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Homework and Emergencies Homework & Coursework Questions Cannot correctly connect multi-stage C command pipe (among others) (FYI: a lot of code) Post 302787959 by Corona688 on Sunday 31st of March 2013 02:15:19 PM
Old 03-31-2013
Code:
      dup2(prev[0],0);
      dup2(prev[1],1);

You are attaching the reading end of the pipe to standard input. This makes sense.

You are attaching the writing end of the pipe to standard output. This makes sense.

However, you are doing both to the same pipe. This does not make sense. To picture the loop you've made:

Code:
 /-->grep---\
 \----------/

Where what you actually want is:

Code:
command1 --> command2 --> command3

command1 writes into the write end of pipe A. command2 reads from pipe A, writes to pipe B. command3 reads from pipe B, writes to the terminal.

Last edited by Corona688; 03-31-2013 at 03:26 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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