Srinivas B T

Process Management

 

ü     Program -A program is an executable file. It is created by a link editor and resides on a disk file. The program is executed by exec system call.

ü     Process –A process is an instance of a program. The new process is created by issuing fork system call.

ü     Each process has own area of memory, protected against other process

ü     Internal view -running as one task and has access to all resources.

 

 

 

A part of task runs in user mode (less privileged)

 

The concepts of tasks and process are same

 

A task running in privileged mode, can take one of the number of states: running, interrupt routine, system call, waiting, return from system calls, ready.

 

 

Process Control Block

 

ü     Description of a process in process structure

ü     All processes are entered in a doubly linked list and has family relationship

ü     Each process has its own process ID (pid)

ü     Files

ü     Timers

ü     Inter process communication like System V

 

 

 

task_struct structure

 

ü     Process ID: process id and parent pid

ü     Scheduling: priority, CPU utilization, time spent sleeping

ü     Process state: run state of the process, status flags, wait info if

 

Sleeping:

 

ü     Signal state: signals pending delivery, current mask, and actions

ü     Tracing information

ü     Machine-dependent state

ü     Real-time timer and CPU utilization counters

 

task_struct structure…

 

ü     Process group ID: process group and session for this process

ü     User credentials: real, effective, and saved UID and GID

ü     Memory Management: virtual address space management (page tables)

ü     File descriptors: pointers to open file entries and cwd

ü     Resource Accounting: system/user time used, memory allocation, paging and disk I/O, etc.

ü     Statistics (kept in user structure): accounting information.

ü     Signal actions (kept in user structure): pointers to signal handlers, list of blocked signals, etc

 

Process Identifiers

 

Every process has a unique process ID, a non-negative integer

 

Process Ids are reused. As processes terminate, their Ids become candidate for reuse. Most Unix systems has algorithms to delay reuse.

 

Process ID 0 is usually the scheduler process, known as swapper.

 

Process ID 1 is the init process, invoked by the kernel at the end of the bootstrap procedure.

 

#include <unistd.h>

pid_t getpid (void);

returns process ID of the calling process

pid_t getppid (void);

returns parent process ID of the calling process

 

Process Control

 

The system calls for process management (create, terminate and priority) are:

fork(), exec(), exit(), wait(), and nice()

 

fork System call:

 

The only way in which a new process is created by Linux is for an existing process to execute the fork system call.

 

intfork();

 

The fork system call creates a copy of a process that was executing. The process that executed the fork is called the parent process and the new process is called the child process. The fork system call is called once (by the parent process) but it returns twice (once in the parent and once in the child). The only difference in the returns from the fork system call is that the return value in the parent process is the process id of the newly created child process, while the return value in the child process is zero. If fork it is not successful, -1 is returned.

 

exec System call

 

The only way in which a program is executed by Linux is for an existing process to issue the exec system call. The exec system call replaces the current process with the new program. The process does not change. There are six different versions of the exec function:

int execlp(char *filename, char *arg0, char arg1,..... , char *argn, (char *) 0);

int execl(char *pathname, char *arg0, char arg1,..... , char *argn, (char *) 0);

int execle(char *pathname, char *arg0, char arg1,..... , char *argn, (char *) 0, char **envp);

int execvp(char *filename, char **argv);

int execv(char *pathname, char **argv,);

int execve(char *pathname, char **argv, char **envp);

 

The exec functions return to the caller only if an error occurs. Otherwise control passes to the start of the new program.

 

wait System call

 

A process can wait for one of its child process to finish by executing the wait system call.

int wait(int*status);

 

The value returned by wait is the process id of the child process that terminated. If the process that calls wait function does not have any child processes, wait returns a value of -1 immediately. If the process that calls wait has one or more child processes that has not yet terminated, then the calling process is suspended by the kernel until one of its child processes terminates.

 

Exit functions

 

A process can terminate normally in five ways:

 

1.     Executing a return from the main function

2.     Calling the exit function

3.     Calling the _exit or _Exit function

4.     Executing a return from the start routine of last thread in the process

5.     Calling the pthread_exitfunction from the last thread in the process