Lecture 5 Process, Thread and Task September 22, 2015 Kyu Ho Park

Major Topics of Lect. 5  How to create a process.  How to create a thread.  What is the task? 2

creating a process 3

4

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

6 Process in Memory Stack Heap Data Text SP PC

Memory Map of a process

4GB 3GB Stack text data heap

Virtualization 9 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

11 Diagram of Process State terminated running ready new admitted waiting interrupt scheduler dispatch I/O or event waitI/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

13 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 Process id Parent process Process group Real uid Effective Real gid Effective gid Bit maps for signals Various flag bits Files management UMASK mask Root directory Working directory File descriptors Effective uid Effective gid System call parameters Various flag bits Some of the fields of the MINIX process table Process Control Block(PCB)

14/39 Process Descriptor  Overview of the Linux Process Descriptor ( struct task_struct )  This seminar (chapter 3) focus on  Process State TASK_RUNNING TASK_INTERRUPTIBLE TASK_UNINTERRUPTIBLE TASK_STOPPED TASK_TRACED TASK_ZOMBIE EXIT_DEAD  Process parent/child Relationship Process Descriptor Process Switching Creating Processes Destroying Processes

Task List  Representation of a process: A process descriptor of the type struct task_struct // 1.7KBytes on a 32-bit machine  Task list: A circular doubly linked list of task_struct 15

16 CPU Switch From Process to Process P 0P 0 Save state into PCB0 reload from PCB1 Save to PCB1 Reload from PCB0 OSP 1P 1 idle

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 duplicate of 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

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

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

fork() 21

forkOut 22

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

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); } 24

fork( ) output 25

Creation of a process 26 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;

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

forkWait

fork()-wait() output

forkWaitExeclp

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

forkWait.c 33

Duplicating a process image #include 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. 34

fork() 1. #include 2. #include 3. #include 4. #include 5. main() 6. { pid_t pid; 7. printf("Start!\n"); 8. pid = fork(); 9. if( pid == 0) printf("I am the Child !\n"); 10. else if (pid > 0) printf("Parent pid=%d, Child pid=%d\n", (int)getpid(),pid); 11. else printf("fork() failed!\n");} #include 2. #include 3. #include 4. #include 5. main() 6. { pid_t pid; 7. printf("Start!\n"); 8. pid = fork(); 9. if( pid == 0) printf("I am the Child !\n"); 10. else if (pid > 0) printf("Parent pid=%d, Child pid=%d\n", (int)getpid(),pid); 11. else printf("fork() failed!\n");} 1. #include 2. #include 3. #include 4. #include 5. main() 6. { pid_t pid; 7. printf("Start!\n"); 8. pid = fork(); 9. if( pid == 0) printf("I am the Child !\n"); 10. else if (pid > 0) printf("Parent pid=%d, Child pid=%d\n", (int)getpid(),pid); 11. else printf("fork() failed!\n");} Original process Parent processChild process PC PC+1 pid ==0 pid > 0

Replacing a process image #include 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[]); 36

Examples of exe*() #include 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); 37

Waiting for a process #include 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 38

status  status: it is the value transferred to the parent by exit(int status). 39 Parent process: wait(&status) Child process: exit(1); 0x00 0x640x00 0x64 exit(1) Start! Parent pid=25820, Child pid=25821 I am the Child ! status=256

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”); } 40

41 #include 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"); } Start! Parent pid=25789, Child pid=25790 I am the Child ! status=25600 vi forkwait.c./forkwait forkwait.c

exit() #include 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. */ 42

waitpid( ) #include 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. 43

Zombie Processes 44 Terminated running ready new admitted waiting interrupt scheduler dispatch I/O or event waitI/O or event completion exit TTerminated Exit_Zombi e Wait( )

forkZombie.c 45

forkZombie.c 46

47 Threads 47 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.

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

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

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). 50

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

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

Creating Threads: clone( ) #include 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) { …. } 53 If the flag is set to CLONE_CHILD_CLEARID|CLONE_CHILD_SETTID, the creates one is a process.

clone() example #include 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); sleep(1); printf("End!\n"); return 0; } 54

pthread example #include 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"); 55 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; }

POSIX Threads (pthread) Kyu Ho Park CORE Lab. Embedded Software

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) 57 NPTL POSIX(Portable OS Interface) UNIX

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

pthread functions #include 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( ). 59

pthread_create, exit, and join 60 pthread_create() pthread_exit() pthread_join() Master_threa d created_ threads

pthread 61

pthread_create 62

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. 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. 63

Simultaneous execution of threads 1. #include 2. #include 3. #include 4. #include 5. void *thread_function(void *arg); 6. int run_now = 1; 7. char message[] = "Hello World"; 8. int main() { 9. int res; 10. pthread_t a_thread; 11. void *thread_result; 12. int print_count1 = 0; 13. res = pthread_create(&a_thread, NULL, thread_function, (void *)message); 14. if (res != 0) { 15. perror("Thread creation failed"); 16. exit(EXIT_FAILURE); 17. } 18. while(print_count1++ < 20) { 19. if (run_now == 1) { 20. printf("1"); 21. run_now = 2; 22. } 23. else { 24. sleep(1); 25. } 26. } printf("\nWaiting for thread to finish...\n"); 2. res = pthread_join(a_thread, &thread_result); 3. if (res != 0) { 4. perror("Thread join failed"); 5. exit(EXIT_FAILURE); 6. } 7. printf("Thread joined\n"); 8. exit(EXIT_SUCCESS); 9. } 10. void *thread_function(void *arg) { 11. int print_count2 = 0; 12. while(print_count2++ < 20) { 13. if (run_now == 2) { 14. printf("2"); 15. run_now = 1; 16. } 17. else { 18. sleep(1); 19. } 20. } 21. sleep(3); 22. }

semaphore #include int sem_init(sem_t *sem, int pshared, unsigned int value); - sem: points to a semaphore object, pshared: if 0, the semaphore is local to the current process, else the semaphore may be shared between processes(Linux does not support it). value: initial value of the semaphore. int sem_wait(sem_t *sem); -This function waits until the semaphore is nonzero, and decreases the value of semaphore by 1. int sem_post(sem_t *sem); -This function increases the value of the semaphore by 1. int sem_destroy(sem_t *sem); 65

Synchronization with semaphore #include void *thread_function(void *arg); sem_t bin_sem; #define WORK_SIZE 1024 char work_area[WORK_SIZE]; int main() { int res; pthread_t a_thread; void *thread_result; res = sem_init(&bin_sem, 0, 0); if (res != 0) { perror("Semaphore initialization failed"); exit(EXIT_FAILURE); } res = pthread_create(&a_thread, NULL, thread_function, NULL); if (res != 0) { perror("Thread creation failed"); exit(EXIT_FAILURE); } 66 printf("Input some text. Enter 'end' to finish\n"); while(strncmp("end", work_area, 3) != 0) { fgets(work_area, WORK_SIZE, stdin); sem_post(&bin_sem); } printf("\nWaiting for thread to finish...\n"); res = pthread_join(a_thread, &thread_result); if (res != 0) { perror("Thread join failed"); exit(EXIT_FAILURE); } printf("Thread joined\n"); sem_destroy(&bin_sem); exit(EXIT_SUCCESS); } void *thread_function(void *arg) { sem_wait(&bin_sem); while(strncmp("end", work_area, 3) != 0) { printf("You input %d characters\n", strlen(work_area) -1); sem_wait(&bin_sem); } pthread_exit(NULL); }