Process Control in Unix

1. Process Control in Unix Operating Systems Hebrew University Spring 2004

2. Unix Process Model • What is a processes? • Properties of a process • Processes organization • Interacting with a process

3. Resources • Advanced Programming in the Unix Environment, Stevens [243.51 St 48] • POSIX.1 Spec

4. What is a process • An entry in the kernel’s process table • Most common unit of execution • Execution state • Machine instructions, data and environment

5. Properties of a process • Process ID • Parent Process ID • Process group ID • Session ID • User ID of the process • Group ID of the process • Effective user ID • Effective group ID

6. Properties of a Process - cont • Controlling terminal • Current working directory • Root directory • Open files descriptors • File mode creation mask • Resource limits • Process times

7. Only an existing process can create a new process Parent-Child relations Process Trees 0 1 init

8. getpid Returns the PID of the current process #include <sys/types.h> #include <unistd.h> pid_t getpid(void); Who am I?

9. getppid Returns the PID of the parent of the current process #include <sys/types.h> #include <unistd.h> pid_t getppid(void); Who is my parent?

10. Send a signal The ONLY way to talk to a process Lots of signals More next week #include <sys/types.h> #include <signal.h> int kill(pid_t pid, int sig) Talking directly to a process

11. Creating a process • Only an existing process can create a new process • Step 1: Make a clone of yourself • Step 2: Have the clone change itself

12. Make a clone of myself ONLY difference is PID and PPID! Very cheap, Copy on Write -1 = failure Reason via ERRNO #include <sys/types.h> #include <unistd.h> pid_t fork(void); Fork

13. Fork Example if ( (pid = fork()) == 0 ) { code for child } else { code for parent }

14. Changing a process • Execute a new program in this space • argv and envp terminated by null pointer • NEVER returns on success, -1 and errno on failure #include <unistd.h> int execve(const char *filename, char *const argv[], char *const envp[]);

15. Exec example if ((pid = fork()) == 0 ){ exec( arguments ); exit(-1); } // parent continues here

16. Better ways #include <unistd.h> extern char **environ; int execl(const char *path, const char *arg, …); int execlp(const char *file, const char *arg, …); int execle(const char *path, const char *arg, …, char *const envp[]); int execv(const char *path, char *const argv[]); int execvp(const char *file, char *const argv[]);

17. Rendezvous • Meeting up with your child processes • Resources are freed only when the parent acknowledges the death of the child #include <sys/types.h> #include <sys/wait.h> pid_t wait(int *status); pid_t waitpid(pid_t pid, int *status, int options);

18. wait status returns • WIFEXITED(status) • WEXITSTATUS(status) • WIFSIGNALED(status) • WIFTERMSIG(status) • WIFSTOPPED(status) • WSTOPSIG(status)

19. Leave and tell my parent why I left NEVER returns Flush output buffers Close all open streams Call exit handlers #include <stdlib.h> void exit(int status); Leaving a process

20. Just like exit, but…. Don’t call the exit handlers Don’t flush output buffers Close file descriptors #include <stdlib.h> void _exit(int status); Leaving a process (quickly)

21. When to use _exit • In the fork branch of a C++ child • Fail Fast!

22. Orphans • A process whose parent has exited. • Orphaned processes are inherited by init • Its slot in the process table is immediately released when an orphan terminates.

23. Zombies • A process that no longer exists, but still ties up a slot in the system process table • Equivalently: • A process that has terminated, but whose parent exists and has not waited/acknowledged the child's termination

24. Daemons • Leaving Home: Disconnecting from my parent so that I can live my own life • Somewhat tricky to do correctly • Examples: • inetd • atd • nfsd

25. How to make a daemon • Fork -- Create pid-1 • Fork again – Create pid-2 • pid-1 exits, pid-2 is now an orphan • Chdir(“/”) – free current directory • Close all file descriptors (0…MAXINT)