Delivery-Date: Tue Oct 28 16:04:59 2008
Message-Id: <200810282316.m9SNG7eX004151@bourbon.usc.edu>
To: cs551@merlot.usc.edu
Subject: Re: reagarding select()
Date: Tue, 28 Oct 2008 16:16:07 -0700
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > So i am receiving in an while(1) loop .So i will have call :
  >  FD_ZERO(&fds);
  >    FD_SET(fd, &fds);
  > before calling every select ?

Please don't ask me if this is right.  You need to understand
what each statement is doing and figure out if it's right
for you.
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>


  On Tue, Oct 28, 2008 at 2:25 PM, Bill Cheng <william@bourbon.usc.edu> wrote:
  > Someone wrote:
  >
  >  > The syntax for select is
  >  > int select( int nfds,fd_set *read,fd_set *write, fd_set *error,Struct
  >  > timeval *timeout))
  >  >
  >  > Now , i have multiple  readind threads calling select().Each select() scans
  >  > socket descriptors from 0 to nfds-1.
  >  > Will a reading thread 's(with sock fd =5) select() unblock if another
  >  > reading thread (with sock  fd = 3) receives data .
  >  >    Beacause the former's select will scan from sock fds 0-4.
  >
  > You need to use FD_SET() to specify which file descriptors
  > are you interested in for a specific select() call.  For
  > example, if your thread is only interested in reading fd=5,
  > then you should do:
  >
  >    fd_set fds;
  >    struct timeval timeout;
  >
  >    FD_ZERO(&fds); /* it's imperative that you initialize all variables */
  >    FD_SET(fd, &fds);
  >    .... /* set timeout */
  >    return_code = select(fd+1, &fds, NULL, NULL, &timeout);
  >
  > Although it will *scan* from file descriptors 0 through 5, it
  > will skip the ones that you did not call FD_SET() on.
  > --
  > Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>