler treats it as 0 by default.

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