How the heck does tail work?

[Joe]

Hi.  I had too much time on my hands the other night and I started
thinking about how to write the program "tail".

It all seemed pretty simple.  Just read through the file saving the last
n lines in a circular buffer and then print them out when done.

But, that has problems.  If the file has *really* long lines, it could
run out of memory or be really nasty for the system.

So, just read down the file with a circular buffer of pointers to lines
and then print from the right pointer when done.  Well, that got rid of
the memory problem and would work fine for a static file.

What I finally got stumped on was standard input.  As far as I know,
once it's read, it's gone - no going back to a pointer and reading again.

I found a version of tail.c on my system and, although it's a bit more
elegant than what I would have written, it's basically the circular
buffer of pointers to lines approach.  (I don't know if this is "the"
tail.c.  It was just in an obscure directory.)

I understand how this works for a static file, but how does it work for
a pipe?

This is really an operating system question rather than an application
programming issue.  It doesn't matter what language the "tail" program
is coded in.

Joe

-- 
We travel together, passengers on a little spaceship, dependent on its 
vulnerable reserves of air and soil; all committed for our safety to its 
security and peace; preserved from annihilation only by the care, the 
work and the love we give our fragile craft. We cannot maintain it half 
fortunate; half miserable; half confident, half despairing; half slave 
of the ancient enemies of man, half free in a liberation of resources 
undreamed of until this day. No craft, no crew can safely travel with 
such vast contradictions. ~ Adlai Stevenson