Why does ls // hang in Cygwin

Question

When I run the command ls // is Cygwin, the shell hangs for about 12 seconds, then gives the error ls: reading directory '//': Permission denied. I remember reading somewhere that /some//path is interpreted the same as /some/path in POSIX paths, and accordingly, when I run the command in an Ubuntu virtual machine, there is no delay, and ls outputs the contents of /.

I can't find anything about // in the Cygwin special filenames documentation, so why does this happen, and how can I reduce the delay?

Edit:

It turns out \\* paths are network paths on Windows. When I disconnect my computer from all networks, ls \\ returns an empty listing immediately. When I connect to a some networks, I get a listing of hosts on the network, but on other networks, I encounter the problem described above. Considering this, is the some way to configure Windows so that the delay is reduced, even in the case of a permission error?

Answer 1

It’s not a Cygwin issue, it’s a Unix issue.  In the early days of computer networking, before Sun (Solaris) introduced the /net/hostname syntax for accessing remote filesystems¹, and NFS allowed remote filesystems to be mounted at arbitrary mount points (e.g., /mnt or /home/username), some versions of Unix implemented a //hostname/pathname syntax for the same purpose.  See On what systems is //foo/bar different from /foo/bar?  Over the years, that syntax has fallen out of favor, yielding (mostly?) to NFS.

However, to allow these legacy systems to be POSIX-compliant, the standard explicitly allows // to mean something different from / (at the beginning of a path):

If a pathname begins with two successive <slash> characters, the first component following the leading <slash> characters may be interpreted in an implementation-defined manner, although more than two leading <slash> characters shall be treated as a single <slash> character.
        ^{(from near the end of the “Pathname Resolution” section)}

— although it doesn’t specify what it should mean (if anything).

Cygwin interprets // much the same as the old /net, as documented here.  It references Windows’ “Network” (pseudo-)folder — the one that contains “This Computer” and other discoverable machines.

See also Unix: difference between path starting with / and //.
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¹ … and, optionally, /net could be listed to produce a list of the names of accessible hosts with exported filesystems — i.e., fileservers.

Answer 2

https://cygwin.com/cygwin-ug-net/using.html#unc-paths

There's also a virtual directory // which allows to enumerate the fileservers known to the local machine with ls.

Answer 3

On my computer, ls // also runs slowly, printing a couple of cannot access '//some-computer-name': No such file or directory, and in the end printing a few computer names. These are Windows PCs on the local network, and apparently ls tries to connect to them (don't know what for).

On the other hand, dir \\ runs quickly and prints the same computer names (no error messages).

I don't think there's a way to configure Windows to prevent that, looks to me like a Cygwin issue (or feature). Best thing would be to write to Cygwin's mailing list about that, asking to fix it.

Anyway, can I ask why do you run ls // at all?