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Full Discussion: Controlling child processes
Top Forums Programming Controlling child processes Post 49307 by Driver on Tuesday 30th of March 2004 04:29:38 PM
Old 03-30-2004
You cannot share a single pipe among all children and the parent in this case, because it's impossible to direct the data to a particular child, so the reader of next message will be picked more or less at random out of the bunch of children blocked on the pipe.
You would have to have one pipe per child.

Each child can block on its pipe and wait for a message by the parent. The problem here is that a pipe is generally uni-directional, i.e. both descriptors for I/O permit only input or output, but not both. Some pipe implementations do support bi-directional transfers, but this feature is not specified by the various POSIX and UNIX standards and should not be relied on.

It follows that you will need an additional means to notify the parent of the completition of command interpretation and execution, such as an additional pipe, or a semaphore, or a signal handler, or a message queue, or ...

Last edited by Driver; 03-30-2004 at 05:35 PM..
 

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PIPE(2) 							System Calls Manual							   PIPE(2)

NAME
pipe - create an interprocess channel SYNOPSIS
pipe(fildes) int fildes[2]; DESCRIPTION
The pipe system call creates an I/O mechanism called a pipe. The file descriptors returned can be used in read and write operations. When the pipe is written using the descriptor fildes[1] up to 4096 bytes of data are buffered before the writing process is suspended. A read using the descriptor fildes[0] will pick up the data. Writes with a count of 4096 bytes or less are atomic; no other process can inter- sperse data. It is assumed that after the pipe has been set up, two (or more) cooperating processes (created by subsequent fork calls) will pass data through the pipe with read and write calls. The Shell has a syntax to set up a linear array of processes connected by pipes. Read calls on an empty pipe (no buffered data) with only one end (all write file descriptors closed) returns an end-of-file. SEE ALSO
sh(1), read(2), write(2), fork(2) DIAGNOSTICS
The function value zero is returned if the pipe was created; -1 if too many files are already open. A signal is generated if a write on a pipe with only one end is attempted. BUGS
Should more than 4096 bytes be necessary in any pipe among a loop of processes, deadlock will occur. ASSEMBLER
(pipe = 42.) sys pipe (read file descriptor in r0) (write file descriptor in r1) PIPE(2)
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