PCB Task Struct

PCB Task Struct

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What is PCB task struct in Linux?

A PCB or process control block is a data structure in the operating system kernel that contains the information needed to manage a particular process. The PCB is also called task controlling block, task struct, or switchframe[^1^]. In Linux, each process is represented by a task_struct in a doubly-linked list, the head of which is init_task (pid 0, not pid 1) [^2^]. This is commonly known as the process table.

The task_struct contains various fields that store information about the process identification, state, control, and resources. Some of these fields are:

struct thread_info thread_info: a pointer to a structure that holds low-level information about the thread, such as stack pointer, flags, and CPU number.

volatile long int state: the current state of the process, such as running, sleeping, stopped, or zombie.

void * stack: a pointer to the kernel stack of the process.

struct thread_struct thread: a structure that holds architecture-specific information about the thread, such as registers, instruction pointer, and page table.

pid_t pid: the process ID.

struct list_head children: a list of child processes.

struct files_struct * files: a pointer to a structure that holds information about the open files of the process.

struct mm_struct * mm: a pointer to a structure that holds information about the memory management of the process.

struct signal_struct * signal: a pointer to a structure that holds information about the signals received by the process.

struct cred * cred: a pointer to a structure that holds information about the credentials of the process, such as user ID and group ID.

The task_struct can be accessed by various kernel functions and macros. For example, current is a macro that returns a pointer to the task_struct of the currently running process. The for_each_process() macro can be used to iterate over all processes in the system. The sched.h header file contains the definition of task_struct and other related structures and functions.

In user mode, the process table is visible to normal users under /proc. Each process has a directory named after its PID that contains various files that provide information about its attributes and status. For example, /proc/1/cmdline contains the command line arguments of the init process (pid 1), and /proc/1/stat contains various statistics about its CPU usage and scheduling [^2^]. The /proc/self directory is a symbolic link to the directory of the current process.

How to Create a New Process in Linux?

A process is created when a program is executed. There are two main ways to execute a program in Linux: from the command line or from a graphical user interface (GUI). When you execute a program from the command line, you can either run it in the foreground or in the background. A foreground process occupies the terminal until it finishes, while a background process runs without blocking the terminal.

To run a program in the foreground, simply type its name and press Enter. For example, to run the top program, type:

top

To run a program in the background, append an ampersand (&) to the end of the command. For example, to run the shutter program in the background, type:

shutter &

You can also use the nohup command to run a program in the background and prevent it from being terminated if you close the terminal. For example, to run the firefox program in the background and ignore any hangup signals, type:

nohup firefox &

To bring a background process to the foreground, use the fg command with the job number of the process. You can use the jobs command to list the background processes and their job numbers. For example, to bring the shutter process to the foreground, type:

jobs

[1]- Running shutter &

[2]+ Running nohup firefox &

fg 1

To create a new process from a GUI, you can use the menu, launcher, or file manager of your desktop environment. For example, on GNOME, you can click on Activities and search for an application name or browse through categories. You can also right-click on a file and choose Open With to select an application to run it.

How to Terminate a Process in Linux?

Sometimes, you may need to terminate a process that is unresponsive or consuming too many resources. Linux provides several commands to kill a process by sending it a termination signal. The most common termination signals are SIGTERM and SIGKILL. SIGTERM attempts to kill a process gracefully by allowing it to perform cleanup operations before exiting. SIGKILL forces a process to quit immediately without any cleanup.

The most basic command to kill a process is kill, which takes the process ID (PID) as an argument. You can use the ps, pidof, or pgrep commands to find out the PID of a process. For example, to kill the firefox process with SIGTERM, type:

pidof firefox

2099

kill 2099

To kill a process with SIGKILL, you need to specify the signal number as -9 or -SIGKILL. For example, to kill the shutter process with SIGKILL, type:

pidof shutter

2100

kill -9 2100

You can also use the pkill or killall commands to kill a process by name instead of PID. The pkill command matches processes based on patterns, while the killall command matches processes based on exact names. For example, to kill all processes whose names contain "cat", type:

pkill cat

To kill all processes whose names are exactly "cat", type:

killall cat

If you want to kill a graphical application with your mouse cursor, you can use the xkill command. This command will turn your cursor into an X shape and allow you to click on any window to terminate its process. For example, to kill the xeyes 061ffe29dd