FAST_CWD warning on 64-bit Cygwin + Windows for ARM64

Tres Finocchiaro
Thu Jan 7 22:13:35 GMT 2021

Running cygwin64 on Windows for ARM64 using the new, experimental
x86_64 emulation layer, the following warning is shown:

> Cygwin WARNING:
>    Couldn't compute FAST_CWD pointer.  This typically occurs if you're using
>    an older Cygwin version on a newer Windows.  Please update to the latest
>    available Cygwin version from  If the problem persists,
>    please see

So far, the problem seems to be mostly benign, however was observed by
a build system which doesn't properly sanitize stderr (the project
used grep through ant's ExecTask which combines stdout and stderr by

I don't require help with this issue, but wanted to report it.  As
Windows on ARM64 becomes more popular, the number of users moving from
cygwin32 to cygwin64 will increase, and the problem is likely to be
reported (or at least experienced) more and more.

I did browse the archives and found this message:

However it didn't mention Windows ARM64 by name, so I wanted to share,
I wasn't sure if this is due to a Win32 API change, a bug in x86_64
emulation for this particular OS or an unforeseen edge-case within the
cygwin codebase.

Slightly unrelated, but if a particular project volunteer requires
ARM64 hardware to reproduce and resolve the above issue, please reply
to me privately, I'd be happy to help.  Also, thanks for the hard work
maintaining the project!


More information about the Cygwin mailing list