Process Control in Unix

Operating Systems

Hebrew University

Spring 2004

Unix Process Model

• What is a processes?

• Properties of a process

• Processes organization

• Interacting with a process

Resources

• Advanced Programming in the Unix Environment, Stevens [243.51 St 48]

• POSIX.1 Spec

What is a process

• An entry in the kernel’s process table

• Most common unit of execution

• Execution state

• Machine instructions, data and environment

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

Properties of a Process - cont

• Controlling terminal

• Current working directory

• Root directory

• Open files descriptors

• File mode creation mask

• Resource limits

• Process times

Process Trees

• Only an existing process can create a new process

• Parent-Child relations

1

0

init

Who am I?

• getpid

• Returns the PID of the current process

#include <sys/types.h>

#include <unistd.h>

pid_t getpid(void);

Who is my parent?

• getppid

• Returns the PID of the parent of the current process

#include <sys/types.h>

#include <unistd.h>

pid_t getppid(void);

Talking directly to a process

• 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)

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

Fork

• 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 Example

if ( (pid = fork()) == 0 )

{ code for child }

else

{ code for parent }

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[]);

Exec example

if ((pid = fork()) == 0 ){

exec( arguments );

exit(-1);

}

// parent continues here

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[]);

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);

wait status returns

• WIFEXITED(status)

• WEXITSTATUS(status)

• WIFSIGNALED(status)– WIFTERMSIG(status)

• WIFSTOPPED(status)– WSTOPSIG(status)

Leaving a process

• 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 (quickly)

• 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);

When to use _exit

• In the fork branch of a C++ child

• Fail Fast!

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.

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

Daemons

• Leaving Home: Disconnecting from my parent so that I can live my own life

• Somewhat tricky to do correctly• Examples:

– inetd– atd– nfsd

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)