[syslinux] Syslinux fwrite support

[Gene Cumm]

On Sun, May 22, 2016 at 8:31 PM, Tal Lubko <tallubko at yahoo.com> wrote:
>
>
>> -----Original Message-----
>> From: Gene Cumm [mailto:gene.cumm at gmail.com]
>> Sent: Saturday, May 21, 2016 4:44 PM
>> To: Tal Lubko
>> Cc: For discussion of Syslinux and tftp-hpa
>> Subject: Re: [syslinux] Syslinux fwrite support
>>
>> On Fri, May 20, 2016 at 11:14 PM, Tal Lubko <tallubko at yahoo.com> wrote:
>> >
>> >
>> >> -----Original Message-----
>> >> From: Gene Cumm [mailto:gene.cumm at gmail.com]
>> >> Sent: Tuesday, May 17, 2016 1:07 PM
>> >> To: Tal Lubko
>> >> Cc: syslinux at zytor.com
>> >> Subject: Re: [syslinux] Syslinux fwrite support
>> >>
>> >> On Tue, May 17, 2016 at 2:39 AM, Tal Lubko via Syslinux
>> >> <syslinux at zytor.com> wrote:
>> >> > Hi
>> >> > Does Syslinux support fwrite for fat filesystem?Is it supported
>> for
>> >> other filesystem?As far as I understand the function exist but isn't
>> >> fully implemented.Is that correct?
>> >> > I also found this thread that says about the
>> >> > samehttp://www.syslinux.org/archives/2011-August/017093.html
>> >> > Thanks,Tal
>> >>
>> >> At this time, writing should only be supported for the ADV and raw
>> >> sectors unless you're writing to a file over the network from a
>> >> network boot like TFTP upload.  Writing to a filesystem beyond
>> >> strictly overwriting contents of existing files byte for byte
>> >> involves understanding allocation and checksum mechanisms involved.
>> >> To my knowledge, no file system that Syslinux supports stores hashes
>> >> of file contents into the file system metadata.
>> >>
>> >> Traditionally, Syslinux has taken the approach that we can not trust
>> >> the BIOS calls for any writes beyond the ADV.
>> >>
>> >> Tal, what's the goal/use case for using fwrite()?  Is it perhaps
>> >> something that could be solved using the ADV?  For example, the
>> >> --once parameter to syslinux/extlinux changes the default for one
>> boot.
>> >>
>> >> --
>> >> -Gene
>> > Hi
>> >
>> > The goal is some kind of boot log.
>>
>> What about the real OS's logs?  What kind of a log?  timestamps of
>> executing a COM32 and perhaps what command executed?  Reboot count?  A
>> single entry of something?
>>
>> > I understand it is possible by using raw sectors writing.
>> > I couldn't find much documentation about ADV, but my guess that it
>> wouldn’t do the job.
>>
>> I'd say the ADV is likely not appropriate for this.
>>
>> > BTW: Where can I find some explanation about ADV and --once
>> parameter?
>>
>> For --once:
>> http://www.syslinux.org/wiki/index.php?title=Install#options
>>
>> --
>> -Gene
> Hi Gene
>
> Thanks a lot for your help.
> OS's log won't do the job but raw writing sectors is good enough.
>
> I think I understand the --once option but there aren't many details about it (for example where it is stored).
> I also couldn't find any detailed documentation about the ADV option.

The "--once" option is stored in the ADV.  To my knowledge, not much
is written about the ADV outside of the code itself.

-- 
-Gene