Creating 3 process and piping a message Post: 302330539

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Top Forums Programming Creating 3 process and piping a message Post 302330539 by p00ndawg on Wednesday 1st of July 2009 02:30:03 PM

07-01-2009

Registered User

Quote:

Originally Posted by tetsujin

Sure. The reason you wound up with a total of four processes is because each call to fork() returns twice: once in the original process and once in the new one. So after you make the first call to fork(), there are two processes - both of which go on to call the second fork().

To create a total of three processes, make sure you only fork twice. You can do this by checking the return value of fork() to make sure you're in the process which should fork.

Code:

pid_b = fork();
if (pid_b == 0)
{
    /* fork() returned 0, so this is the new process, which I call process B. */
    pid_c = fork();
    if (pid_c == 0)
    {
        /* This is process C... */
    }
}

Now, if your communication channels only need to be such that A can send messages to B, and B can send messages to C, you wind up with something like this:

Code:

pipe(channel_ab);
 pid_b = fork();
if (pid_b > 0)
{
    /* Process A...  Send messages to B using channel_ab[1] */
    close(channel_ab[0]);
    write(channel_ab[1], msg, size);
} else if (pid_b == 0) {
    /* Process B */
    close(channel_ab[1]);

    pipe(channel_bc);
    pid_c = fork();
    if (pid_c > 0)
    {
        /* Still process B.  Receive messages from A using channel_ab[0], send messages to C using channel_bc[1]. */
        close(channel_bc[0]);
        msg_size = read(channel_ab[0], buffer, size);
        write(channel_bc[1], msg, size);
    } else if (pid_c == 0) {
        /* Process C.  Receive messages from B using channel_bc[0]. */
        msg_size = read(channel_bc[0], buffer, size);
    }
}

'Course there's various error checking and stuff you'd want to do in there that's not shown above - but that's the basic idea.

oh ok thanks alot, I think i understand now . I have a project coming up where ill need to use some pipes and I this really fills in some of my blanks.

thanks again.

p00ndawg

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LEARN ABOUT DEBIAN

pvm_advise

ADVISE(3PVM)							  PVM Version 3.4						      ADVISE(3PVM)

NAME

       pvm_advise() - Controls use of direct task-to-task routing.
       [In Version 3.2: Replaced by pvm_setopt]

SYNOPSIS

       C    int info = pvm_advise( int route )

       Fortran	 call pvmfadvise( route, info )

PARAMETERS

       route   Integer advising PVM to set up direct task-to-task links.
									  route options
	       PvmDontRoute	   1	Don't allow direct links to this task
	       PvmAllowDirect	   2	Allow but don't request direct links
	       PvmRouteDirect	   3	Request direct links

       info    Integer returning error status.

DESCRIPTION

       The routine pvm_advise advises PVM on whether or not to set up direct task-to-task links (using TCP) for all subsequent communication. Once
       a link is established it remains until the application finishes.  If a direct link can not be established because one of the two tasks  has
       requested  PvmDontRoute	or  because no resources are available, then the default route through the PVM daemons is used.  pvm_advise can be
       called multiple times to selectively establish direct links, but is typically set only once near the beginning of each  task.   PvmAllowDi-
       rect  is  the default advise setting.  This setting on task A allows other tasks to set up direct links to A.  Once a direct link is estab-
       lished between tasks both tasks will use it for sending messages.  pvm_advise returns the error status in info.	The performance of  direct
       task-to-task  links can be up to a factor of two better than the default route. The draw back is a lack of scalability of the direct links.
       Some versions of UNIX limit the number of links to no more than 30.

EXAMPLES

       C:
			   info = pvm_advise( PvmRouteDirect );

       Fortran:
			   CALL PVMFADVISE( PVMROUTEDIRECT, INFO )

ERRORS

       This error condition can be returned by pvm_advise

       PvmBadParam
	      giving an invalid route value.

SEE ALSO

       pvm_setopt(3PVM)

								  30 August, 1993						      ADVISE(3PVM)

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