Re: [libvirt] [PATCH 05/11] Add a generic internal API for handling any FD based stream
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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