From: <sten.kristian.ivarsson@gmail.com>
To: "'Ken Brown'" <kbrown@cornell.edu>, <moss@cs.umass.edu>,
"'Jonathan Yong'" <10walls@gmail.com>,
"'The Cygwin Mailing List'" <cygwin@cygwin.com>
Subject: Sv: Sv: Sv: Sv: Sv: Sv: g++ and c++17 filesystem
Date: Wed, 25 Nov 2020 09:30:48 +0100 [thread overview]
Message-ID: <002001d6c305$475e8d40$d61ba7c0$@gmail.com> (raw)
In-Reply-To: <11a20f55-46db-c9b4-1f30-d2181a3aeb9e@cornell.edu>
Thanx for the insightful thoughts Ken
See more below
> >>> all the std::filesystem implementations I've seen for Windows
> >>
> >> The implementation on top of Cygwin is not "for Windows", it's "for
> >> Cygwin", i.e., "for Posix". And for Cygwin that's the right thing to
> do.
> >> And that's where we keep talking at cross purposes.
> >
> >
> > Maybe it is the right thing to do, but what is your take of why it must
> be so (all the way) ?
> >
> > I also do understand that it have several advantages in the
> > implementation of std::filesystem but it also imply an extra layer of
> > abstraction to the underlaying platform, but to just remove the
> > _CYGWIN_ macro when building it would make std::filesystem to not
> > understand /cygdrive at all (and I guess that would confuse other
> > users;-) so I guess it would require some more sophisticated
> > implementation (or extend the number of exceptions already existing in
> > the underlaying Cygwin-Posix-implementation)
>
> I'd like to try to make this discussion more concrete by looking at what's
> actually going on in the test program main.cpp that you posted at the
> beginning of that thread. (I ran it under strace and gdb to see this for
> myself.)
>
> First, your program calls std::filesystem::exists, which ultimately calls
> stat(2) in the Cygwin DLL. The work for this is done in the
> path_conv::check function and various functions that it calls. These
> functions have code that recognizes Win32 paths like C:\Temp, and
> std::filesystem::exists therefore reports "true". This is consistent with
> the statement at
> https://cygwin.com/cygwin-ug-net/using.html#pathnames-win32 about how the
> Cygwin DLL treats Win32 paths.
>
> Next, your program calls std::filesystem::canonical. This calls
> std::filesystem::absolute, which reports that C:\Temp is not an absolute
> path.
> It therefore tries to treat it as a relative path and fails with "No such
> file or directory" because <current directory>/C:\Temp does not exist.
> Note that the Cygwin DLL never sees the original path C:\Temp in this
> case. Again, this is consistent with the statement in the documentation
> that Cygwin applications do not necessarily recognize Win32 paths.
>
> You say this is a bug, because first you're told that C:\Temp exists and
> then you're told it doesn't. But I think it just illustrates the hazards
> of using
> Win32 paths in Cygwin, which tries hard to emulate Posix. Sometimes you
> can get away with using Win32 paths and sometimes you can't, depending on
> how and when the Cygwin DLL is involved.
Well, to call it a bug in this context was maybe wrong, but in a
ISO-standard perspective it would be considered a bug ;-)
> I don't see a good way to avoid this inconsistency. We could change
> Cygwin so that it rigidly recognizes only Posix paths. Cygwin would then
> be consistent, but we would be removing a feature that many users have
> become accustomed to
I guess so too, but they are already there for some reason
I don't expect that the posix-functions should accept any non-posix-paths
but it would be nice to have the platform independent std::filesystem to
work transparently but I guess, since the cygstdc++-library uses the
underlaying posix-functions the only reasonable way to accomplish this is to
extend the list of features that already exists
The sole reason for std::filesystem was to avoid platform dependent code
So I guess in practical, it is up to the community of how well motivated
they are to extend those Posix-functions but my take in this thread is that
many rather would like it to become more ... like an emulator
> And it wouldn't help you. Or we could ask all Cygwin package maintainers
> to try to patch their packages so that they recognize Win32 paths, but
> that's simply not feasible, nor would many package maintainers be willing
> to invest the required time.
I'm not sure what package maintainers you're referring to here now ?
Applications ? Cygwin ? GCC ?
> I tried it once with emacs, which I maintain, in response to a user
> request. I failed miserably. Every change I made broke something else,
> and I finally gave up.
>
> I don't think g++ will be any easier than emacs, and I don't think you can
> expect the g++ maintainer to work on this.
Yeah, I noticed that the _CYGWIN_ macro actually was a part of the real
libstdc++-distro
Thanx Ken and keep up the good work
Kristian
> Ken
next prev parent reply other threads:[~2020-11-25 8:30 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-17 15:15 sten.kristian.ivarsson
2020-11-17 16:45 ` René Berber
2020-11-18 9:00 ` Sv: " sten.kristian.ivarsson
2020-11-18 16:24 ` René Berber
2020-11-18 16:31 ` Eliot Moss
2020-11-18 20:46 ` Kristian Ivarsson
2020-11-18 20:56 ` Eliot Moss
2020-11-18 21:18 ` Kristian Ivarsson
2020-11-18 23:47 ` Eliot Moss
2020-11-19 8:10 ` Sv: " sten.kristian.ivarsson
2020-11-18 21:45 ` Norton Allen
2020-11-19 0:08 ` Doug Henderson
2020-11-19 6:23 ` Brian Inglis
2020-11-19 10:03 ` Sv: " sten.kristian.ivarsson
2020-11-19 15:27 ` Brian Inglis
2020-11-20 9:37 ` Sv: " sten.kristian.ivarsson
2020-11-20 15:29 ` Brian Inglis
2020-11-20 16:11 ` Sv: " sten.kristian.ivarsson
2020-11-19 15:36 ` Eliot Moss
2020-11-20 8:31 ` Sv: " sten.kristian.ivarsson
2020-11-20 18:28 ` Jonathan Yong
[not found] ` <000601d6c173$aa55d540$ff017fc0$@gmail.com>
2020-11-23 11:09 ` Sv: " Jonathan Yong
2020-11-24 9:32 ` Sv: " sten.kristian.ivarsson
2020-11-24 10:24 ` Jonathan Yong
2020-11-24 11:35 ` Sv: " sten.kristian.ivarsson
2020-11-24 12:33 ` Jonathan Yong
2020-11-24 14:01 ` Sv: " sten.kristian.ivarsson
2020-11-25 2:25 ` Jonathan Yong
2020-11-24 13:22 ` Eliot Moss
2020-11-24 14:31 ` Sv: " sten.kristian.ivarsson
2020-11-24 20:06 ` Ken Brown
2020-11-24 20:39 ` Eliot Moss
2020-11-25 8:02 ` Sv: " sten.kristian.ivarsson
2020-11-25 8:30 ` sten.kristian.ivarsson [this message]
2020-11-25 0:23 ` Brian Inglis
2020-11-25 9:00 ` Sv: " sten.kristian.ivarsson
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