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Process Management

Process Management. CS3320 Spring 2008. Process. A process is an instance of a running program. Not the same as “program” or “processor” Process provides each program with two key abstractions: Logical control flow Each program seems to have exclusive use of the CPU.

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Process Management

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  1. Process Management CS3320 Spring 2008

  2. Process • A process is an instance of a running program. • Not the same as “program” or “processor” • Process provides each program with two key abstractions: • Logical control flow • Each program seems to have exclusive use of the CPU. • Private address space • Each program seems to have exclusive use of main memory. • How are these Illusions maintained? • Process executions interleaved (multitasking) • Address spaces managed by virtual memory system

  3. Multi-processes in modern systems • Image that you are now a system architect to design a new multi-process systems. • Q1: How does the systems create a new process and terminate a process? • Q2: What processes you need when the system starts? • Q2: Should you impose a communist or democratic system on your system?

  4. Program Counter main Function int main(int argc, char *argv[ ]) • The starting point of a C program. Programs written in different languages have a different name. • A special start-up routine is called to set things up first before call main() • Set up by the link editor (invoked by the compiler) Memory Disk

  5. How a C program is started and terminated. user functions exit handler _exit or _Exit exit (does not return) call user process … return _exit or _Exit return call exit handler call main function exit function exit (does not return) return call return call exit (does not return) _exit or _Exit standard I/O cleanup return C start-up routine exec kernel

  6. Booting • Bootstrap program • Initialize all aspects of the system, e.g., CPU registers, device controllers, memory, etc. • Load and run the OS

  7. Process Control • Special Processes • PID 0 – Swapper (I.e., the scheduler) • Kernel process • No program on disks correspond to this process • PID 1 – init responsible for bringing up a Unix system after the kernel has been bootstrapped. • User process with superuser privileges • Initializing system processes, e.g., various daemons, login processes, etc. • /etc/rc* & init or /sbin/rc* & init • PID 2 - pagedaemon responsible for paging • Kernel process

  8. A Tree of Processes on a Typical Solaris

  9. Init Process When UNIX starts (boots), there is only one process, called init, with process ID = 1. The only way to create a new process is to duplicate an existing process So init is the ancestor of all subsequent processes. Initially, init duplicates (forks) several times and each child process replaces its code (execs) with the code of the executable getty which is responsible for user logins. 2014/4/1 cs3320 10

  10. General Structure of Operating System Control Tables Process image Memory tables Process 1 I/O tables Memory File tables Process image Devices Files Primary process table Process 3 Processes Process 1 Process 2 Process 3

  11. 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

  12. Process States

  13. Discussion How can your systems have multiple processes?

  14. Process Creation & Termination

  15. What’s a Fork() Parent Child • Child is an exact copy of the parent process. • They have their own memory space. if ((pid=fork() == 0){ { … } else { } exit(0); If ((pid=fork()) == 0){ { … } else { } exit(0); fork()

  16. fork #include <sys/types.h> #include <unistd.h> pid_t fork(void); • The only way beside the bootstrap process to create a new process. • Call once but return twice • 0 for the child process (getppid) • Child pid for the parent (1:n) • Cannot predict which process runs first • Process scheduling • Copies of almost everything but no sharing of memory, except text • Copy-on-write (fork() – exec())

  17. fork • fork(), race conditions, write vs standard I/O functions • File sharing • Sharing of file offesets (including stdin, stdout, stderr)

  18. As before, suppose foobar.txt consists of 6 ASCII characters "foobar". Then what is the output of the following program? int main() { int fd; char c; fd = open("foobar.txt", O_RDONLY, 0); if(fork() == 0) {read(fd, &c, 1); exit(0);} wait(NULL); read(fd, &c, 1); printf("c = %c\n", c); exit(0); } Child inherit’s the parent’s descriptor table. So child and parent share an open file table entry. Hence they share a file position. c=‘o’ 2014/4/1 cs3320 21

  19. Example Program with getpid() #include <stdio.h> main () { int pid; printf ("I'm the original process with PID %d and PPID %d.\n", getpid (), getppid ()); pid = fork (); /* Duplicate. Child & parent continue from here */ if (pid != 0) /* pid is non-zero, so I must be the parent */ { printf ("I'm the parent process with PID %d and PPID %d.\n", getpid (), getppid ()); printf ("My child's PID is %d\n", pid); } else /* pid is zero, so I must be the child */ { printf ("I'm the child process with PID %d and PPID %d.\n", getpid (), getppid ()); } printf ("PID %d terminates.\n", getpid () ); /* Both processes execute this */ } 2014/4/1 cs3320 22

  20. Example Program with getpid() $ myfork I'm the original process with PID 639 and PPID 416. I'm the parent process with PID 639 and PPID 416. My child's PID is 640 PID 639 terminates. I'm the child process with PID 640 and PPID 1. PID 640 terminates. $ 2014/4/1 cs3320 23

  21. fork • Normal cases in fork: • The parent waits for the child to complete. • The parent and child each go their own way (e.g., network servers). • Inherited properties: • Real/effective [ug]id, supplementary gid, process group ID, session ID, controlling terminal, set[ug]id flag, current working dir, root dir, file-mode creation mask, signal mask & dispositions, FD_CLOEXEC flags, environment, attached shared memory segments, resource limits • Differences on properties: • Returned value from fork, process ID, parent pid, tms_[us]time, tms_c[us]time, file locks, pending alarms, pending signals

  22. fork • Reasons for fork to fail • Too many processes in the system • The total number of processes for the real uid exceeds the limit • CHILD_MAX • Usages of fork • Duplicate a process to run different sections of code • Network servers • Want to run a different program • shells (spawn = fork+exec)

  23. Orphan Processes If a parent process terminates before its child terminates, the child process is automatically adopted by the init process The following program shows this. 2014/4/1 cs3320 26

  24. Example Program of Orphan #include <stdio.h> main () { int pid; printf ("I'm the original process with PID %d and PPID %d.\n", getpid (), getppid ()); pid = fork (); /* Duplicate. Child & parent continue from here */ if (pid != 0) /* Branch based on return value from fork () */ { /* pid is non-zero, so I must be the parent */ printf ("I'm the parent process with PID %d and PPID %d.\n", getpid (), getppid ()); printf ("My child's PID is %d\n", pid); } else { /* pid is zero, so I must be the child */ sleep (5); /* Make sure that the parent terminates first */ printf ("I'm the child process with PID %d and PPID %d.\n", getpid (), getppid ()); } printf ("PID %d terminates.\n", getpid () ); /* Both processes execute this */ } 2014/4/1 cs3320 27

  25. Example Program of Orphan $ orphan I'm the original process with PID 680 and PPID 416. I'm the parent process with PID 680 and PPID 416. My child's PID is 681 PID 680 terminates. $ I'm the child process with PID 681 and PPID 1. PID 681 terminates. 2014/4/1 cs3320 28

  26. Example of exit() % cat myexit.c #include <stdio.h> #include <stdlib.h> /* needed for exit */ main() { printf("I am going to exit with status code 42\n"); exit(42); } % myexit I am going to exit with status code 42 % echo status is $? 42 2014/4/1 cs3320 29

  27. Zombie Processes A process cannot leave the system until parent process accepts its termination code even if it has exit-ed If parent process is dead; init adopts process and accepts code If the parent process is alive but is unwilling to accept the child's termination code because it never executes wait() the child process will remain a zombie process. 2014/4/1 cs3320 30

  28. Zombie Processes Zombie processes do not take up system resources But do use up an entry in the system's fixed-size process table Too many zombie processes is a problem 2014/4/1 cs3320 31

  29. Zombie Example #include <stdio.h> main () { int pid; pid = fork (); /* Duplicate */ /* Branch based on return value from fork () */ if (pid != 0) { /* Never terminate, never execute a wait () */ while (1) sleep (1000); } else { exit (42); /* Exit with a silly number */ } } 2014/4/1 cs3320 32

  30. Zombie Example $ zombie & [1] 684 $ ps 684 p4 S 0:00 zombie 685 p4 Z 0:00 (zombie <zombie>) 686 p4 R 0:00 ps $ kill 684 [1]+ Terminated zombie $ ps 688 p4 R 0:00 ps $ 2014/4/1 cs3320 33

  31. Waiting for a Child: wait() pid_t wait(int *status) Causes a process to suspend until one of its child processes terminates. A successful call to wait() returns the PID of the child process that terminated places a status code into status 2014/4/1 cs3320 34

  32. Waiting for a Child: wait() Status code is encoded as: If the rightmost byte of status is zero the leftmost byte contains the low 8 bits of the value returned by the child's exit() or return() call If the rightmost byte of status is non-zero the rightmost 7 bits are equal to the Signal Number that caused the child to terminate the last bit is set to 1 if the child core dumped 2014/4/1 cs3320 35

  33. Waiting for a Child: wait() If a process executes a wait() and has no children, wait() returns immediately with a value of -1 If a process executes a wait(), And one or more children are already zombies, Then wait() returns immediately with the status of one of the zombies 2014/4/1 cs3320 36

  34. Example Program using wait() #include <stdio.h> main () { int pid, status, childPid; printf ("I'm the parent process and my PID is %d\n", getpid()); pid = fork (); /* Duplicate */ if (pid != 0) /* Branch based on return value from fork () */ { printf ("I'm the parent process with PID %d and PPID %d\n", getpid (), getppid ()); /* Wait for a child to terminate. */ childPid = wait (&status); printf("A child with PID %d terminated with exit code %d\n", childPid, status >> 8); } else { printf ("I'm the child process with PID %d and PPID %d\n", getpid (), getppid ()); exit (42); /* Exit with a silly number */ } printf ("PID %d terminates\n", getpid () ); } 2014/4/1 cs3320 37

  35. Example Program using wait() $ mywait I'm the parent process and my PID is 695 I'm the parent process with PID 695 and PPID 418 I'm the child process with PID 696 and PPID 695 A child with PID 696 terminated with exit code 42 PID 695 terminates 2014/4/1 cs3320 38

  36. Differentiating a Process: exec With the exec family, a process replaces Its current code Data Stack PID and PPID stay the same Only the code that the process is executing changes 2014/4/1 cs3320 39

  37. Differentiating a Process: exec Use the absolute or relative name int execl(const char* path, const char* arg0, ..., const char* argn, NULL) int execv(const char* path, const char* argv[ ]) 2014/4/1 cs3320 40

  38. Differentiating a Process: exec Use $PATH variable to find the executable: int execlp(const char* path, const char* arg0, ..., const char* argn, NULL) int execvp(const char* path, const char* argv[ ]) 2014/4/1 cs3320 41

  39. Differentiating a Process: exec argi or argv[i]: ith command line argument for executable (arg0: name of executable) If executable is not found, -1 is returned otherwise the calling process replaces its code, data, and stack with those of the executable and starts executing the new code A successful exec() never returns 2014/4/1 cs3320 42

  40. Example Using execl() #include <stdio.h> main () { printf ("I'm process %d and I'm about to exec an ls -l\n", getpid ()); execl ("/bin/ls", "ls", "-l", NULL); /* Execute ls */ printf ("This line should never be executed\n"); } $ myexec I'm process 710 and I'm about to exec an ls -l total 38 -rw-rw-r-- 1 raj raj 187 Jul 22 20:24 alarm.c -rw-rw-r-- 1 raj raj 226 Jul 22 20:22 background.c -rw-rw-r-- 1 raj raj 284 Jul 22 20:22 mychdir.c -rw-rw-r-- 1 raj raj 2058 Jul 22 20:23 vount.c -rwxrwxr-x 1 raj raj 4174 Jul 24 12:08 zombie -rw-rw-r-- 1 raj raj 298 Jul 22 20:20 zombie.c 2014/4/1 cs3320 43

  41. Changing Directories: chdir() Every process has a current working directory Used when processing a relative path name A child process inherits the current working directory from its parent Example: when a utility is run from a shell, the utility process inherits the shell's current working directory 2014/4/1 cs3320 44

  42. Changing Directories: chdir() int chdir(const char* pathname) Sets a process' current working directory to pathname The process must have execute permission from the directory for chdir() to succeed chdir() returns -1 if it fails 0 if it succeeds 2014/4/1 cs3320 45

  43. Example Using chdir() #include <stdio.h> main () { /* Display current working directory */ system ("pwd"); /* Change working directory to root directory */ chdir ("/"); /* Display new working directory */ system ("pwd"); chdir ("/home/naveen"); /* Change again */ system ("pwd"); /* Display again */ } $ mychdir /home/raj/3320/ch12 / /home/naveen $ 2014/4/1 cs3320 46

  44. Changing Priorities: nice() Child process inherits priority of parent process Can change the priority using nice() int nice(int delta) Adds delta to the priority Legal values of priority -20 to +19 Only super user can change to negative priority. Smaller the priority value, faster the process will run. 2014/4/1 cs3320 47

  45. Getting and Setting User and Group IDs: The get functions always succeed The set methods will succeed only when executed by super user or if id equals real or effective id of the process. uid_t getuid() uid_t setuid(uid_t id) uid_t geteuid() uid_t seteuid(uid_t id) gid_t getgid() uid_t setgid(gid_t id) gid_t getegid() uid_t setegid(gid_t id) 2014/4/1 cs3320 48

  46. Example Using fork() and exec() Executes a program in background Original process creates a child process to exec the specified executable and terminates. The orphaned child process is adopted by init. 2014/4/1 cs3320 49

  47. Example Using fork() and exec() #include <stdio.h> main (int argc, char* argv[]) { if (fork () == 0) /* Child */ { /* Execute other program */ execvp (argv[1], &argv[1]); fprintf (stderr, "Could not execute %s\n", argv[1]); } } $ background gcc mywait.c 2014/4/1 cs3320 50

  48. exit • Termination • The same code in the kernel is finally executed. • Close all open descriptors, release memory, and the like. • Exit status vs. termination status • Exit status (arg from exit, _exit, or return)  termination status • In abnormal case, the kernel generates it. • wait & waitpid to retrieve the termination status.

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