Re: [Qemu-devel] [PATCH] suppress 'warn_unused_result' warning

[Paul Brook <paul@codesourcery.com>]

On Monday 11 May 2009, Stuart Brady wrote:
> On Sun, May 10, 2009 at 07:53:35PM -0600, M. Warner Losh wrote:
> > When a signal is received and you are waiting for data, you get
> > EINTR.  If there's data available, then I believe the behavior is to
> > return that data and not EINTR.  That's the way Unix works.
>
> So if I do a read() from a file over NFS, and there's an awful lot of
> latency (and perhaps even connection problems), and the process gets a
> signal -- does that mean that the signal will only be delivered once
> data is returned?

The signal should be delivered immediately. Depending on various factors the 
kernel will either restart the syscall, fail with EINTR or return a partial 
read.

> If not, then I would really start to wonder whether /all/ code dealing
> with read(), write(), etc. should be written to cope with EINTR (and
> also partial reads/writes?) regardless of whatever is done with threads
> and signal masks, as doing otherwise seems only to be asking for trouble
> at some point.  (I'd be especially concerned about signals intended for
> libraries that are not under the developer's control...)

If your application uses signals (directly or indirectly via libraries) then 
all uses of read/write need to be tolerant of EINTR and partial reads.

Paul