Concurrent ProcessesConcurrent Processes

Processes can concurrently run same Processes can concurrently run same program.program.

Processes can start other processes.Processes can start other processes. UNIX provides concurrency functions UNIX provides concurrency functions

Creating New ProcessesCreating New Processes

system -- makes a shellsystem -- makes a shell

Exec -- replace this process with a new Exec -- replace this process with a new oneone

Fork -- start a concurrent process Fork -- start a concurrent process

Example Example ““SystemSystem””

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

The exec( ) FamilyThe exec( ) Family

char *path, *file;char *path, *file;

char *arg0, *arg1, ..., *argn;char *arg0, *arg1, ..., *argn;

char *argv[ ];char *argv[ ];

int ret;int ret;

ret = execl (path, arg0, arg1, ... , argn, (char *)0);ret = execl (path, arg0, arg1, ... , argn, (char *)0);

ret = execv (path, argv);ret = execv (path, argv);

ret = execlp (file, arg0, arg1, ... , argn, (char *) 0);ret = execlp (file, arg0, arg1, ... , argn, (char *) 0);

ret = execvp (file, argv);ret = execvp (file, argv);

The fork( ) FacilityThe fork( ) Facility

Basic process creation primitive. Basic process creation primitive. Duplicates the calling process. Duplicates the calling process. New process New process child processchild process Initiating process is Initiating process is parentparent.. Returns childReturns child’’s s process-idprocess-id

Inherited Data and File Inherited Data and File DescriptorsDescriptors

Forked child has instances of current Forked child has instances of current values of the variables and open file values of the variables and open file descriptors.descriptors.

Variables; Variables; ““passpass”” by copy by copy Read/write pointers for a file; Read/write pointers for a file; referencereference

wait( )wait( )

int wait (int * status)int wait (int * status) Suspends execution of process while child Suspends execution of process while child

runsruns Parent resumes upon child terminationParent resumes upon child termination When the When the firstfirst child process terminates child process terminates Normally returns process-id of exiting child. Normally returns process-id of exiting child. On error, wait( ) returns a -1, usually meaning On error, wait( ) returns a -1, usually meaning

no child exists on which to wait.no child exists on which to wait.

Zombies and Premature ExitsZombies and Premature Exits

Zombie processes are ones that somehow Zombie processes are ones that somehow lose connection with the parent. How?lose connection with the parent. How?

Zombie occupies a slot in process table, Zombie occupies a slot in process table, but uses no system resources. but uses no system resources.

Zombie cleared when parent Zombie cleared when parent ““claimsclaims”” child child Wait removes chance of zombie???Wait removes chance of zombie???

Parent exits B4 children Parent exits B4 children premature exit premature exit

Process and Group IdProcess and Group Id’’ss

pid = getpid( ); its own process idpid = getpid( ); its own process id pid = getppid( ); parent process idpid = getppid( ); parent process id newpg = setpgrp( ); change groupnewpg = setpgrp( ); change group pgid = getpgrp( ); access group pgid = getpgrp( ); access group

UNIX SignalsUNIX SignalsSIGHUPSIGHUP - The hangup signal, sent upon user logout- The hangup signal, sent upon user logout

SIGINTSIGINT - Interrupt signal, sent on user break signal - Interrupt signal, sent on user break signal

SIGQUITSIGQUIT - Quit signal, sent when user hits quit key- Quit signal, sent when user hits quit key

SIGILL SIGILL - Illegal Instruction signal- Illegal Instruction signal

SIGTRAPSIGTRAP - Trace trap signal used by debuggers- Trace trap signal used by debuggers

SIGFPESIGFPE - Floating-Point exception signal- Floating-Point exception signal

SIGKILLSIGKILL - Sent process n to terminate process m- Sent process n to terminate process m

SIGSYSSIGSYS - Bad argument to a system call signal- Bad argument to a system call signal

SIGPIPESIGPIPE - Write on a pipe with no one to read it- Write on a pipe with no one to read it

SIGALRMSIGALRM - Alarm clock signal used for software timers- Alarm clock signal used for software timers

SIGTERMSIGTERM - Software signal to terminate process- Software signal to terminate process

SIGUSR1SIGUSR1 - user signal- user signal

SIGUSR2SIGUSR2 - user signal- user signal

pause( )pause( )

int pause( )int pause( )

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