[Mesa-dev] [PATCH v2 3/5] util: add anon_file.h for all memfd/temp file usage

[Greg V]

On 01/24/2018 15:16, Emil Velikov wrote:
> On 24 January 2018 at 11:58, Greg V <greg at unrelenting.technology> wrote:
>
>>>> +#ifndef __FreeBSD__
>>> mkstemp does exist on FreeBSD, right? If so you can drop this guard
>>> and simplify the one in os_create_anonymous_file().
>> The guard is there because this function (create_tmpfile_cloexec) goes
>> unused on FreeBSD. os_create_anonymous_file uses shm_open, which does not
>> take a filename.
>> I could've used shm_open inside create_tmpfile_cloexec but that would cause
>> compiler warnings about unused arguments…
>>
>> I guess I'll just merge create_tmpfile_cloexec into
>> os_create_anonymous_file, it's currently separated for no good reason.
> I was thinking that FreeBSD can fallback to create_tmpfile_cloexec, if
> shm_open fails.
> Skimming through shm_open and mk{o,}stemp manuals - they are both
> POSIX 2001 standard (kind of, mkostemp is a GNU extension).
> Can't we use one across all platforms?
We don't want shm_open semantics in general though. That wouldn't be an 
anonymous fd. Shared memory actually gets garbage collected when you 
unlink it.

Only FreeBSD extended shm_open to accept the SHM_ANON constant to 
provide anonymous memory file descriptors, i.e. the same thing that 
Linux did with a separate syscall (memfd_create).

I can't imagine a situation when shm_open(SHM_ANON) would fail but 
mkostemp would succeed.
> Unused variables/arguments can be annotated with the UNUSED/MAYBE_UNUSED macros.
> I won't merge the two functions through. As is they're shorter and
> easier to read - plus the compiler can inline things where needed.
More functions — more terrible ifdefs…