Do comments prevent a header guard optimization?

Question

Does C compiler cache a header file and parse it only once if the file starts with a comment?

// Some comment
#ifndef HEADER_GUARD
#define HEADER_GUARD

#endif

The question is about compilers, that can cache headers to not parse them more than once.

This is implementation dependent. The compiler may or may not cache this information. Some ocmpilers have the `#pragma once` that may be more efficient than header guards. Aso read this: https://stackoverflow.com/questions/1143936/pragma-once-vs-include-guards — Jabberwocky, Nov 23 '17 at 08:16

score 9 · Accepted Answer · answered Nov 23 '17 at 08:23

Yes, this optimization is done. gcc documents it quite explicitly here: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

Basically, if you have a well-written include guard, gcc won't touch the header twice. And a comment outside of the include guard does not stop the optimization (it would be worthless otherwise with all the license comments that precede virtually all open-source header files).

Do comments prevent a header guard optimization?

1 Answers1