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Using #pragma once in headers

Andrew Pinski pinskia@gmail.com
Wed May 7 02:13:28 GMT 2025 
On Tue, May 6, 2025 at 7:01 PM Fangrui Song <i@maskray.me> wrote:
>
> On Tue, May 6, 2025 at 5:59 PM Alan Modra <amodra@gmail.com> wrote:
> >
> > On Tue, May 06, 2025 at 11:58:54AM +0100, Matthieu Longo wrote:
> > > Would it make sense to accept "#pragma once" and deviate one millimeter away
> > > from the C99 standard instead of using #ifndef ... #define ... #endif ?
> >
> > I'm mildly against the proposal.  Consistency matters.  If you were
> > starting a new sub-project of binutils then I wouldn't mind.
> >
> > --
> > Alan Modra
>
> #pragma once  does not deduplicate when symlinks are involved (as
> explained by the Wikipedia page). I do not know whether the point is
> relevant to binutils.
>
> ln -s a.h b.h
> #include "a.h"
> #include "b.h"
>
> https://thamara.dev/posts/pragma-once-vs-header-guards/

Actually symlinks work with GCC's implementation. It is clang and
MSVC's implementation which does not.

Now `#pragma once` should not be used at all. In fact it was at one
point deprecated in GCC and then undeprecated years later when a new
(current) implementation was done.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=11569
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45599) was the bug
recording the undeprcation of `#pramga once` and the new
implementation mentioned the fix of the symlinks issue. Later on when
clang was being designed it didn't handle the symlink issue.

But GCC's implementation does treat the 2 different files with the
same contents as the same:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52566  (is this a bug
depending on the definition of once means).

Thanks,
Andrew

>
>
> The only other technical issue is related to C++ modules
> (https://github.com/llvm/llvm-project/issues/77995 fixed by newer
> Clang), which is not relevant to us.
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