Linux Job Six--description of the process and creation of the process

Description of the process and creation of the process

One, Process descriptor task_struct

To manage the process, the kernel must have a clear description of each process, and the process descriptor provides the process information that the kernel needs to understand.

Code key points:

1.Struct list_head task process linked list, bidirectional loop link table connection.

2.Struct mm_struct *mm,*active_mm process address space, memory management

3. each process has its own independent 4G process address space.

4.Struct thread Struct Thread current task-related CPU code

5.Struct fs_struct *fs file system data structure

6.Struct files_struct *files Open file descriptor list

7.struct signal_stract *signal Signal Processing

Ii. creation of the process

A process is the ancestor of all user-state processes, and the second process is the ancestor of all kernel threads.

Fork,vfork , and clone Three system calls can create a new process, all by calling do_fork To implement the process creation;

1.fork system call, the user state is used to create a child process. (Part of the code below)

if (PID < 0) error handling

10 {

One/* error occurred */

fprintf (stderr, "Fork failed!");

Exit (-1);

14}

All else if (PID = = 0)

16 {

+/ * Child process */subprocess pid=0 if and else will execute the C7>fork system calls are returned once in the parent and child processes

printf ("This is a child process!\n");

19}

Else

21 {

*/* Parent Process */

printf ("This is the Parent process!\n");

*/* Parent'll wait for the child to complete*/

Wait (NULL);

-printf ("Child complete!\n");

27}

2. Understanding of the Code

The function performs for else and else if , that is , it becomes two processes after the fork , and the PID in the child process The return value is 0 , in the parent process PID The return value of the child process is the ID , Fork system calls are returned once in the child process and in the parent process.

Creating a process is accomplished by replicating the current process. It is broadly divided into three steps:

1. Duplicating the PCB(copyprocess)

2. Modify the PCB

3. Allocating a new kernel stack (partial data copies the parent process to return)

*childregs = *current_pt_regs (); Copy the kernel stack

Childregs->ax = 0; Why the fork of the subprocess returns 0, here is the reason!

P->THREAD.SP = (unsigned long) childregs; top of the kernel stack when dispatched to a child process

P->thread.ip = (unsigned long) ret_from_fork; the address of the first instruction when dispatched to a child process

3. Where does the child process start?

Childregs The kernel stack dispatched to the child process

Ret_from_fork The first instruction address dispatched to the child process

↓

Syscall_exit

↓

Restore_all normal Return to user state

Third, the experiment

Start tracking debugging after updating to the latest version:

Note here: Executing a fork, you will find that only a fork command description is output, and it is not executed because it is parked in the sys_clone position.

20135123 Qin

Linux Job Six--description of the process and creation of the process