[PATCH] Cygwin: Have tmpfile(3) use O_TMPFILE
Corinna Vinschen
corinna-cygwin@cygwin.com
Wed Feb 10 10:12:41 GMT 2021
On Feb 10 01:07, Mark Geisert wrote:
> Hi Corinna,
>
> Corinna Vinschen via Cygwin-patches wrote:
> > Hi Mark,
> >
> > On Feb 9 02:50, Mark Geisert wrote:
> > > Per discussion on cygwin-developers, a Cygwin tmpfile(3) implementation
> > > has been added to syscalls.cc. This overrides the one supplied by
> > > newlib. Then the open(2) flag O_TMPFILE was added to the open call that
> > > tmpfile internally makes.
> > > ---
> > > winsup/cygwin/release/3.2.0 | 4 ++++
> > > winsup/cygwin/syscalls.cc | 20 ++++++++++++++++++++
> > > 2 files changed, 24 insertions(+)
> > >
> > > diff --git a/winsup/cygwin/release/3.2.0 b/winsup/cygwin/release/3.2.0
> > > index f748a9bc8..d02d16863 100644
> > > --- a/winsup/cygwin/release/3.2.0
> > > +++ b/winsup/cygwin/release/3.2.0
> > > @@ -19,6 +19,10 @@ What changed:
> > > - A few FAQ updates.
> > > +- Have tmpfile(3) make use of Win32 FILE_ATTRIBUTE_TEMPORARY via open(2)
> > > + flag O_TMPFILE.
> > > + Addresses: https://cygwin.com/pipermail/cygwin/2021-January/247304.html
> > > +
> > > Bug Fixes
> > > ---------
> > > diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
> > > index 52a020f07..b79c1c7cd 100644
> > > --- a/winsup/cygwin/syscalls.cc
> > > +++ b/winsup/cygwin/syscalls.cc
> > > @@ -5225,3 +5225,23 @@ pipe2 (int filedes[2], int mode)
> > > syscall_printf ("%R = pipe2([%d, %d], %y)", res, read, write, mode);
> > > return res;
> > > }
> > > +
> > > +extern "C" FILE *
> > > +tmpfile (void)
> > > +{
> > > + char *dir = getenv ("TMPDIR");
> >
> > This isn't what Linux tmpfile does. Per the man page, it tries to
> > create the file in P_tmpdir first, and if that fails, it tries
> > "/tmp".
>
> Oops, I was following newlib's code here. I'll adjust this.
Oops, I didn't check with newlib, sorry. Hmm, so we're stuck with
either Linux-compat or backward-compat but not both. Decisions,
decisions...
Let's stick to backward-compat then, so no changes here.
> > > + if (!dir)
> > > + dir = P_tmpdir;
> > > + int fd = open (dir, O_RDWR | O_CREAT | O_BINARY | O_TMPFILE,
> >
> > You have to specify O_EXCL here. The idea is that this file cannot be
> > made permanent, and missing the O_EXCL flag allows exactly that. See
> > https://man7.org/linux/man-pages/man2/open.2.html, the lengthy
> > description in terms of O_TMPFILE.
>
> I started out with O_EXCL as you suggested, but found syscalls.cc:1504
> reporting EEXIST. Is there some clash there between fh->exists() and
> O_TMPFILE? Hmm.
You specify O_CREAT as well. O_TMPFILE replaces O_CREAT. The
combination O_CREAT|O_EXCL still checks for existence of the file, in
that case, the dir.
Thanks,
Corinna
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