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

[Daniel P. Berrange]

On Mon, May 11, 2009 at 11:16:24AM -0500, Anthony Liguori wrote:
> 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?
> >
> >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...)
> >  
> 
> Any system call can return EINTR just about.  It's not just read/write.

For many system calls you can have them auto-restarted after EINTR
by using sigaction() with the SA_RESTART flag. IIRC read/write/poll 
won't support restarts in this way though, because of the problem of
partial data read/writes and partial timeouts for poll meaning you
can't auto-restart them without app developer help

Daniel
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