Concurrent Processes

1. Concurrent Processes • Processes can concurrently run same program. • Processes can start other processes. • UNIX provides concurrency functions

2. Creating New Processes • system -- makes a shell • Exec -- replace this process with a new one • Fork -- start a concurrent process

3. Example “System” int main() { char buffer[80]; strcpy(buffer, “wc *.*”); system(buffer); }

4. The exec( ) Family char *path, *file; char *arg0, *arg1, ..., *argn; char *argv[ ]; int ret; ret = execl (path, arg0, arg1, ... , argn, (char *)0); ret = execv (path, argv); ret = execlp (file, arg0, arg1, ... , argn, (char *) 0); ret = execvp (file, argv);

5. The fork( ) Facility • Basic process creation primitive. • Duplicates the calling process. • New process child process • Initiating process is parent. • Returns child’s process-id

6. Inherited Data and File Descriptors • Forked child has instances of current values of the variables and open file descriptors. • Variables; “pass” by copy • Read/write pointers for a file; reference

7. wait( ) • int wait (int * status) • Suspends execution of process while child runs • Parent resumes upon child termination • When the first child process terminates • Normally returns process-id of exiting child. • On error, wait( ) returns a -1, usually meaning no child exists on which to wait.

8. Zombies and Premature Exits • Zombie processes are ones that somehow lose connection with the parent. How? • Zombie occupies a slot in process table, but uses no system resources. • Zombie cleared when parent “claims” child • Wait removes chance of zombie??? • Parent exits B4 children  premature exit

9. Process and Group Id’s • pid = getpid( ); its own process id • pid = getppid( ); parent process id • newpg = setpgrp( ); change group • pgid = getpgrp( ); access group

10. UNIX Signals SIGHUP - The hangup signal, sent upon user logout SIGINT - Interrupt signal, sent on user break signal SIGQUIT - Quit signal, sent when user hits quit key SIGILL - Illegal Instruction signal SIGTRAP - Trace trap signal used by debuggers SIGFPE - Floating-Point exception signal SIGKILL - Sent process n to terminate process m SIGSYS - Bad argument to a system call signal SIGPIPE - Write on a pipe with no one to read it SIGALRM - Alarm clock signal used for software timers SIGTERM - Software signal to terminate process SIGUSR1 - user signal SIGUSR2 - user signal

11. pause( ) int pause( ) pause( ) suspends the calling process, without wasting resources, until some kind of signal is received.