FILE_ATTRIBUTE_TEMPORARY ?

[Cygwin list mailing list]

Hi

Does the cygwin
     tempfile()
API use the Win32
     FILE_ATTRIBUTE_TEMPORARY
flag on the files it creates?

Asking because that makes a fairly significant performance difference when creating very short lived temp files.

Thanks, Noel Grandin

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[Mark Geisert]

Hi Noel,

Noel Grandin via Cygwin wrote:

> Hi
>
> Does the cygwin
>      tempfile()
> API use the Win32
>      FILE_ATTRIBUTE_TEMPORARY
> flag on the files it creates?
>
> Asking because that makes a fairly significant performance difference when
> creating very short lived temp files.

Good question!  If one specifies O_TMPFILE as one of the flags on an open() call,
Cygwin does set that Win32 attribute as desired.  Unfortunately, Cygwin's
tmpfile() is supplied by newlib and it does not specify O_TMPFILE on its
underlying open() call.

That looks like a bug to me, unless I'm missing something subtle there; I'll
submit a patch and find out.
Thanks!

..mark
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[Cygwin list mailing list]

On 2021/01/06 11:44 am, Mark Geisert wrote:
> Hi Noel,
> That looks like a bug to me, unless I'm missing something subtle there; I'll submit a patch and find out.
> Thanks!
>

Cool thanks!
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[Brian Inglis]

On 2021-01-06 02:44, Mark Geisert wrote:

> Hi Noel,
>
> Noel Grandin via Cygwin wrote:
>> Hi
>>
>> Does the cygwin
>>      tempfile()
>> API use the Win32
>>      FILE_ATTRIBUTE_TEMPORARY
>> flag on the files it creates?
>>
>> Asking because that makes a fairly significant performance difference when
>> creating very short lived temp files.
>
> Good question!  If one specifies O_TMPFILE as one of the flags on an open()
> call, Cygwin does set that Win32 attribute as desired.  Unfortunately, Cygwin's
> tmpfile() is supplied by newlib and it does not specify O_TMPFILE on its
> underlying open() call.
>
> That looks like a bug to me, unless I'm missing something subtle there; I'll
> submit a patch and find out.

O_TMPFILE is only implemented under winsup/cygwin, and only defined for newlib
under Cygwin:

https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=newlib/libc/include/sys/_default_fcntl.h

see:

https://sourceware.org/git/?p=newlib-cygwin.git;a=commitdiff;h=0aa99373c1d01b19a7f8ba53e8c7749358480f3e

        https://cygwin.com/pipermail/cygwin/2018-February/236018.html

and:

        https://cygwin.com/pipermail/cygwin/2018-February/235869.html

https://sourceware.org/git/?p=newlib-cygwin.git;a=commitdiff;h=7ae73be14194ccadc9a7557be33c3940ca4667cb

You should allow linkat to reverse the applied attributes to make the file
permanent, which may be taken care of in the patch above to
fhandler_disk_file::link;
from Linux open(2) see

        https://man7.org/linux/man-pages/man2/open.2.html:

"O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and,
optionally, O_EXCL. If O_EXCL is not specified, then linkat(2) can be
used to link the temporary file into the filesystem, making it
permanent, using code like the following:

        char path[PATH_MAX];
        fd = open("/path/to/dir", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR);

        /* File I/O on 'fd'... */

        snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd);
        linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
                                        AT_SYMLINK_FOLLOW);
"

although the implementation may not currently handle O_EXCL.

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Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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