[Development] Using #pragma once

[Thiago Macieira]

On Sunday, 7 October 2018 15:17:30 PDT Henry Skoglund wrote:
> > I recommend against changing Qt.
> 
> Hi, but isn't C++17's __has_include preprocessor cmd an implicit
> endorsement of #pragma once? I mean, they both assume that the file
> namespace is stable and idempotent.

No, I don't see how one thing has to do with the other.

__has_include imples that the #include will not fail with "No such file or 
directory" and hopefully won't fail either with "Is a directory". Other checks 
are welcome, but I wouldn't be too picky if they didn't get done.

#pragma once only applies after the #include and changes nothing about whether 
the #include succeeds or not.

> The example with ~/src as a bind-mount to ~/dev/src above, reminds me of
> the old pointer aliasing problems in C++. But cannot it be solved by
> requiring #pragma once to always call realpath() first, i.e. always test
> on "the canonicalized absolute pathname" that realpath() returns?

No.

$ realpath ~/dev/src/qt/ ~/src/qt 
/home/tjmaciei/dev/src/qt
/home/tjmaciei/src/qt

Bind-mounts are not symlinks. For all intents and purposes they are different 
filesystems: you can't link() from one to the other, for example. You may be 
able to tell that they are the same because the dev_t is the same:

$ stat -c '%D %i %n' ~/dev/src ~/src 
fe03 4456449 /home/tjmaciei/dev/src
fe03 4456449 /home/tjmaciei/src

But to tell that the file is the same you'd need to compare inodes and those 
are not guaranteed on network mounts.

Another option is to use name_to_handle_at(), a Linux-specific function, but 
that will also fail with bind mounts and multiple mounts of the same 
filesystem. The manpage suggests resolving mount IDs to actual disks by 
looking for them in /dev/disks/by-uuid.

So you see why this is not a very easy solution.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center