II$.$ Reading from the $/$proc File System

Once the process identifier has been stored, any reads from $/$proc$/$pid will return the name of the command, its process identifier, and its state. As illustrated in Section $3\mathrm{.}1,$ the PCB in Linux is represented by the structure task$\_$struct, which is found in the include file. Given a process identifier, the function pid$\_$task$()$ returns the associated task$\_$struct. The signature of this function appears as follows:

struct task$\_$struct pid$\_$task$($struct pid $*$pid$,$

$$enum pid$\_$type type$)$

The kernel function find$\_$vpid$($int pid$)$ can be used to obtain the struct pid, and PIDTYPE$\_$PID can be used as the pid$\_$type.

For a valid pid in the system, pid$\_$task will return its task$\_$struct. You can then display the values of the command, pid, and state. $($You will probably have to read through the task$\_$struct structure in to obtain the names of these fields.$)$

If pid$\_$task$()$ is not passed a valid pid, it returns NULL. Be sure to perform appropriate error checking to check for this condition. If this situation occurs, the kernel module function associated with reading from $/$proc$/$pid should return $0.$

In the source code download, we give the C program pid.c$,$ which provides some of the basic building blocks for beginning this project.