Question

Round 2: pipeline.c

This program takes the same input as sequence.c, but executes the commands as a pipeline, where the output of each command is piped to the input of the next command in line. The input of the first command, and the output of the final command, should not be altered. For example, if the file "cmdpipe" contains the lines

ls -s1

sort -n

tail -n 5

then running

./pipeline < cmdpipe

should output the 5 largest files in the current directory, in order of size.

Suggested approach: set it up so that the parent process forks a child to execute each command in turn, and keeps track of the pipes that the current child should read from and write to. The child redirects its input and output to these pipes, and then executes the current command. Here is a pseudo-code version of the setup:

// For each command except the final one:// new_pipe = the output of the current command// For each command except the first one:// prev_pipe = the output of the previous command, input of current commandfor cmd in cmds:
    if this is not the final command:
        pipe (new_pipe)    // Create a new pipe

    fork and handle errors    if (child)
        if this is not the first command:
            Redirect input to prev_pipe        if this is not the final command:
            Redirect output to new_pipe
        execute command    else // parent
        if this is not the first command:
            close prev_pipe        if this is not the final command:
            prev_pipe = new_pipe

close any remaining pipes, clean up

Remember that once you call fork, both parent and child have their own copy of all variables, including pipes! Have a look at the pattern for setting up processes to read and write from pipes, from lectures.

Make sure you close all pipes when they are no longer needed as not doing so can cause behaviour that is hard to debug.

Note your code should work with any command. For example:

cal

cat pipeline.c

grep .

wc -l

should print out the number of non-blank lines in your source file (the first line has no effect - why?).