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Date:	Fri, 8 Aug 2025 22:33:25 -0700 (PDT)
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To:	Brian Inglis via Cygwin <cygwin AT cygwin DOT com>
cc:	brian DOT inglis AT systematicsw DOT ab DOT ca
Subject:	Re: getting the on-disk case for a path
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From:	Jeremy Drake via Cygwin <cygwin AT cygwin DOT com>
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On Fri, 8 Aug 2025, Brian Inglis via Cygwin wrote:

> On 2025-08-08 00:53, Thomas Wolff via Cygwin wrote:
> > Am 08.08.2025 um 02:31 schrieb Jeremy Drake via Cygwin:
> >> On a case-insensitive but case-preserving filesystem, is there a Cygwin
> >> API to get the on-disk case for a given path?  It seems like `realpath`
> >> ought to do it but running
> >> $ touch case-test
> >> $ realpath CASE-TEST
> >> returns CASE-TEST.
> > On the command line, you could use
> > ls | grep -i
> >
> >> Regardless, canonicalize_file_name or realpath may not
> >> be what I want because it would dereference symlinks.
> >>
> >> Background: I'm trying to debug some test failures in Clang, due to a
> >> warning that's supposed to be issued when you #include "foo.h" but the
> >> file on disk that it opened is "Foo.h".
>
> Looks like if you use wildcards, it should work correctly:
>
> $ lsattr -dl .
> .                            ---
> $ l *_exit*
> _Exit.2  _exit.3
> $ l _exit.?
> _Exit.2  _exit.3
> $ l _exit.[23]
> _Exit.2  _exit.3
> $ l *EXIT-TEST*
> exit-test
> $ l *exit*
> _Exit.2  _exit.3  EXIT  exit-test
>
> also, you could just opendir(3)/readdir(3)/closedir(3) and strcasecmp(3).
>

Yeah, globbing is opendir/readdir/closedir, but it'd have to be done on
each path component to recover the on-disk case for a path.  I'll explain
now that I have a better idea what clang is doing under the hood.

llvm has an API that opens a file with an out parameter for the "real"
path.  On Windows, it uses GetFinalPathNameByHandleW
(FILE_NAME_NORMALIZED|VOLUME_NAME_DOS), with some massaging for UNC.  On
Unix, it first would prefer fcntl with F_GETPATH.  It doesn't look like
Cygwin provides that.  If that's not available, available, it tries
readlink on /proc/self/fd/%d.  If that's not available, it falls back to
realpath on the name input.  I had a breakpoint on realpath that was not
hit, so it appears that readlink is what it's doing.

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