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Re: /dev/fd/N not synonymous with file descriptor N; it is on Linux




On Jan 27 19:39, Houder wrote:
> On Tue, 22 Jan 2019 11:39:28, Corinna Vinschen  wrote:
> > 
> > On Jan 22 11:20, Houder wrote:
> > > On Tue, 22 Jan 2019 10:41:57, Corinna Vinschen  wrote:
> > > > On Jan 22 10:25, Houder wrote:
> > > [snip]
> > >
> > > > > Curious! It fails (for me) on W7 ...
> > > >
> > > > It works for me just as well on W7:
> [snip]
> 
> > Maybe you should run the above shell session under strace and see if
> > something unusual crops up.  BLODA?
> 
> NO BLODA.
> 
> Ok, for the record (as this is W7, i.e. pre-pre-W10 :-)
> 
> Using my original STC again: (source code included below)
> 
>  - create file (in /tmp) write-only, write "Hello, world!" to file, close fd
>  - open file once more read-only
>  - unlink file
>  - open file, using /dev/fd/N, read-write <==== succeeds (and the handle shown by fcntl is read-write)
>  - write "*****" to file (using the fd obtained in the previous line), lseek to begin of file
>   - write fails w/ "Permission denied" <==== so ... the file cannot be written to?

Yes, that scenario fails on W7 but works on W10 1709 and later.  Keep in
mind that the OS doesn't allow to reopen a file which has been deleted.
Cygwin tries a best effort by duplicating the handle.  A duplicated file
handle can't have more permissions than the original handle, so if the
original handle was opened for reading only, the duplicated handle can't
have write perms.

> Btw, earlier on, when /tmp/stc.txt is unlinked, an "Sharing violation" occurs:
>    32   21456 [main] stc 1368 unlink_nt: Trying to delete \??\E:\Cygwin64\tmp\stc.txt, isdir = 0
> ----- # Sharing violation? in unlink_nt() in winsup/cygwin/syscalls.cc, after the 3rd call to NtOpenfile()

Yes, that's expected.  Cygwin first tries to open the file for DELETE
only.  If that works, it can just delete the file and be done with it.

However, if there's any other open handle to the file with more perms
than just DELETE, this first call fails with STATUS_SHARING_VIOLATION.
Now Cygwin knows that the file is in-use and that the job is not just
done with deleting the file.  More effort is required.


Corinna

-- 
Corinna Vinschen
Cygwin Maintainer

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