제 11 장 프로세스 시스템 프로그래밍 1. Objectives Process Start and Exit Create a child process Execute a new program within a process Signal System boot 2

제 11 장 프로세스 시스템 프로그래밍

1

Objectives

Process Start and Exit Create a child process Execute a new program within a process Signal System boot

2

© 숙대 창병모 3

프로세스 시작 /종료


Process Start

Kernel exec system call

C start-up routineuser process

int main(int argc, char * argv[]);

call return


main() int main(int argc, char *argv[]);

argc : the number of command-line arguments argv[] : an array of pointers to the arguments

C start-up routine Started by the kernel (by the exec system call) Take the command-line arguments and the

environment from the kernel Call main exit( main( argc, argv) );


Process Termination Normal termination

return from main() calling exit() : calling _exit()

Abnormal termination calling abort() terminated by a signal


exit()

프로세스를 정상적으로 종료한다

cleanup processing 을 수행한다 모든 열려진 스트림을 닫고 (fclose) 출력 버퍼의 내용을 디스크에 쓴다 (fflush)

status the exit status of a process 이 값은 UNIX shell 에 의해서 사용됨

#include <stdlib.h>

void exit(int status);


_exit()

프로세스를 정상적으로 종료한다

커널로 즉시 리턴한다

#include <unistd.h>

void _exit(int status);


atexit()

exit handler 를 등록한다 프로세스당 32 개까지

func an exit handler a function pointer

exit() 는 exit handler 들을 등록된 역순으로 호출한다

#include <stdlib.h>

void atexit(void (*func)(void)); returns: 0 if OK, nonzero on error


C Program Start and Termination

retu

rn ca

ll

exitfunction

userfunction

mainfunction

C start-uproutine

exit handler

exit handler

standard I/Ocleanup

kernel

user process

exit(does not return)exit

(does not return)

exit

(does not re

turn)

call

return

call

return

callreturn

……

_exit

exec

_exit

_exit

retu

rn ca

ll


/* doatexit.c */static void my_exit1(void), my_exit2(void);

int main(void) { if (atexit(my_exit2) != 0) perror("can't register my_exit2"); if (atexit(my_exit1) != 0) perror("can't register my_exit1"); if (atexit(my_exit1) != 0) perror("can't register my_exit1"); printf("main is done\n"); return 0;}

static void my_exit1(void) { printf("first exit handler\n");}

static void my_exit2(void) { printf("second exit handler\n");}

Example of exit handlers


프로세스 구조

What is a process ?

a process is an executing program together with the program's data, stacks, user-area information about the program in kernel

a process image is the layout of a process in memory

a process is not a program by itself

13

Components of Process Code area

the executable code of a process

Data area static and global variables heap area

Stack area runtime stack to hold activation records including

local variables

Kernel data structures to keep information

one process table in kernel a user area per process

information like open fds, signal actions, etc 14

Components of Process

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Heap

Stack

Code

User-area

Kernel processes and user processes

User process process in user mode a system call make it into kernel mode

Kernel process process in kernel mode the code is linked directly into the kernel

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프로세스 생성

System Calls : Creating a new process

새로운 프로세스 생성 자기복제

int fork( ) Create a new process (child process)

An almost-exact duplicate of the parent process

code, data, stack, open file descriptors, etc.

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fork()

fork() is called once, but returns twice! returns 0 in child process returns the child process ID in parent process Returns -1 on failure.

Execution Parent and child continue executing instructions

following the fork() call

Often, fork() is followed by exec()

System Calls : Creating a new process

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main() {...rtn = fork();...}

HEAP

STACK

USER AREA

CODE

main() {...rtn = fork();...}

HEAP

STACK

USER AREA

CODE

BEFORE fork()

AFTER fork()

pid: 12791

pid: 12791 pid: 12792

main() {...rtn = fork();...}

HEAP

STACK

CODE

USER AREA

System Calls

Process idint getpid( )

return a process's idint getppid( )

return a parent process's id

int exit (int status) a process may terminate process by executing exit() frees its code, data, stack and closes a process's fds, sends its parent a SIGCHLD signal

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System Calls

Orphan process If a parent dies before its child, the child is called an

orphan process and is adopted by "init" process, PID 1. it is dangerous for a parent to terminate without waiting

for the death of its child.

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System Calls

int wait(int* status) wait() causes a process to suspend until one of its

children terminate wait() returns the pid of the child that terminated and places a status code into *status

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fork()

exit()exec()

wait()

Example : fork and wait

#include <stdio.h>main( ){

int pid, status, childPid;printf(“I’m the parent process and my PID is %d\n”, getpid( ));pid = fork( );if (pid != 0) { printf(“I’m the parent process with PID %d and PPID %d\n”,

getpid( ), getppid( )); childPid = wait(&status); printf(“A child with PID %d terminated with exit code %d\n”

childPid, status>>8);} else {

printf(“I’m the child process with PID %d and PPID %d\n”,getpid( ), getppid( ));

exit(42);}

printf(“PID %d terminates\n”);}

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프로그램 실행


Program Execution: exec()

프로세스 내의 프로그램을 새 프로그램으로 대치 exec() 시스템 호출 사용 보통 fork() 후에 exec( )

When a process calls an exec function that process is completely replaced by the new

program (text, data, heap, and stack segments) and the new program starts at its main function Successful exec never returns !

프로세스 내에서 새 프로그램 실행

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...

execl(“newpgm”, …);...

HEAP

STACK

USER AREA

CODE

main() // newpgm {...

...}

HEAP

STACK

USER AREA

CODE

BEFORE exec( ) AFTER exec() pid: 12791 pid: 12791

System Calls:Exec

Replace the calling process's code, data, and stack with

the new program path and execute the new code.

Return a successful exec( ) never returns failure : -1

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int execl(char* path, char* arg0, char* arg1, ... , char* argn, NULL)int execv(char* path, char* argv[ ])

Example : fork and exec

Call exec( ) after calling fork( ) Create a new process and execute a program


int fork( );if (fork( ) == 0) {

execl("/bin/echo", "echo", "this is message one", NULL);perror("exec one faild"); exit(1);

}if (fork( ) == 0) {

execl("/bin/echo", "echo", "this is message two", NULL);perror("exec two faild"); exit(2);

}if (fork( ) == 0) {

execl("/bin/echo","echo", "this is message three",NULL);perror("exec three faild"); exit(3);

}printf("Parent program ending\n");

}29

Etc

Changing directoryint chdir (char* pathname)

set a process's current working directory to pathname the process must have execute permission for the directory returns 0 if successful; otherwise -1

Accessing user and group ids int getuid() int setuid(int id) int geteuid() int seteuid(int id) int getgid() int setgid(int id) int getegid() int setgid(int id)

returns the calling process's real and effective user or group ids

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Example

#include <stdio.h>

main( ){

system(“pwd”);chdir(“/”);system(“pwd”);chdir(“/user/faculty/chang”);system(“pwd”);

}

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Example: Background Processing

Execute a program in the background using fork( ) and exec( )

No waiting

#include <stdio.h>main(argc, argv)int argc, argv[ ];{

if (fork( ) == 0) { execvp(argv[1], &argv[1]); fprintf(stderr, “Could not execute %s\n”,argv[1]);}

}32

Redirection 구현

%redirect output command

1) The parent forks and then waits for the child to terminate

2) The child opens the file "output“, and

3) Duplicates the fd of “output" to the standard output(fd=1), and close the fd of the original file

4) The child execute the “command”.

All of the standard output of the command goes to "output“

5) When the child shell terminates, the parent resumes

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Example: Redirection

%redirect out ls –l#include <stdio.h>main(argc, argv)int argc, argv[ ];{

int fd, status;if (fork() == 0) {

fd=open(argv[1], O_CREAT|O_TRUNC|O_WRONLY, 0600)

dup2(fd, 1);close(fd);execvp(argv[2], &argv[2]);perror(“main”);

} else {

wait(&status);printf(“Child is done\n”);

}}

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시그널 (Signal)

Signals

Unexpected events a floating-point error a power failure an alarm clock the death of a child process a termination request from a user (Ctrl-C) a suspend request from a user (Ctrl-Z)

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ProcessSignal #8

Predefined signals

31 signals /usr/include/signal.h See the table in page 427

Actions of the default signal handler terminate the process and generate a

core(dump) ignores and discards the signal(ignore) suspends the process (suspend) resume the process

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Terminal signal

Terminal signals Control-C

SIGINT signal Control-Z

SIGSTP signal

Requesting an alarm signal SIGALRM signal Default handler

display message “Alarm clock”

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Example : Alarm

Alarm.c


alarm(3);printf(“Looping forever …\n”);while (1);printf(“This line should never be

executed\n”);}

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Handling Signals: signal( )

Signal handler 등록

signal(int sigCode, void (*func)( ))) specify the action for a signal

sigCode func func

SIG_IGN SIG_DFL user-defined function

return the previous func

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Handling Signals: Example

#include <stdio.h>#include <signal.h>int alarmFlag=0;alarmHandler( );main( ) {

signal(SIGALRM, alarmHandler);alarm(3);printf(“Looping …\n”);while(!alarmFlag) {

pause( ); } printf(“Loop ends due to alarm signal \n”);} /* main */

alarmHandler( ) {printf(“An alarm clock signal was received\n”);alarmFlag = 1;

} 41

Example: Protecting Critical Code

#include <stdio.h>#include <signal.h>main ( ) {

int (*oldHandler)( );printf(“I can be Control-C’ed\n”);sleep(3);oldHandler = signal(SIGINT, SIG_IGN);printf(“I’m proctected from Control-C now\n”);sleep(3);signal(SIGINT, oldHandler);printf(“I can be Control-C’ed again\n”);sleep(3);

}

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Sending signals: kill()

프로세스에 Signal 보내기

int kill(int pid, int sigCode) send signal sigCode to the process pid condition for success

the owner of the sending process = the owner of pid

the owner of the sending process = super-user

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Example: Time Limit #include <stdio.h>#include <signal.h>int delay;main(argc, argv)int argc; char *argv[ ];{

int pid;signal(SIGCHLD,childHandler);pid = fork();if (pid == 0) {

execvp(argv[2], &argv[2]);perror(“Limit”);

} else {sscanf(argv[1], “%d”, &delay);sleep(delay);printf(“Child %d exceeded limit and is being killed\n”, pid);kill(pid, SIGINT);

}} 44

Example: Time Limit

childHandler( ) /* Executed if the child dies before the parent */{

int childPid, childStatus;childPid = wait(&childStatus);printf(“Child %d terminated within %d seconds\n”, childPid, delay);exit(0);

}

Usage%limit 5 ls%limit 4 sleep 100

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Example: Suspending and Resume#include <signal.h>#include <stdio.h>main( ) {

int pid1, pid2;

pid1 = fork( );if (pid1 == 0) {

while(1) { printf(“pid1 isalive\n”); sleep(1); }}

pid2 = fork( );if (pid2 == 0) {

while (1) { printf(“pid2 is alive\n”); sleep(1); }}

sleep(3); kill(pid1, SIGSTOP);sleep(3); kill(pid1, SIGCONT);sleep(3); kill(pid1, SIGINT);kill(pid2, SIGINT);

} 46


시스템 시작


시스템 시작

The First Process the first process(sched) with pid =0 is created during boot

time, and fork/exec twice.

0 sched1 init2 pagedaemon

Process ID 0 : swapper (scheduler process) system process part of the kernel no program on disk corresponds to this process


시스템 시작 Process ID 1: init process

invoked by the kernel (/etc/init or /sbin/init) reads the system initialization files (/etc/rc*)

brings the system to a certain state (multi-user) creates processes based upon script files getty, login process, mounting file systems, start daemon processes

Process ID 2: pagedaemon supports the paging of the virtual memory system kernel process


The process hierarchy

getty process(/etc/getty) detects line activity and prompts with login exec /bin/login, after entering a response to the

login prompt.

login process checks a user's login and password against

/etc/passwd exec /bin/sh or /bin/csh set the evnironment variables like HOME,

LOGNAME, PATH...

shell process initially read commands form start-up files like

/etc/.cshrc and $HOME/.cshrc shell starts its job


The process hierarchy

sched(0)

pagedaemon(2)init(1)

getty(10)

login(10)

shell(10)

httpd(5) …


Process Identifiers

none of these functions has an error return

#include <sys/types.h>#include <unistd.h>

pid_t getpid(void); // returns process IDpid_t getppid(void); // returns parent process IDuid_t getuid(void); // returns real user IDuid_t geteuid(void); // returns effective user IDgid_t getgid(void); // returns real group IDgid_t getegid(void); // returns effective group ID

Documents

제 11 장 프로세스 시스템 프로그래밍 1. Objectives Process Start and Exit Create a child process Execute a new program within a process Signal System boot 2