https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58770
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Manuel López-Ibáñez from comment #4) > Sorry, I don't understand this. The #ifdef include-guards do prevent a > second inclusion, even if done by a different file name, no? No, including the file via a different filename (e.g. via a symlink) will re-open the file and preprocess it again. The include guards mean you don't get re-definition of the file contents, but the file still gets included twice. You can confirm that with strace. With #pragma once the file is not even re-opened, so the compiler needs to do extra checking.
