Question

In Unix, the first process is called init. All the others are descendants of “init”. The init process spawns a sshd process that detects a new secure ssh requested connection (WKPort 22). Upon a new connection, sshd spawns a login process that then loads a shell on it when a user successfully logs into the system. Now, assume that the user types

who | grep <uwnetid> | wc –l

Draw a process tree from init to those three commands. Add fork, exec, wait, and pipe system calls between any two processes affecting each other.

Answer 1

Please find the explanation for process tree and how its linked to init process. Also i have given sample program which will create the process under init.

Process Tree:

It is a schematic diagram which shows the process as tree and we can easily understand how process are interrelated. We can also identify how the parent/child processes are related.

Please let me know if you require more info.

Answer:

Following is the output from pstree command when we execute "who|grep <uwnetid>|wc -l"

init

├─sshd───sshd //This represents sshd daemon is running under init which will handle ssh connections
│─sshd──sshd───sshd───bash─┬─grep // shows ssh connection established and the cmd is ran from bash
│ ├─wc
│ └─who

Process tree diagram:

Following is the process tree diagram when we use fork, pipe, wait system calls.

├─sshd───sshd
├─sshd─┬─sshd───sshd───bash───forkpipie.exe───forkpipie.exepstree

We have got the above process tree from below sample c program:

#include <stdio.h>
#include <unistd.h>
#define MSGSIZE 16

char* msg1 = "fork calls";
char* msg2 = "exec calls";
char* msg3 = "pipe calls";

int main()
{
char inbuf[MSGSIZE];
int p[2], pid, nbytes;

if (pipe(p) < 0)
exit(1);

/* continued */
if ((pid = fork()) > 0) {
write(p[1], msg1, MSGSIZE);
write(p[1], msg2, MSGSIZE);
write(p[1], msg3, MSGSIZE);

/* close(p[1]); */
wait(NULL);
}

else {

/* close(p[1]); */
while ((nbytes = read(p[0], inbuf, MSGSIZE)) > 0)
printf("% s\n", inbuf);
if (nbytes != 0)
exit(2);
printf("Finished reading\n");
}
return 0;
}

Output: You can run this program and this hangs after the execution. Then you can execute pstree to identify how process tree looks like.

Process tree diagram for Fork, wait, pipe: