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Writing c code for who | sort | lp

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# 1  
Old 01-18-2003
Bug Writing c code for who | sort | lp
Hi,

Can any one please let me know the code for implementing
who | sort | lp

Basically I want to implent pipe here. The program can use the existing who, sort and lp


Also please let me know How many processes are created in Unix when the following commands are executed?
$ ls | more
$ ls > more

Thanks in Advance
Yeheya Ansari
# 2  
Old 01-18-2003
system("who|sort|lp"); will do it. But why would you do this? Just type:
who|sort|lp
at the shell prompt.

"ls| more" is two processes. "ls > more" is just one process. But they don't do the same thing. The second one sends the output to a file called "more".
# 3  
Old 01-19-2003
Thanks for the Reply.

What u suggest is just calling the command from C. As written earlier, I want to implenet "PIPE" in C, using pipe command and forking different processes.

Can u please suggest the solution for that.

Also if u know some document on Advanced Unix Programming, pls refer it to me.

Regards,
Yeheya
# 4  
Old 01-19-2003
If you want to do it the hard way, that is your choice I guess. But I'm certainly not going to code that for you. I do hope that you have read our rules about homework. I will point out that that if you use system(), you will indeed be implementing pipes and forking processes.

You seem to know most of the required system calls. The only other one you'll need is exec(). I suggest that you start with just "who | lp". Once you get that working, add in the sort.

I suggest "Advanced Programming in the UNIX Environment" by Richard Stevens. I think it's the best book on the subject that was ever written.
# 5  
Old 01-20-2003
To implement "who|sort|lp" command feature using a C code you might have to work with Standard C Library Functions like :
getutent ( ) / getutline ( ) , setutent ( ) , endutent ( ) , qsort ( ) and some basics of terminal I/O.
See man pages for more information.
# 6  
Old 01-20-2003
Well, I have some time to kill...so
Code:
#include <unistd.h>

main()
{
        int pipe1[2], pipe2[2];
        pipe(pipe1);

        if(fork()) {
                pipe(pipe2);
                if(fork()) {
                        close(0);
                        dup(pipe2[0]);
                        close(pipe1[0]);
                        close(pipe1[1]);
                        close(pipe2[0]);
                        close(pipe2[1]);
                        execl("/usr/bin/sort", "sort", NULL);
                } else {
                        close(0);
                        dup(pipe1[0]);
                        close(1);
                        dup(pipe2[1]);
                        close(pipe1[0]);
                        close(pipe1[1]);
                        close(pipe2[0]);
                        close(pipe2[1]);
                        execl("/usr/bin/sed", "sed", "s/tty/TTY/g", NULL);
                }
        } else {
                close(1);
                dup(pipe1[1]);
                close(pipe1[0]);
                close(pipe1[1]);
                execl("/usr/bin/who", "who", NULL);
        }
        exit(0);
}

The system that I'm on doesn't have lp so I use a sed to get to 3 processes...
# 7  
Old 01-29-2003
I was just wondering that had the requirement been like this :
who | sort | lp 2> log.dat
then how would we modify the code you have written.Is it possible that the purpose can be attended without forking more processes.

How can we do ">" (redirection) of the output to a file using exec utility.
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