[SOLVED] socket IPC..

vendtagain · 12-24-2009, 01:01 AM

I am trying to get some IPC working within a program. Basically I need iostream to print text every so many seconds, and at the same time be able to accept input from the keyboard (which would execute a function).

Currently I have a select() function which executes some code when it times out. It is set within a loop, so it executes every few seconds, which works well.
I (thought I had) set it up to execute a different piece of code if iostream input is received, but this part never gets executed.

I am vaguely familiar with IPC methods, would a different IPC method be more(or evenly) practical for this situation? A pipe maybe?, or message queues?

Do I need to use fork() to have one process that controls the timeout function, and another which checks for input?

Some pseudocode and keywords would be appreciated.

ta0kira · 12-25-2009, 12:00 PM

Please reduce your code as much as possible (while still reproducing the problem) and post it.
Kevin Barry

Sergei Steshenko · 12-25-2009, 12:26 PM

Quote:

Originally Posted by vendtagain

I am trying to get some IPC working within a program. Basically I need iostream to print text every so many seconds, and at the same time be able to accept input from the keyboard (which would execute a function).

Currently I have a select() function which executes some code when it times out. It is set within a loop, so it executes every few seconds, which works well.
I (thought I had) set it up to execute a different piece of code if iostream input is received, but this part never gets executed.

I am vaguely familiar with IPC methods, would a different IPC method be more(or evenly) practical for this situation? A pipe maybe?, or message queues?

Do I need to use fork() to have one process that controls the timeout function, and another which checks for input?

Some pseudocode and keywords would be appreciated.

IIRC there are socket related functions which tell how much data is in the socket and do not block. So, one doesn't even need a separate thread/fork, i.e. one can more or less implement the following concept:

Code:

unsigned desired_number_of_items = ...; // put real value here
some_type time_to_sleep_between_check = ...; // put real value here
for(;;)
  {
  if(number_of_items_in_socket(socket) < desired_number_of_items)
    {
    sleep(time_to_sleep_between_checks);
    }
  else
    {
    read_data_from_socket(socket);
    break;
    }
  }

You may easily add timeout to the above pseudocode.

paulsm4 · 12-25-2009, 02:41 PM

vendtagain -

Quote:

Currently I have a select() function which executes some code when it times out. It is set within a loop, so it executes every few seconds

You're polling.

Polling is Bad.

You're already using "select()". "Select()" does multiplexing.

Multiplexing is Good.

Give it a try.

IMHO .. PSM

vendtagain · 12-25-2009, 07:06 PM

Code:

   fd_set fdset;
   struct timeval timeout;
struct timeval timer1,timer2;
   int  rc;
   char  val;

   timeout.tv_sec = 0;   // wait for 1 seconds for data
   timeout.tv_usec = 500000;
   FD_ZERO(&fdset);
   FD_SET(0, &fdset);
bool done = 0;
tmap.cursorMap();

 while(done == 0)
{
gettimeofday(&timer1,NULL);

   rc = select(1, &fdset, NULL, NULL, &timeout);
   if (rc == -1)  // select failed
   {
       std::cout<<"error: select failed with -1\n";
	done = 1;
   }
   else if (rc == 0)  //select timed out
   {
      std::cout<<"timeout\n";
	syncmove();
	tmap.cursorMap();
	timeout.tv_sec = 0;
	timeout.tv_usec = 500000;
   }
   else   //character is entered
   {
      if (FD_ISSET(0, &fdset)) 
      {
        val = getchar();
	std::cout<<"val == "<<val;
           gettimeofday(&timer2,NULL);
           timeout.tv_sec = timeout.tv_sec - (timer2.tv_sec - timer1.tv_sec);
           timeout.tv_usec = timeout.tv_usec - (timer2.tv_usec - timer1.tv_usec);
      }
   }

}

Sergei Steshenko · 12-25-2009, 07:23 PM

Quote:

Originally Posted by vendtagain

Code:

   fd_set fdset;
   struct timeval timeout;
struct timeval timer1,timer2;
   int  rc;
   char  val;

   timeout.tv_sec = 0;   // wait for 1 seconds for data
   timeout.tv_usec = 500000;
   FD_ZERO(&fdset);
   FD_SET(0, &fdset);
bool done = 0;
tmap.cursorMap();

 while(done == 0)
{
gettimeofday(&timer1,NULL);

   rc = select(1, &fdset, NULL, NULL, &timeout);
   if (rc == -1)  // select failed
   {
       std::cout<<"error: select failed with -1\n";
	done = 1;
   }
   else if (rc == 0)  //select timed out
   {
      std::cout<<"timeout\n";
	syncmove();
	tmap.cursorMap();
	timeout.tv_sec = 0;
	timeout.tv_usec = 500000;
   }
   else   //character is entered
   {
      if (FD_ISSET(0, &fdset)) 
      {
        val = getchar();
	std::cout<<"val == "<<val;
           gettimeofday(&timer2,NULL);
           timeout.tv_sec = timeout.tv_sec - (timer2.tv_sec - timer1.tv_sec);
           timeout.tv_usec = timeout.tv_usec - (timer2.tv_usec - timer1.tv_usec);
      }
   }

}

Why do you set

timeout.tv_sec
timeout.tv_usec

in three places ?

Where the values of

Code:

timeout.tv_sec = timeout.tv_sec - (timer2.tv_sec - timer1.tv_sec);
           timeout.tv_usec = timeout.tv_usec - (timer2.tv_usec - timer1.tv_usec);

are used ?

If they are meant to be used as the new timeout, why the timeout should change ?

Why is 'rc' defined outside the 'while' body ? Is it needed outside the 'while' body ?

vendtagain · 12-25-2009, 07:38 PM

Quote:

Why do you set

timeout.tv_sec
timeout.tv_usec

in three places ?

If a key is pressed, an action occurs and the loop begins again. I would like to keep the structure in timeout section to occur every .5 secs, so if a key is pressed I dont want the timer to start over at .5, but begin where it left off when the key is pressed. I assumed this would be the best way to do this.
Do you mean the timeout section of the nested if structure would occur every .5 seconds, if a key was pressed or not?

Quote:

Why is 'rc' defined outside the 'while' body ? Is it needed outside the 'while' body ?

I can't give you a good answer here, I'm am still trying to figure this stuff out.

Sergei Steshenko · 12-25-2009, 08:04 PM

Quote:

Originally Posted by vendtagain

If a key is pressed, an action occurs and the loop begins again. I would like to keep the structure in timeout section to occur every .5 secs, so if a key is pressed I dont want the timer to start over at .5, but begin where it left off when the key is pressed. I assumed this would be the best way to do this.
Do you mean the timeout section of the nested if structure would occur every .5 seconds, if a key was pressed or not?

I can't give you a good answer here, I'm am still trying to figure this stuff out.

Suppose you initialized the timeout value to be 0.5 seconds and a key was pressed 0.25 seconds after the initialization.

If I understand you correctly, you want to wait for the next key to be pressed again at most 0.5 seconds. If it's the case, which part of your code will set timeout to become 0.5 seconds again ?

Regarding 'rc' and stuff like that - the rule of thumb is: one creates an object (in human, not necessarily OO) sense as late as possible and disposes of it as early as possible. Believe me, it really helps to keep mess away regardless of programming language.

vendtagain · 12-26-2009, 01:31 PM

I think I need to find some guides and docs relating to sockets, IPC or (multiplexing?). to read through. I haven't been able to find any good ones yet.

Any suggestions?

paulsm4 · 12-26-2009, 05:16 PM

Hi -

Beej's guide is a *superb* resource:

http://beej.us/guide/bgnet/

He also wrote one on IPC:

http://beej.us/guide/bgipc/

The single best book I've ever read on sockets programming (IMHO) is Stevens:

http://www.amazon.com/dp/0131411551/

He also wrote a brilliant book on Unix programming in general (including IPCs):

http://www.amazon.com/dp/0321525949/

'Hope that helps .. PSM

ta0kira · 12-26-2009, 07:41 PM

Quote:

Originally Posted by vendtagain

If a key is pressed, an action occurs and the loop begins again. I would like to keep the structure in timeout section to occur every .5 secs, so if a key is pressed I dont want the timer to start over at .5, but begin where it left off when the key is pressed. I assumed this would be the best way to do this.

Maybe you should use setitimer to have it executed every .5s, then use select with a timeout of 0s when the timer is triggered. To do that you might start a thread blocked with a pthread condition that the timer releases, that way you don't interrupt a read in progress.
Kevin Barry

edit: Take a look at this for timing.
https://developer.berlios.de/snippet...ppet&id=100040

vendtagain · 01-01-2010, 07:23 PM

Quote:

Beej's guide is a *superb* resource:

http://beej.us/guide/bgnet/

I agree this is a superb guide.
This is actually the one I've been using, I'm on my 3rd go-around.

I still can't get my head around a client that would be ready to receive info from a server, and also send info if something is inputted.

code below is a server I've developed based on beej's guide which should send info to all clients when it is received from any client.

Now I still can't figure how to implement a client that would wait for info being sent from the server, but would also be able to send info inputted via local cin input.

*main server loop...

Code:

int sockfd, new_fd;
fd_set master,read_fds,write_fds;
int fdmax;
struct sockaddr_storage remoteaddr;
socklen_t addrlen;

sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol);
fdmax = sockfd;  //max so far
FD_SET(sockfd, &master);

for(;;)
{
write_fds = read_fds = master;

select(fdmax+1, &read_fds, &write_fds, NULL, &time_out);

   for(i = 0; i <= fdmax; i++) {
   {
      if (FD_ISSET(i, &read_fds))
      {

        if (i == sockfd)// handle new connections
	{  addrlen = sizeof remoteaddr;
         newfd = accept(sockfd,(struct sockaddr *)&remoteaddr,&addrlen);
         FD_SET(newfd, &master); //add new connection to master set
          fdmax = newfd;
	}
        else
        {
         //client on socket i is "send()"ing info
          nbytes = recv(i,bufdata,sizeof bufdata,0);
            for(jj=0;jj<=fdmax;jj++)
            {
                if (jj!=sockfd && jj!=i)
                {  //send data to all but listener and sender
                     send(jj,bufdata,nbytes,0);
                }
            }
        }

      }

      if (FD_ISSET(i, &write_fds))
      {

      }

   }
}

I'm not even sure how to look at getting the client set up (to wait for keyboard input and server socket recv()), or what to look into, so thought maybe posting the server would help someone give me some insight.

vendtagain · 01-02-2010, 12:35 AM

if (cin >> jj_input)
send(...);
else
recv(...);
?
?Seems like a fairly simple logic...just need cin and recv to be ready for action, simultaneously...without blocking each other

can cin, getline(), or keyboard inputstream be a sort of socket descriptor?

vendtagain · 01-03-2010, 04:15 PM

I didnt quite realize how threading worked, but from a few tests with pthreads, it seems I should be able to use them in this case effectively. Thanks for the input.