1. Lecture 4 Process, Thread and Task
September 20, 2016
Kyu Ho Park

2. Major Topics of Lect. 4 How to create a process.
How to create a thread.
What is the task?

3. creating a process

4. creating a process

5. Process Concept An operating system executes a variety of programs:
Batch system – jobs
Time-shared systems – user programs or tasks
Process – a program in execution; process execution must progress in sequential fashion
A process includes:
program counter
stack
data section
heap
text
In a batch system, a single job is executed at a time.
To get a high utilization of CPU, the multiprogramming system had appeared. Also give fair opportunity to all processes, time sharing system was made.
In MS window which is a single user system, a word processor, a web browser, e_mail process are running. At the same time, memory management process is running in the OS.

6. Process in Memory Stack SP Heap Data Text PC Program: text part
Data: global variables
Heap: Dynamic data area such as allocated during program execution of ‘new’ and ‘malloc’.
Stack: temporary data such as function parameters, return address, local variables.
Process

7. Memory Map of a process

8. Memory Map of a process
4GB
3GB
Stack
heap
data
text

9. Virtualization Processes are provided with 2 virtualizations:
Virtualized Processor
Virtualized Memory

10. Process State As a process executes, it changes state
new: The process is being created
running: Instructions are being executed
waiting: The process is waiting for some event to occur
ready: The process is waiting to be assigned to a process
terminated: The process has finished execution
Now, we defined what the process is.
We talked about the running process. Are all processes running?
There are several states in a process.
When you compile your program and get an executable file. When a program becomes a process when an executable file is loaded into memory.
By double clicking or entering the name of the executable file on the command line( as in prog.exe, or a.out)

11. Diagram of Process State
terminated
running
ready
new
admitted
waiting
interrupt
scheduler dispatch
I/O or event wait
I/O or event completion
exit

12. Process Control Block (PCB)
Information associated with each process
Process state
Program counter
CPU registers
CPU scheduling information
Memory-management information
Accounting information
I/O status information
Each process is represented in the OS by a PCB. PCB is also called a task control block.
CPU scheduling information: priority

13. Process Control Block(PCB)
Process management
Registers
Program counter
Program status word
Stack pointer
Process state
Time when process started
CPU time used
Children’s CPU time
Time of next alarm
Message queue pointers
Pending signal bits
Process id
Various flag bits
Memory management
Pointer to text segment
Pointer to data segment
Pointer to bss segment
Exit status
Signal status
Parent process
Process group
Real uid
Effective
Real gid
Effective gid
Bit maps for signals
Files management
UMASK mask
Root directory
Working directory
File descriptors
Effective uid
System call parameters
Some of the fields of the MINIX process table
PCB is given one entry per process.
PCB is all and every information that must be saved when the process is switched from running to ready state so that it can be restarted later as if it had never stopped before.

14. Process Descriptor
Process Descriptor
Process Switching
Creating Processes
Destroying Processes
Overview of the Linux Process Descriptor ( struct task_struct )

15. Task List
Representation of a process: A process descriptor of the type
struct task_struct //<linux/sched.h>
1.7KBytes on a 32-bit machine
Task list: A circular doubly linked list of
task_struct

16. CPU Switch From Process to Process
Save state into PCB0
reload from PCB1
Save to PCB1
Reload from PCB0 OS
P 1
idle
Make a process run means that the state of PCB is changed to ‘running’ from ‘ready’ and the PC value of the PCB is loaded to the PC of CPU.
User mode/ Kernel Mode

17. Operations on Processes: Process Creation
Parent process create children processes, which, in turn create other processes, forming a tree of processes
Possibilities of Resource sharing
Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources
Possibilities of Execution
Parent and children execute concurrently
Parent waits until children terminate

18. Process Creation (Cont.)
Possibilities of Address space
Child is a duplicate of the parent
Child has a program loaded into it
UNIX examples
fork system call creates a new process
exec system call used after a fork to replace the process’ memory space with a new program

19. fork() Initial Process fork() Returns a new pid(child process) Returns
zero
Original Process
Continues
New Process:Child

20. fork() Child Parent PC PC PC Registers SP PC Stack Resources
Heap
BSS
Data
Text .
fork( ) SP PC
open files
Registers
Resources
pid=1000 ID PC
Stack
Heap
BSS
Data
Text .
fork( ) SP PC
open files
Registers
Resources
pid=1001 ID
Stack
Heap
BSS
Data
Text .
fork( ) SP PC
open files
Registers
Resources
pid=1000 ID
Child
Parent PC PC

21. fork()

22. forkOut

23. fork() pid=999; fork() pid=1000; running parent pid=1234; pid=1345;
child
pid=1234;
pid=1345;
pid=1452;
ready

24. fork()
int main(){ int i; for(i=0; i<10; i++){ printf(“Process_id=%d, i=%d\n”, getpid(), i); if(i==5){ printf(“Process_id=%d: fork() to start\n”,getpid()); int forkValue=fork(); printf(“forkValue=%d\n”, forkValue); }

25. fork( ) output

26. Creation of a process
fork() system call : It creates a new process by duplicating an existing one. The process that calls fork() is the parent, and the new process is the child.
pid = fork();
in the parent process, pid is the child process ID and in the child process, pid=0;
execve() system call : It creates a new address space and load a new program into it.
int execve(const char *filename, char *const argv[],
char *const envp[]);
argv: command line argument;
envp: path name, etc;

27. fork() - exec()-wait()
exit()
exec()
fork()
parent
resumes
child

28. forkWait

29. fork()-wait() output

30. forkWaitExeclp

31. output

32. Process Termination
Process executes last statement and asks the operating system to delete it (exit)
Output data from child to parent (via wait)
Process’ resources are deallocated by operating system
Parent may terminate execution of children processes (abort)
Child has exceeded allocated resources
Task assigned to child is no longer required
If parent is exiting
Some operating system do not allow child to continue if its parent terminates
All children terminated - cascading termination

33. forkWait.c

34. Duplicating a process image
#include <sys/types.h>
#include <unistd.h>
pid_t fork(void);
-return vaule of fork() : pid of the child if successful to the parent, 0 to the child.
If failed, -1 to the parent.
unistd.h : fork() prototype is defined.
types.h : pid_t is defined.

35. fork() Original process PC pid ==0 pid > 0 Parent process
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
main()
{ pid_t pid;
printf("Start!\n");
pid = fork();
if( pid == 0) printf("I am the Child !\n");
else if (pid > 0) printf("Parent pid=%d, Child pid=%d\n", (int)getpid(),pid);
else printf("fork() failed!\n");} PC
pid ==0
pid > 0
Parent process
Child process
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
main()
{ pid_t pid;
printf("Start!\n");
pid = fork();
if( pid == 0) printf("I am the Child !\n");
else if (pid > 0) printf("Parent pid=%d, Child pid=%d\n", (int)getpid(),pid);
else printf("fork() failed!\n");}
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
main()
{ pid_t pid;
printf("Start!\n");
pid = fork();
if( pid == 0) printf("I am the Child !\n");
else if (pid > 0) printf("Parent pid=%d, Child pid=%d\n", (int)getpid(),pid);
else printf("fork() failed!\n");}
PC+1
PC+1

36. Replacing a process image
#include <unistd.h> char **environ; int execl(const char *path, const char *arg0, …,(char *)0); int execlp(const char *file, const char *arg0,…,(char *)0); int execle(const char *path, const char *arg0,…,(char *)0,char *const envp[]); int execv(const char *path, char *const argv[]); int execvp(const char *file, char *const argv[]); int execve(const char *path, char *const argv[],char *const envp[]);

37. Examples of exe*()
#include <unistd.h> char *const ls_argv[]={“ls”,”-l”,0}; char *const ls_envp[]={“PATH=bin:/usr/bin”,”TERM=console”,0}; execl(“/bin/ls”,”ls”,”-l”,0); execlp(“ls”,”ls”,”-l”,0); execle(“/bin/ls”,”ls”,”-l”,0,ls_envp); execv(“/bin/ps”,ls_argv); execvp(“ls”,ls_argv); execve(“/bin/ls”, ls_argv, ls_envp);

38. Waiting for a process
#include <sys/types.h> #include <sys/wait.h> pid_t wait(int *status); The parent process executing wait(), pauses until its child process stops. The call returns the pid of the child process

39. status
status: it is the value transferred to the parent by exit(int status).
Parent process: wait(&status)
0x00
0x64
Child process: exit(1);
0x00
0x64
exit(1)
./forkwait
Start!
Parent pid=25820, Child pid=25821
I am the Child !
status=256

40. If(pid !=0){ int stat; pid_t pid_child; pid_child = wait(&status); printf(“Child has finished,pid_child=%d\n”,pid_child); if(status !=0) printf(“Child finished normally\n”); else printf(“Child finished abnormally\n”); }

41. wait(&status) forkwait.c ./forkwait #include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
main() {
pid_t pid;
int status;
printf("Start!\n");
pid = fork();
if( pid == 0) {
printf("I am the Child !\n");
exit(100); }
else if (pid > 0){
printf("Parent pid=%d, Child pid=%d\n", (int)getpid(),pid);
wait(&status);
printf("status=%d\n", status);
else printf("fork() failed!\n");
./forkwait
./forkwait
Start!
Parent pid=25789, Child pid=25790
I am the Child !
status=25600
vi forkwait.c

42. exit()
#include <stdlib.h> void exit(int status); /* Terminating current process and transfer the status to the parent. The value of status ranges from 0 to 255 integer value. */

43. waitpid( )
#include <sys/types.h> #include <sys/wait.h> pid_t waitpid(pid_t pid, int *status, int options); pid : pid of the child, status : transferred from the child executing exit(status), options 0 : usual wait state, that is , the parent waits until the child process finishes, WNOHANG :the parent process does not stay at ‘wait’ state.

44. Zombie Processes Terminated running admitted interrupt
ready
new
admitted
waiting
interrupt
scheduler dispatch
I/O or event wait
I/O or event completion
exit
Exit_Zombie
Wait( )
TTerminated

45. forkZombie.c

46. forkZombie.c

47. Threads So far a process is a single thread of execution.
The single thread of control allows the process to perform only one task .
The user cannot simultaneously type in characters and run the spell checker with the same process.
Therefore modern OSs have extended the process concept to allow a process to have multiple threads of execution.
In a traditional process, there is a single thread of control and a single PC in each process.
However in modern OSs, support is provided for multiple threads of control.
The thread will be discussed in Lecture 3. 47

48. Process and Threads Computer Program Thread Process counter
(b)
Three processes each with one thread.
One process with three threads.

49. Thread Usage[Tanenbaum]
A word processor with three threads

50. Linux Implementation of Threads
In the Linux, each thread has a unique task_struct . Linux implements all threads as standard processes.
A thread in Linux is just a process that shares certain resources( such as an address space).

51. A Thread Stack Heap BSS Data Text SP PC open files Registers Resources.
open files
Registers
Resources

52. Sharing of threads Registers Thread1 Stack1 SP PC Stack2 Thread2 Heap
open files . SP PC
Registers
Resources
Stack1
Heap
BSS
Data
Stack2
Text
Thread1
Thread2

53. Creating Threads: clone( )
#include <sched.h> int clone( int (*func)(void *), void *child_stack, int flags, void *func_arg,…); /* The caller must allocate a suitably sized block in the argument child_stack. The stack grows downward, so the child_stack argument should point to the high end of the allocated block. */ int main() { int child_stack[4096]; …. clone(thread_func, (void *)(child_stack+4095), CLONE_THREAD, NULL); ….. } int thread_func(void *arg) {
If the flag is set to CLONE_CHILD_CLEARID|CLONE_CHILD_SETTID,
the created one is a process.

54. clone() example
#include <unistd.h> #include <stdio.h> #include <stdlib.h> #include <linux/unistd.h> #include <sched.h> int thread_func(void *arg) { printf("Child :TGID(%d), PID(%d)\n",(int) getpid(),(int) syscall(__NR_gettid)); sleep(1); return 0; } int main(void) int pid; int child_stack[4096]; printf("Start!\n"); printf("Parent:TGID(%d), PID(%d)\n",(int)getpid(),(int) syscall(__NR_gettid)); clone (thread_func, (void *)(child_stack+4095), CLONE_VM | CLONE_THREAD | CLONE_SIGHAND, NULL); printf("End!\n");

55. pthread example
if((pid = pthread_create(&p_thread[0], NULL, pthread_func, (void*)&a)) < 0){
perror("Creation0 failed! ");
exit(1); }
if((pid = pthread_create(&p_thread[1], NULL, pthread_func, (void*)&b)) < 0){
perror("Creation1 failed");
exit(2);
pthread_join(p_thread[0], (void **)&status);
printf("pthread_join0\n");
pthread_join(p_thread[1], (void **)&status);
printf("pthread_join1\n");
printf(“The Parent.\n”);
printf("End!\n");
return 0;
#include <unistd.h> #include <stdio.h> #include <stdlib.h> #include <linux/unistd.h> #include <pthread.h> void *pthread_func(void *data) { int id; int i=0; pthread_t t_id; id = *((int *)data); printf(“I am the created pthread.\n”); sleep(2); } int main(void) int pid, status; int a = 1; int b = 2; pthread_t p_thread[2]; printf("Start!\n");

56. POSIX Threads (pthread)
Embedded Software
Kyu Ho Park
POSIX Threads (pthread)

57. Pthread and LinuxThread
Thread got attention with the advent of the POSIX 1003.c specification. But not so popular.
LinuxThreads which was very close to the POSIX standard:
POSIX Thread:
NPTL(Native POSIX Thread Library)
NGPT(New Generation POSIX Threads)
NPTL
POSIX(Portable OS Interface)
UNIX

58. Drawbacks of thread program
Difficult to write multi-threaded programs.
Difficult to debug multi-threaded programs.

59. pthread functions
#include <pthread.h> int pthread_create(pthread_t *thread, pthread_attr_t *attr, void *(*start_routine)(void *), void *arg); void pthread_exit(void *retval); Int pthread_join(pthread_t th, void **thread_return); //equivalent to wait( ).

60. pthread_create, exit, and join
Master_thread
pthread_create()
pthread_join()
created_
threads
pthread_exit()

61. pthread

62. pthread_create

63. Task ?
In Linux, Processes and threads are all represented by task_struct. Every task has its own unique id and it is represented at pid field of <task_struct>. But, according to the POSIX standard, all threads of a process shoud have the same pid. Linux adopts tgid(thread group id) to satisfy the POSIX standard. Therefore,a task if it is a process : pid == tgid, if it is a thread : pid != tgid and the child’s tgid has the same tgid of the parent.