[Click] Element::selected()

[Ian Rose]

Eddie Kohler wrote:
> Hi Ian,
> 
> It's this way because even if a file descriptor was selected for both 
> reading and writing, selected() is called only once.

True, but you could just use flags right?  So the function could be 
redefined as Element::selected(int fd, int ops)

and called like
elt->selected(fd, Element::SELECT_READ)
or elt->selected(fd, Element::SELECT_WRITE)
or elt->selected(fd, Element::SELECT_READ | Element::SELECT_WRITE)

> 
> It might be useful to say whether reading or writing was enabled...but 
> of course in between select()'s return and the call of selected(), 
> something might have happened to make the fd selectable in other ways.  

Also true, but if it was really that important to someone, they could 
always do that re-checking themselves.  E.g. if their selected() was 
called for reading, they could also check the fd for writing "just in 
case".  That seems rather inefficient to me, but I guess there could be 
some apps that really want to know ASAP when the fd is ready.

> I don't feel that strongly.

I think that adding a second function something like the following would 
retain existing functionality - do you agree?

(in element.hh)
virtual void selected(int fd, int ops);

(in element.cc)
void
Element::selected(int fd, int ops)
{
     selected(fd);
}


> 
> Eddie
> 
> 
> Ian Rose wrote:
>> Hi list,
>>
>> Is there any rational for why Element::selected() doesn't tell you 
>> what the file descriptor was selected for (reading vs. writing)?  Or 
>> is there a way to get this info that I don't know about?  It seems to 
>> me that if you have a FD that you are both reading and writing to 
>> (like a socket) you are pretty much stuck with something like:
>>
>> void
>> Foo::selected(int fd)
>> {
>>     // perhaps fd was selected for writes...
>>     int rv = send(fd, ...);
>>     if (rv == -1) {
>>         if (errno == EAGAIN) {
>>             // oops - I guess fd was actually selected for reads
>>             int rv = recv(fd, ...);
>>             (etc)
>>         } else {
>>             // this is a "real" send failure
>>         }
>>     }
>>     // send succeeded!
>>     (etc)
>> }
>>
>> Seems kinda ugly and inefficient to me...  Am I missing something?
>>
>> I guess an alternative is to call select() on your fd to figure out 
>> which it was selected for, but that's rather redundant!
>>
>> - Ian
>>
>>
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