Answer by Keith Thompson for Can #endif in an included file be used to close a #if in the including file?

The C standard defines 8 translation phases. A source file is processed by each of the 8 phases in sequence (or in an equivalent manner).

Phase 4, as defined in N1570 section 5.1.1.2, is:

Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. If a character sequence that matches the syntax of a universal character name is produced by token concatenation (6.10.3.3), the behavior is undefined. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively. All preprocessing directives are then deleted.

The relevant sentence here is:

A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively.

which implies that each included source file is preprocessed by itself. This precludes having a #if in one file and the corresponding #endif in another.

(As "A wild elephant" mentioned in comments, and as rodrigo's answer says, the grammar in section 6.10 also says that an if-section, which starts with a #if (or #ifdef or #ifndef) line and ends with a #endif line, can only appear as part of a preprocessing-file.)