Re: Tail -f in lua, truncate aware?

[Coda Highland <chighland@...>]

On Fri, Jun 28, 2013 at 1:27 PM, Egil Hjelmeland
<privat@egil-hjelmeland.no> wrote:
> On 28. juni 2013 21:01, William Ahern wrote:
>>
>> On Fri, Jun 28, 2013 at 02:42:19PM +0200, Bernd Eggink wrote:
>>>
>>> On 28.06.2013 13:47, ms2008vip wrote:
>>>
>>>>            I'd like to make the output of tail -F or something similar
>>>>            available to me in Lua without blocking or blocking.
>>>>        If the file gets truncated or log rotated, the program will
>>>> detect
>>>>        it and will return to the start. This seems to be
>>>>        a level 1 question but looks strange to me. I just can't figure
>>>> it
>>>>        out. Does anyone could share some code? TKS
>>>>
>>>
>>> In Linux you can use poll() or rpoll() from the luaposix library
>>> (https://github.com/luaposix/luaposix)
>>
>> poll doesn't work on regular files. Or rather, it's defined to immediately
>> signal readiness, which makes it useless for this case.
>>
>>         "Regular files shall always poll TRUE for reading and writing."
>>
>>         -- http://www.opengroup.org/susv3xsh/poll.html
>>
>>
>>
> yes, by Bernd mentions output of process 'tail -F'. If he want output from
> an other program into Lua, it will most likely involve a pipe or a fifo, and
> pipes and fifos can be poll()'ed or select()'ed. Use a pipe if you want to
> fork and exec the 'tail -F' directly from Lua, or just make a named
> /path/to/fifo and do 'tail -F > /path/to/fifo'
>
>
>
> Egil
>
>

For the record:

tail -F works by polling the file. Every 1.0 seconds (configurable),
it stat()s the file to see if the size has changed. If it's gotten
larger, it reads from the end and outputs; if it's gotten smaller, it
outputs the entire file.

If it's been updated to use something like inotify, the docs on
gnu.org haven't been updated to match (don't want to go dig out the
source code at the moment), but I've got my suspicions it still just
polls.

/s/ Adam