discussion related to implementation of File descriptor functions in RTEMS

[Joel Sherrill]

On Fri, Mar 27, 2020 at 12:12 PM Eshan Dhawan <eshandhawan51 at gmail.com>
wrote:

>
>
> On Thu, Mar 26, 2020 at 11:18 AM Sebastian Huber <
> sebastian.huber at embedded-brains.de> wrote:
>
>> On 25/03/2020 20:33, Joel Sherrill wrote:
>>
>>
>>
>> On Wed, Mar 25, 2020 at 12:17 AM Eshan Dhawan <eshandhawan51 at gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Wed, Mar 25, 2020 at 4:01 AM Joel Sherrill <joel at rtems.org> wrote:
>>>
>>>>
>>>>
>>>> On Tue, Mar 24, 2020 at 4:57 PM Eshan Dhawan <eshandhawan51 at gmail.com>
>>>> wrote:
>>>>
>>>>> Hello everyone,
>>>>> As @Vaibhav Gupta <vaibhav.varodek at gmail.com> suggested I have also
>>>>> added adding file descriptor functions to my GSOC project.
>>>>> I went through the mailing list archives for more information.
>>>>> RTEMS as its own file descriptor so the functions need to be
>>>>> implemented from scratch.
>>>>> I wanted to get more information related to it.
>>>>>
>>>>
>>>> What's the set of functions you are proposing for those not tracking
>>>> your draft proposal?
>>>>
>>> Link:
>>> https://docs.google.com/document/d/1n-JOFUbFn6V1kViAGWsEGbVHL9MxlMyKP0BbZhEA1Rs/edit?usp=sharing
>>> I haven't searched about the functions in the list yet. The list was
>>> made by Vaibhav, last year and he told me that it could be added to
>>> proposal this year as well.
>>> I read the archives that these need to be written from scratch.
>>>
>>
>>
>> Maybe not. I found at least this implementation of renameat() which was
>> implemented on top of existing calls:
>>
>>
>> https://github.com/lattera/freebsd/blob/master/contrib/openbsm/bin/auditdistd/renameat.h
>>
>> It should be in a C file but that shows it can be done. That directory
>> has a lot of these methods.
>>
>> Adding the *at() functions with an RTEMS-specific implementation would be
>> nice (and not difficult). The generic renameat() implementation for example
>> changes the current directory. One of the goals of this API is to avoid
>> exactly this. In glibc/Linux for example a system call is used:
>>
>>
>> https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/renameat.c;h=901d61f37e10d0c2df245c01bb2ef980d00e8f52;hb=HEAD
>>
>> https://github.com/torvalds/linux/blob/master/fs/namei.c#L4590
>>
>
I'm not disagreeing that a generic implementation is undesirable long-term
but these methods have not been present for 30 years and no one has
complained about their absence. I would propose generic implementations be
added and a ticket filed to add RTEMS specific implementations if someone
complains about the performance.

I see adding them now as a higher effort task that brings little value.
These are for standards compliance and not performance critical.

Unless as Eshan says, you have a simple pattern in your head to add them as
system calls. But that still requires a fair amount of implementation
versus just porting these.

>
>> Can you tell where to find details related to these function as well as
> how to implement them?
> :)
>
> thanks
> -Eshan
>
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