Unix Process Model What is a processes? Process Control in Unix - - PDF document

• 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 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) SIGILL – illegal instruction SIGKILL – kill (cannot catch

• r ignore)

SIGTERM - terminate

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)