Nested #else handling in false #ifdef in C

Asked Aug 15 '16 at 22:14

Active Aug 16 '16 at 10:51

Viewed 115 times

How should a C compiler handle a #else directive that corresponds to a #ifdef directive inside another #ifdef that evaluates to false? In other, words in the code below should code C get executed because the compiler thinks the #else corresponds with the first #ifdef? Or should it be 'smart' enough to know that all of the 2nd #ifdef is nested inside the false evaluation of the 1st #ifdef?

#undefine FLAG1

#ifdef FLAG1
//code section 1
#ifdef FLAG2
//code section 2
#else
//code section 3
#endif
#endif

edited Aug 16 '16 at 10:51

Brian Tompsett - 汤莱恩

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asked Aug 15 '16 at 22:14

Mr. Gravy

As noted in the dup, the else will go with the FLAG2 check. Use your #endifs at the right places to dictate the path you want. – Joe Aug 15 '16 at 22:17
@Joe Nice dup find! Nice question, too bad its already answered. – Fantastic Mr Fox Aug 15 '16 at 22:18
1

The dup is about C++ not C, but the preprocessor behaves the same way as far as this example is concerned. – Keith Thompson Aug 15 '16 at 22:20

Nested #else handling in false #ifdef in C

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