[libvirt] [PATCH 05/11] Add a generic internal API for handling any FD based stream
Daniel P. Berrange
berrange at redhat.com
Wed Nov 3 16:44:03 UTC 2010
On Wed, Nov 03, 2010 at 10:38:33AM -0600, Eric Blake wrote:
> On 11/03/2010 07:23 AM, Daniel P. Berrange wrote:
> >> I was wondering about reusing saferead/write but they have a different
> >> semantic on blocking,
> >
> > Yep, saferead/write cannot be used on any FD with O_NONBLOCK
> > set, because they'll just spin in a 100% CPU loop whenever
> > EAGAIN occurs, until they get nbytes worth of data.
>
> Is it worth teaching saferead/write to use fcntl() to determine if an fd
> is O_NONBLOCK? And if so, should it outright reject an O_NONBLOCK fd
> (to help us diagnose bugs) or be documented as allowing an EAGAIN
> failure on non-blocking fds?
The API design doesn't lend itself to this, because it would result in
a horribly inefficient way of reading data
fcntl(fd)
read(fd, buf, 1024);
fcntl(fd)
read(fd, buf, 1024);
fcntl(fd)
read(fd, buf, 1024);
fcntl(fd)
read(fd, buf, 1024);
fcntl(fd)
read(fd, buf, 1024);
fcntl(fd)
read(fd, buf, 1024);
fcntl(fd)
read(fd, buf, 1024);
...
Daniel
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