Path prefix //./ in Cygwin

[Sam Edge]

On 12/06/2025 00:01, Jeremy Drake via Cygwin wrote:
> While making some tests for a path parser in rust
> (https://github.com/rust-lang/rust/pull/141864), an interesting corner
> case in Cygwin path handling came to light:
> 
> Works:
> \\.\C:
> //.\C:
> //./C:\foo
> 
> Doesn't work:
> //./C:
> //./C:/foo
> 
> It appears to be able to use the \\.\ prefix, there must be at least 1
> backslash in the path.  Otherwise, it seems that . and .. are being
> normalized like a normal path and //./C:/foo is treated like
> //C:/foo.  The delay attempting to access it makes me think it is looking
> for a server/share style path.
> 
> My question is whether this behavior is intentional or a bug.  I don't
> want to see rust codify this in their path parser and tests, only to have
> it change later as a bugfix in a corner case of path handling.
> 

Cygwin is a POSIX layer. In POSIX //x/y is allowed to mean the same as 
/x/y as it does in Linux or it can give special significance to 'x' in 
this context.

Cygwin treads a tricky line with backslash. Technically in POSIX it has 
no special significance - a backslash is not a path separator and is a 
legitimate character for a directory or file name but for convenience 
and backwards compatibility sometimes - and others will be able to give 
you the circumstances better than I - it will attempt to treat the path 
as a Windows one.

I would think that if you're building something against Cygwin, it's 
probably best to assume it's POSIX where only forward-slash is special 
and not try to second-guess.

Just my two penn'orth.


-- 
Sam Edge
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