Re: [PATCH] Properly handle short writes in sigint handlers

[Tomi Ollila <tomi.ollila@iki.fi>]

On Fri, 23 Dec 2011 08:10:33 +0000, David Edmondson <dme@dme.org> wrote:
> Sorry for being slow.
> 
> Can you describe the situation in which you expect a write to stderr to
> be a short write? (Without error.)

In the following hypothetical case (correct me if I'm wrong :):

* There is 4096 byte buffer in tty driver.
* Stderr is in blocking-mode (the usual case).
* There is already 4090 bytes in that buffer that has not been read.
* One attemtps to write "Stopping...         \n" there (blocks).
* Somehow the system call is interrupted (and SA_RESTART not set)
  -- write() should return 6 bytes written.

But, if the buffer is full already, does the write() system call return
with -1 and EINTR set ?

If there is enough space for all data in that buffer to begin with, 
write() should be atomic.

> In that situation, what guarantee is there that the loop you've written
> will terminate?

If write() keeps returning 0 then it will not terminate (I guess this never
happens). Also, it never terminates if write blocks indefinitely 
(with or without that loop).

> We're not talking about safeguarding a users' data here - this is a
> short message to indicate that a tool is terminating due to a signal.
> I'm concerned that the solution is worse than the problem.

I'm also in favor of "opportunistic" write *in this particular case*

In case that write fails there is most probably more serious things going
on (all resources eaten, hardware problem, etc) and trying to push these
writes forward doesn't help.

Tomi