Re: I/O multiplexing among socket descriptors

["Lakshmi Gopinath" <lakbala@xxxxxxxx>]

Hi,

Thanks for helping me out. I am yet to use poll. But I am still having
problem with select. In the following code both the FD_ISSET macro is
returning 1 for both the calls to select even before data has arrived on
them ! So the recvfrom calls are proceeding sequentially and until data
arrives in the recvfrom call it is blocking.

for(;;)
    {
      FD_SET(sockfd1,&set);
      FD_SET(sockfd2,&set);
      maxfd1 = max(sockfd1,sockfd2) + 1;
      select(maxfd1,&set,&set,NULL,&time);
      printf("\n sokcfd1 status = %d",FD_ISSET(sockfd1,&set);
      printf("\n sokcfd 2 status = %d",FD_ISSET(sockfd2,&set);

      if(status = FD_ISSET(sockfd1,&set))
       {
         bzero(buff1,sizeof(buff1));
        count = recvfrom(sockfd1,(char *)buff1,sizeof(buff1),0,(struct
sockaddr *)&client,&addrlen);
         printf("\n %s",buff1);
       }

         if(status = FD_ISSET(sockfd2,&set))
        {
         bzero(buff2,sizeof(buff2));
        count = recvfrom(sockfd2,(char *)buff2,sizeof(buff2),0, (struct
sockaddr *)&client,&addrlen);
         printf("\n %s ",buff2);
         }
     }
}

So i decided to make the recvfrom call non-blocking by using the flag
MSG_DONTWAIT flag option. and i changed the code for both the sockets as
follows :

count = recvfrom(sockfd2,(char *)buff2,sizeof(buff2),MSG_DONTWAIT, (struct
sockaddr *)&client,&addrlen);
 if(errno != EWOULDBLCOK)
         printf("\n %s ",buff2);


but in this case both the sockets are not accepting data from their
respective buffers. I am still not able to identiful the problem. Could you
please help me out. I am refering Unix Network Programming by Richard
Stevens, edition 2.

Regards,
Lakshmi








----- Original Message -----
From: "Ch Rama Krishna Prasad" <rkp@xxxxxxxxxxxxx>
To: <linux-india-programmers@xxxxxxxxxxxxxxxxxxxxx>; "Lakshmi Gopinath"
<lakbala@xxxxxxxx>; <linux-india-programmers@xxxxxxxxxxxxxxxxxxxxx>
Cc: <ilugc@xxxxxxxxxxxxxxxxxx>
Sent: Monday, February 12, 2001 10:03 AM
Subject: Re: [LIP] I/O multiplexing among socket descriptors


> Hi Lakshmi gopinath,
>    I want to say one thing is that in select also we can set the events
> intersted in so taht when the data comes on the socket it will set the
revents
> .. If the revents is read event then we can read the data and if the event
is
> write event you can send the data, For more see man on poll . if you have
any
> doubts contact me.
>    On more thing poll is better than select so use poll.
>
> From
> Ch.R.k.Prasad
>
> On Sun, 11 Feb 2001, Lakshmi Gopinath wrote:
> > Hi Rama Krishna Prasad,
> >
> > Thanks for helping me out.But I am still facing some problems. fcntl can
be
> > used to set the socket in non-blocking mode. but the purpose of
select/poll
> > is that the process should block at select or poll instead of call to
read
> > or write or the like , till one of the sockets it is tesing becomes
readable
> > or writable or an exception condition occurs.for this why do i have to
set
> > the socket to non-blocking mode. bcoz anyway the process has to block in
the
> > select system call.
> >
> >  my purpose is to wait on a particular socket for a few seconds for data
to
> > arrive and if data does not arrive i want to move on to the next socket.
i
> > realised that linux does not support RCVTIMEO socket option. so i wanted
to
> > use the timer option in select. so i wrote this piece of code :
> > for(;;)
> >    {
> >       FD_SET(sockfd1,&set);
> >       FD_SET(sockfd2,&set);
> >       maxfd1 = max(sockfd1,sockfd2) + 1;
> >       select(maxfd1,&set,&set,NULL,&time);
> >       if(FD_ISSET(sockfd1,&set))
> >        {
> >          count = recvfrom(sockfd1,(char *)buff1,sizeof(buff1),0,
> >          (struct sockaddr *)&client,&addrlen);
> >          printf("\n %s ",buff1);
> >        }
> >    if(FD_ISSET(sockfd2,&set))
> >        {
> >          count = recvfrom(sockfd2,(char *)buff2,sizeof(buff2),0,
> >          (struct sockaddr *)&client,&addrlen);
> >          printf("\n %s ",buff2);
> >         }
> > }
> >
> > but only after data arrives on sockfd1 does it move onto sockfd2. i
cannot
> > understand the reason for this. could you please help me out.
> >
> > Regards,
> > lakshmi
> >
> >
> >
> > ----- Original Message -----
> > From: "Ch Rama Krishna Prasad" <rkp@xxxxxxxxxxxxx>
> > To: <linux-india-programmers@xxxxxxxxxxxxxxxxxxxxx>; "Lakshmi Gopinath"
> > <lakbala@xxxxxxxx>; <linux-india-programmers@xxxxxxxxxxxxxxxxxxxxx>
> > Cc: <ilugc@xxxxxxxxxxxxxxxxxx>
> > Sent: Saturday, February 10, 2001 9:50 AM
> > Subject: Re: [LIP] I/O multiplexing among socket descriptors
> >
> >
> > > Hi GopiNath,
> > >
> > >    you are trying blocking sockets. Then recvfrom,recv,sen,sendto are
> > blocking
> > > system calls. First open the socket in non blocking mode so taht it
will
> > not
> > > block the sockets.
> > >
> > > Example like this
> > > lSockid=socket(AF_INET,SOCK_DGRAM,0);
> > >       /*
> > >      * Get the flags of the socket
> > >      */
> > >     if((flags = fcntl(lSockid, F_GETFL, 0)) < 0)
> > >     {
> > >        perror("CreateSocket():The getting flags is failed");
> > >        return(-1);
> > >     }
> > >
> > >     /*
> > >      * Set the flag to Non-block mode
> > >      */
> > >     if( fcntl( lSockid, F_SETFL, flags | O_NONBLOCK ) < 0 )
> > >     {
> > >        perror("CreateSocket():theSetting Flags Failed");
> > >        return(-1);
> > >     }
> > >
> > > Now the socket will be non blocking mode and use the getsockopt option
to
> > get
> > > the socket options and bu using the select or poll system call you
will
> > proceed.
> > >
> > > From
> > > Ch.R.K.Prasad
> > >
> > >
> > >
> > >
> > > On Fri, 09 Feb 2001, Lakshmi Gopinath wrote:
> > > > Hello Everybody,
> > > >
> > > > I need some help regarding socket programming.I need to do multiplex
> > between
> > > > multiple sockets. For the same purpose I tried 2 approaches.
> > > >
> > > > 1. using select
> > > > 2. using S_RCVTIMEO socket option.
> > > >
> > > > I wrote 2 simple programs - one that uses socket option and the
other
> > that
> > > > uses select call. The program can accept UDP dgrams on 2 UDP ports.i
> > created
> > > > 3 simple programs.
> > > > selrecv.c - uses select to multiplex between the 2 udp ports
> > > >
> > > > sockrecv.c - uses socket options to multiplex between the 2 udp
ports
> > > >
> > > > sender.c - sends udp dgrams. change the port no in the program to
8000
> > > > ,create an executable and again change the program to 6000 and
create
> > > > another executable.and use them 2 send dgrams to the 2 ports.
> > > >
> > > > But both the approaches failed...
> > > > 1. while using select unless dgram is first got from sockfd1 ( the
first
> > > > recvfrom call) it does not go to the next recvfrom call..ie it
blocks at
> > > > recvfrom then at select
> > > >
> > > > 2. while using socket option. i change the socket options using
> > setsockopt .
> > > > but when i verify it with getsockopt it remains unchanged.
> > > >
> > > > I am not able to understand the reason. Could someone plsssss help
me
> > out
> > > >
> > > > Regards,
> > > > Lakshmi
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > ---------------------------------------------
> > > > The mailing list archives are available at
> > > > http://lists.linux-india.org/cgi-bin/wilma/linux-india-programmers
> > >
> >
> >
> > ---------------------------------------------
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>