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Full Discussion: Creating 3 process and piping a message
Top Forums Programming Creating 3 process and piping a message Post 302330317 by p00ndawg on Tuesday 30th of June 2009 09:05:14 PM
Old 06-30-2009
Quote:
Originally Posted by tetsujin
OK, first off, you've got a total of four processes here, not three... (Is that what you wanted? For the process I call A to spawn off three children? Or did you want three processes total?)

Process A (original process): pid = B, pid1 = C
Process B (first child of A): pid = 0, pid1 = D
Process C (second child of A): pid = B, pid1 = 0
Process D (child of B): pid = 0, pid1 = 0



This if statement will actually run for three of the four processes: A, C, and D:
Process A: (pid | pid1 == 0) = (B | C == 0) = (B | 0) = true
Process B: (pid | pid1 == 0) = (0 | D == 0) = (0 | 0) = false
Process C: (pid | pid1 == 0) = (B | 0 == 0) = (B | true) = true
Process D: (pid | pid1 == 0) = (0 | 0 == 0) = (0 | true) = true
(where true is some nonzero value...)



p[2] is uninitialized. pipe() only creates two file descriptors: p[0], the read end of the pipe, and p[1], the write end of the pipe.

Also, from the problem description it sounded like you wanted each of the three communicating processes to have the ability to write messages to and receive messages from each of the other two communicating processes. This suggests you need to create at least three pipes (three invocations of pipe() in the parent process). I say "at least three pipes" because if these processes are running concurrently, allowing two processes to write to the same file descriptor could have unpredictable results. (Using write() to write a single, whole message will probably be safe - but if the message were large it's possible the write would block waiting for the reader to read some data out - in which case I think it's possible the other process's write() could wind up getting in before the first write() finishes its message. (Not sure, though. write() is a system call so the kernel may synchronize it...)

Remember that pipe() creates two file descriptors but just one pipe. A pipe with an input end and an output end. A file descriptor you get from pipe() is either read-only or write-only - you can never read and write from the same file descriptor, when the file descriptor was obtained with pipe().
yes I actually wanted processes A, B, and C communicating with each other, but I only wanted to pipe a message from a->b->c. I didnt realize I had created 4 processes.
Is there anyway to just create 3 processes? how about a way to just get variable pipes communicating with each other?

Thanks for the awesome breakdown.
Last edited by p00ndawg; 06-30-2009 at 10:29 PM..
 

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LEARN ABOUT DEBIAN

icetdatareplicationgroup

icetDataReplicationGroup(3)					  IceT Reference				       icetDataReplicationGroup(3)

NAME

       icetDataReplicationGroup -- define data replication.

Synopsis
       #include <IceT.h>

       void icetDataReplicationGroup(	IceTInt 	  size,
					const IceTInt *   processes  );

Description
       IceT  has the ability to take advantage of geometric data that is replicated among processes. If a group of processes share the same geome-
       try data, then IceT will split the region of the display that the data projects onto among the processes, thereby reducing the total amount
       of image composition work that needs to be done.

       Each  group  can be declared by calling icetDataReplicationGroup and defining the group of processes that share the geometry with the local
       process. size indicates how many processes belong to the group, and processes is an array of ids of processes that  belong  to  the  group.
       Each process that belongs to a particular group must call icetDataReplicationGroup with the exact same list of processes in the same order.

       You can alternately use icetDataReplicationGroupColor to select data replication groups.

       By default, each process belongs to a group of size one containing just the local processes (i.e. there is no data replication).

Errors
       ICET_INVALID_VALUE
	       processes does not contain the local process rank.

Warnings
       None.

Bugs
       IceT assumes that icetDataReplicationGroup is called with the exact same parameters on all processes belonging to a given group.  Likewise,
       IceT also assumes that all processes have called icetBoundingVertices or icetBoundingBox with the exact same parameters	on  all  processes
       belonging  to  a  given	group. These requirements are not strictly enforced, but failing to do so may cause some of the geometry to not be
       rendered.

Copyright
       Copyright (C)2003 Sandia Corporation

       Under the terms of Contract DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains certain rights in this software.

       This source code is released under the New BSD License.

See Also
       icetDataReplicationGroupColor(3), icetBoundingVertices(3), icetBoundingBox(3)

IceT Reference							September 20, 2010				       icetDataReplicationGroup(3)
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