Thanks for digging up all that useful information, Martin! So `#pragma once` is not the clear winner after all. Too bad since we could eliminate some boilerplate code with that approach.
Is there another way to solve the problem Arthur raised in this thread? The LMKL thread had a python script and people were also talking about adding something to checkpatch.pl. Maybe these days it's sufficient to assume clang will catch any real problems (CB:62173).
On Mon, May 16, 2022 at 1:03 PM ron minnich rminnich@gmail.com wrote:
we have, in the past, used Linux kernel style as our guideline on coreboot style.
I'd say, based on Martin's note, that #pragma once is not such a good idea after all. If we decide NOT to use it, however, let's put a note about it in our style guide?
This is not the first time this question has come up.
On Mon, May 16, 2022 at 12:34 PM Martin Roth gaumless@tutanota.com wrote:
After reading what I've included below, I'm going to have to vote to
keep the guards.
Martin
May 16, 2022, 10:59 by david.hendricks@gmail.com:
On Mon, May 16, 2022 at 8:59 AM ron minnich rminnich@gmail.com
wrote:
btw, sometimes this has gone the other direction .. https://github.com/lowRISC/opentitan/pull/5693
It looks like they did that solely to conform to Google's style guide which, dogmatic as it may appear, makes sense since OpenTitan is a Google-lead project.
The question then is 'why does Google require the use of guards?'.
Whatever you think of google, they're not going to mandate something like this without a good reason.
I went searching for where this rule came from, and found this:
If you trust our in-house C++ compiler gurus, here's the most salientpart of the whole thread linked above.Matt Austern (4/11/2013): If you talk to the authors of [most C++] compilers, I think you'll find that most of them consider "#pragma once" (or the equivalent #import) to be a flaky feature -- something that works almost all of the time and that can cause seriously annoying bugs when it doesn't work.
Chandler Carruth (4/12/2013): As one of the authors of one of those
compilers, I couldn't agree more.
any interested Googlers can find this here: https://yaqs.corp.google.com/eng/q/5768278562045952 Further digging:To support #pragma once, the compiler tries to identify duplicate
encounters with the same file, but the check gcc actually performs to establish the identity of the file is weak. Here's someone who made two copies of the same header with different names, each with a #pragma once, and it screwed up his build.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52566
The two headers had the same size, same content, and same timestamps
because he had run "touch *" on them, but they were intended to both be included. Only one was included, and the other was falsely identified as a re-inclusion and ignored.One might say he was asking for trouble by running "touch *", but timestamp collisions are EASY to come by. First of all, they're only 1sec resolution. You might patch all the relevant files and they'd have matching timestamps. You might be using a network filesystem that just doesn't bother with timestamps (common).
Now both of these are almost a decade old, so things might have changedquite a bit since then.
Linux kernel threads: https://lkml.iu.edu/hypermail/linux/kernel/1401.0/02048.html
https://lore.kernel.org/lkml/CAHk-=wi54DEScexxpMrO+Q2Nag_Tup+Y5YBHc_9_xGLeRf...
On Sun, Feb 28, 2021 at 11:34 AM Alexey Dobriyan <adobriyan@gmail.com>wrote:>> >> > End result: #pragma is fundamentally less reliable than the>
traditional #ifdef guard. The #ifdef guard works fine even if you> >
re-read the file for whatever reason, while #pragma relies on some> > kind of magical behavior.You continue to not even answer this very fundamental question."#pragma once" doesn't seem to have a _single_ actual real advantage.Everybody already does the optimization of not even opening - muchless reading and re-parsing - headers that have the traditional #ifdefguard.And even if you _don't_ do that optimization, the #ifdef guardfundmentally semantically guarantyees the right behavior.So the #ifdef guard is (a) standard (b) simple (c) reliable (d) traditionaland you have yet to explain a _single_ advantage of "#pragma once".Why add this incredible churn that has no upside?So no. We're not using #pragma once unless y9ou can come up with somevery strong argument for itAnd no, having to come up with a name for the #ifdef guard is not astrong argument. It's simply not that complicated. Linus