CoPy to /dev/null fails

CoPy to /dev/null fails

[Christian Franke]

Hi,

this works on Linux:

$ cp file /dev/null

but fails on Cygwin 1.5.25-15:

$ cp file /dev/null
cp: cannot create regular file `/dev/null': Invalid request code


/dev/null exists, so /bin/cp opens it with O_TRUNC only. But this fails 
with EBADRQC (54):

  fd = open("/dev/null", O_WRONLY|O_TRUNC, .)

According to strace, Cygwin calls:

  h = CreateFile("NUL", ., ., TRUNCATE_EXISTING, ., .);

which fails.


Interestingly, this works:

   fd = open("/dev/null", O_WRONLY|O_CREAT|O_TRUNC, .);

Cygwin calls:

  h = CreateFile("NUL", ., ., CREATE_ALWAYS, ., .);

Apparently a subtle (and IMO undocumented) difference between 
TRUNCATE_EXISTING and CREATE_ALWAYS even when file exists.

Christian


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Re: CoPy to /dev/null fails

[Corinna Vinschen]

On Jul 11 22:05, Christian Franke wrote:
> Hi,
>
> this works on Linux:
>
> $ cp file /dev/null
>
> but fails on Cygwin 1.5.25-15:
>
> $ cp file /dev/null
> cp: cannot create regular file `/dev/null': Invalid request code
>
>
> /dev/null exists, so /bin/cp opens it with O_TRUNC only. But this fails 
> with EBADRQC (54):
>
>  fd = open("/dev/null", O_WRONLY|O_TRUNC, .)
>
> According to strace, Cygwin calls:
>
>  h = CreateFile("NUL", ., ., TRUNCATE_EXISTING, ., .);
>
> which fails.
>
>
> Interestingly, this works:
>
>   fd = open("/dev/null", O_WRONLY|O_CREAT|O_TRUNC, .);
>
> Cygwin calls:
>
>  h = CreateFile("NUL", ., ., CREATE_ALWAYS, ., .);
>
> Apparently a subtle (and IMO undocumented) difference between 
> TRUNCATE_EXISTING and CREATE_ALWAYS even when file exists.

That's a weirdness of the Win32 CreateFile call used in 1.5.25 when
accessing NUL.  This will be fixed in 1.7.0.  It uses
NtCreateFile("\Device\Null") which doesn't have that problem.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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