Re: [PATCH] Cygwin: Have tmpfile(3) use O_TMPFILE

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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".

> +  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.


Thanks,
Corinna