[libvirt] PATCH: Allow specific FDs to be kept open in virExec()

[Jim Meyering]

"Daniel P. Berrange" <berrange at redhat.com> wrote:
> On Fri, Aug 22, 2008 at 09:22:06AM +0100, Richard W.M. Jones wrote:
>> On Thu, Aug 21, 2008 at 02:58:30PM +0100, Daniel P. Berrange wrote:
>> > +#define VIR_EXEC_FDSET_SIZE() (sysconf(_SC_OPEN_MAX)/8)
>> > +#define VIR_EXEC_FDSET_CLEAR(set) memset((set), 0, VIR_EXEC_FDSET_SIZE())
>> > +#define VIR_EXEC_FDSET_ON(set, fd) (set[(fd)/8] |= (1 << ((fd)%8)))
>> > +#define VIR_EXEC_FDSET_OFF(set, fd) (set[(fd)/8] &= ~(1 << ((fd)%8)))
>> > +#define VIR_EXEC_FDSET_ISON(set, fd) (set[(fd)/8] & (1 << ((fd)%8)))
>>
>> +1 although I'm interested to know why you didn't just use ordinary
>> FD_SETs for this.
>
> For some reason I have it in my head that fd_set was limited to 32
> but looking at the Linux source its clearly 1024, which is the sme
> as my _SC_OPEN_MAX.   The manpage says its FD_SETSIZE, but doesn't
> clarify whether FD_SETSIZE is guarenteed to be large enough to store
> any FD upto _SC_OPEN_MAX.
>
> Jim, any thoughts on this ?  I'll happily switch to fd_set if we expect
> it'll be suitable

AFAIK, FD_SET must work for any valid file descriptor.

The fd_set bit-manipulation function/macros like FD_SET, FD_CLR
are specified in http://www.opengroup.org/susv3xsh/select.html