Transcript ppt

Process Control in Unix
Operating Systems
Hebrew University
Spring 2004
Unix Process Model
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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
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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
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Controlling terminal
Current working directory
Root directory
Open files descriptors
File mode creation mask
Resource limits
Process times
Process Trees
0
• Only an existing
process can create a
new process
• Parent-Child relations
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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
#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid, int sig)
• Lots of signals
• More next week
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
#include <sys/types.h>
#include <unistd.h>
• ONLY difference is
PID and PPID!
• Very cheap, Copy on
Write
• -1 = failure
pid_t fork(void);
– Reason via ERRNO
Fork Example
if ( (pid = fork()) == 0 )
{ code for child }
else
{ code for parent }
Changing a process
#include <unistd.h>
int execve(const char *filename,
char *const argv[],
char *const envp[]);
• Execute a new program in this space
• argv and envp terminated by null pointer
• NEVER returns on success, -1 and errno on failure
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
#include <stdlib.h>
void exit(int status);
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NEVER returns
Flush output buffers
Close all open streams
Call exit handlers
Leaving a process (quickly)
• Just like exit, but….
#include <stdlib.h>
• Don’t call the exit
handlers
• Don’t flush output
buffers
• Close file descriptors
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
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A process that no longer exists, but still
ties up a slot in the system process table
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Equivalently:
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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
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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)