On Mon, Jan 14, 2013 at 11:00:02AM +0100, Corinna Vinschen wrote:
>On Jan 14 01:17, Christopher Faylor wrote:
>> On Mon, Jan 14, 2013 at 04:21:25PM +1100, Shaddy Baddah wrote:
>> >In investigating this, I believe the issue I am having is due to how
>> >stat() handles tilde prefixed paths. On linux we see:
>> >
>> >linux$ $ python -c 'import os; print os.stat("~/..")'
>> >Traceback (most recent call last):
>> > File "<string>", line 1, in <module>
>> >OSError: [Errno 2] No such file or directory: '~/..'
>> >
>> >and on cygwin we see:
>> >
>> >cygwin$ python -c 'import os; print os.stat("~/..")'
>> >posix.stat_result(st_mode=16832, st_ino=562949953496729L,
>> >st_dev=4174909669L, st_nlink=1, st_uid=42037, st_gid=10513, st_size=0L,
>> >st_atime=1357616166, st_mtime=1357616166, st_ctime=1357616166)
>>
>> It is a bug. It's not just "~". Any nonexistent directory will
>> work, like "foo/..".
>
>And it's a bug which isn't easily fixed. Since about the dawn of time,
>Cygwin's core path handling evaluates the path in a non-POSIX manner,
>mainly for performance reasons.
>
>POSIX demands to evaluate a path from left to right (thus tripping over
>the non-existant "~" or "foo" directory). Windows OTOH skips testing
>all parent directories of a path, and while this can be changed(*), it's
>the default setting since the earliest Windows NT versions.
>
>So, since Cygwin can't rely on the OS to this job when it has to convert
>a POSIX path to a Windows path internally, Cygwin would have to check
>the existence of every single path component from left to right to
>emulate the POSIX requirements. But that would be a big performance
>hit, so Cygwin's path handling code tries to be clever to avoid having
>to call too many OS functions:
>
>The first step of converting a POSIX path to a Windows path is to
>normalize the path. "." and ".." components are simply dropped:
>
> "a/b/./c" -> "a\b\c"
> "a/b/../c" -> "a\c"
>
>Then the path prefix is replaced by the matching mount point.
>
>Eventually it evaluates the path from right to left. Consider a valid,
>normalized path with 10 components. Under POSIX rules this requires 10
>checks for existence. No problem for the Linux kernel since it has
>everything under control anyway and the test is blazingly fast.
>
>But Cygwin is not the OS so it has to call the necessary OS functions 10
>times. By checking from right to left, Cygwin has to call the OS
>functions only once, if the file exists, two times if the file does not
>exist, but its parent dir exists, and so on. On top of that, the entire
>chore has to restart when tripping over a symbolic link.
>
>Since the predominant number of file operations are performed on
>existing paths, or at least paths for which the parent dir exists,
>Cygwin reduces the number of OS operations to convert a POSIX to a
>Windows path. The price we're paying is this very deviation from the
>POSIX standard.
Also:
c:\>dir foo\bar\..\..
Volume in drive S is share Serial number is e620:3c3d
Directory of S:\*
1/11/2013 9:58 <DIR> .
12/26/2012 21:34 <DIR> ..
1/12/2013 16:27 <DIR> bin
1/14/2013 10:20 <DIR> cgf
...
I don't have a foo directory but cmd was happy to just ignore that
fact and show my the root directory. This is YA place where Windows
and Linux differ drastically.
cgf
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