[ale] Select()

[Bob Toxen]

If your open file descriptors are:

  0,1,2,7

nfds is max(0,1,2,7)+1 or 7+1 or 8.

The select() code then does (8-1) to get 7 and that's the bitmask it
uses to watch for events.

It's an efficiency thing to tell select() to test a change of status on
the first 7 file descriptors and then see if any of those are in your
three sets to watch.

On Mon, Aug 16, 2010 at 03:23:31PM -0400, Calvin Harrigan wrote:
> I'm taking a long overdue plunge back into linux C programming.  While 
> reading the man page for select(), I came across a line that has me a 
> bit confused.
> 
> http://linux.die.net/man/2/select
> 
> "nfds is the highest-numbered file descriptor in any of the three sets, 
> plus 1."
> 
> What exactly are they referring to? What is highest-numbered file 
> descriptor?  Are they referring to integer value stored in the file 
> descriptor? Perhaps it's the number of file descriptors added to the 
> set?  It's not very clear to me.  Any insight would be greatly appreciated.
> 
> Thanks


You're welcome.

Bob Toxen
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