Can #endif in an included file be used to close a #if in the including file?

Question

Let's say I have two files, a.h:

#if 1
#include "b.h"

and b.h:

#endif

Both gcc's and clang's preprocessors reject a.h:

$ cpp -ansi -pedantic a.h >/dev/null
In file included from a.h:2:0:
b.h:1:2: error: #endif without #if
 #endif
  ^
a.h:1:0: error: unterminated #if
 #if 1
 ^

However, the C standard (N1570 6.10.2.3) says:

A preprocessing directive of the form

# include "q-char-sequence" new-line

causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters.

which appears to permit the construct above.

Are gcc and clang not compliant in rejecting my code?

Answer 1

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.)