apue (process control)cs.kangwon.ac.kr/~ysmoon/courses/2010_2/us/14.pdf · 2016. 6. 2. · process...

Li /UNIX P iLi /UNIX P iLinux/UNIX ProgrammingLinux/UNIX Programming

APUE (Process Control)APUE (Process Control)

문양세문양세양세양세강원대학교강원대학교 ITIT대학대학 컴퓨터과학전공컴퓨터과학전공

강의강의 내용내용APUE (Process Control)

프로세스 ID

프로세스 생성프로세스 생성

프로세스 종료

레이스 컨디션

프로그램 실행프로그램 실행

기타

UNIX System Programmingby Yang-Sae Moon

Process Identifiers (1/2)Process Identifiers (1/2)APUE (Process Control)

Every process has a unique process ID, a nonnegative integer.(모든 프로세스는 양수의 유일한 식별자(PID)를 가짐)(모든 프로세스는 양수의 유일한 식별자(PID)를 가짐)

Process ID 0: swapper

• scheduler process (it controls time slots for processes)

• system process

• part of the kernel

• no program on disk corresponds to this process


Process Identifiers (2/2)Process Identifiers (2/2)APUE (Process Control)

Process ID 1: init process

invoked by the kernel at the end of the bootstrap procedure• invoked by the kernel at the end of the bootstrap procedure

• /etc/init or /sbin/init

d h d d l f l• reads the system-dependent initialization files (/etc/rc*)

• brings the system to a certain state (multi-user)

• a normal user process but runs with super-user privileges

• PID 1 is immortal

Process ID 2: pagedaemon

• supports the paging of the virtual memory system

• kernel process


p

System Process System Process 예제예제APUE (Process Control)


PID PID 관련관련 함수함수APUE (Process Control)

#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 ID_ g ( ); //uid_t geteuid(void); // returns effective user IDgid_t getgid(void); // returns real group IDgid_t getegid(void); // returns effective group ID

None of these functions has an error return.



프로세스 ID


프로세스 종료

레이스 컨디션


기타


fork() (1/3)fork() (1/3)APUE (Process Control)

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

( )pid_t fork(void);

fork() is the ONLY way to create a process in Unix kernelfork() is the ONLY way to create a process in Unix kernel.

Child process is the new process created by fork().

fork() is called once, but returns twice!

• returns 0 in child process.

• returns the child process ID in parent process.



Child gets a copy of parent’s data space, heap, and stack

Often read only text segment is shared• Often, read-only text segment is shared

Parent and child continue executing instructionsfollowing the fork() call

Often, fork() is followed by exec().Often, fork() is followed by exec().



main()

BEFORE fork() pid: 12791main() {...rtn = fork();...}

TEXT

}DATASTACK

USER AREAUSER AREA

i () i ()

AFTER fork()pid: 12791 pid: 12793

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

TEXTmain() {...rtn = fork();...

TEXT

}DATASTACK

USER AREA

}DATASTACK

USER AREA


USER AREA USER AREA

예제예제: fork.c (1/2): fork.c (1/2)APUE (Process Control)


예제예제: fork.c (2/2): fork.c (2/2)APUE (Process Control)

실행 결과


File Sharing after fork()File Sharing after fork()APUE (Process Control)

Parent and child share the same file descriptors.Parent and child share the same file offset.Therefore, intermixed output will occur from parent and child.

parent process table entry file table

v-node tablev-nodeentry

file descriptorsfd 0:fd 1:

fd flags ptr

file tablefd status flagscurrent file offsetv-node ptr

v nodeinformation

i-nodeinformation

fd 2:. . . . fd status flags

current file offsetv-node ptr

current file size

v-nodeinformation

file descriptorsfd 0: fd flags ptr

child process table entry fd status flags

current file offsetv-node ptr

informationi-node

information

current file sizefd 0:fd 1:fd 2:

. . . .

fd flags ptr v node ptr

v-nodeinformation

i-node


i-nodeinformation

current file size

Properties Inherited to the ChildProperties Inherited to the ChildAPUE (Process Control)

real user and group ID, effective user and group ID

supplementary group IDssupplementary group IDs

process group ID, session ID

set user ID and set group ID flagsset-user-ID and set-group ID flags

current working directory

t di troot directory

file mode creation mask

signal mask and dispositions

the close-on-exec flag for any open file descriptors

environment

attached shared memory segments


resource limits

Properties NOT Inherited to the ChildProperties NOT Inherited to the ChildAPUE (Process Control)

the return value from fork()

the process IDs are differentthe process IDs are different

file locks

pending alarms are cleared for the childpending alarms are cleared for the child

the set of pending signals for the child is set to the empty set



프로세스 ID


프로세스 종료

레이스 컨디션


기타


exit() exit() –– Process TerminationProcess TerminationAPUE (Process Control)

A process can terminate in 5 ways:

Normal Termination

• return from main()

• exit() w/ cleanup procedure() p p

• _exit() w/o cleanup procedure

Abnormal Termination

• calling abort() (generates SIGABRT signal)

• process receives signals


exit() exit() –– Termination Termination StatusStatus –– skip skip APUE (Process Control)

Exit Status:

argument of exit() exit() • argument of exit(), _exit()

• the return value from main()

Termination Status:

• Normal termination: Exit status Termination status

• Abnormal termination: kernel indicates reason Termination status

Parent can obtain the termination status of the child process.p

• by wait() or waitpid()

Wh t if t t i t b f hild?What if parent terminates before child?

• init(PID = 1) becomes the parent of the child process


exit() exit() –– Child Child TerminationTermination –– skip skip APUE (Process Control)

Suppose child terminates first

If child disappeared parent would not be able to check child’s termination status• If child disappeared, parent would not be able to check child’s termination status.

• Zombie: minimal info of dead child process (pid, termination status, CPU time) kept by the kernel for the parent to call wait() or waitpid()by the kernel for the parent to call wait() or waitpid()

Zombie가 되지 않도록 하기 위해서는 Parent가 wait()해 주어야 함


wait(), wait(), waitpidwaitpid() (1/2() (1/2)) –– skip skip APUE (Process Control)

Child terminates

Kernel sends SIGCHLD signal to parent• Kernel sends SIGCHLD signal to parent.

• an asynchronous event

Default action for signal: ignore it.

Signal handlers can be defined by usersg y

• call wait() to fetch the termination status of child.

• (무사히 죽을 수 있도록 배려~)• (무사히, 죽을 수 있도록 배려~)


wait(), wait(), waitpidwaitpid() (2/2() (2/2)) –– skip skip APUE (Process Control)

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

pid_t wait(int *statloc);pid_t waitpid(pid_t pid, int *statloc, int options);

Both returns: process ID if OK, 0, or -1 on error

A process that calls wait or waitpid can

ot etu s: p ocess O , 0, o o e o

• block (if all of its children are still running), or

• return immediately with the termination status of a child, ory

• return immediately with an error (if it doesn’t have any child processes)

statlocstatloc

• a pointer to an integer to store the termination status


options: refer to manual of waitpid()

Macros to examine the termination Macros to examine the termination statusstatus –– skip skip APUE (Process Control)

Macro Description

WIFEXITED(status)True if child is terminated normallyWEXITSATUS(status) : get exit status (low-order 8 bits)

WIFSIGNALED(status)True if child is terminated abnormally (by receipt of a signal)WTERMSIG(status): fetch the signal number that caused the termination.

T if hild i tl t dWIFSTOPPED(status)

True if child is currently stoppedWSTOPSIG(status): fetch the signal number that caused the stop.


예제예제: : nwait.cnwait.c (1/3(1/3)) –– skip skip APUE (Process Control)

#include <stdio.h> // nwait.c#include <unistd.h>#include <sys/types.h>#include <sys/wait.h>

void pr_exit(int status) {if (WIFEXITED(status))printf("normal termination, exit status = %d\n“, WEXITSTATUS(status));

else if (WIFSIGNALED(status))else if (WIFSIGNALED(status))printf("abnormal termination, signal number = %d\n", WTERMSIG(status));

else if (WIFSTOPPED(status))printf("child stopped, signal number = %d\n", WSTOPSIG(status));

}



int main(void) {pid_t pid;int status;

if((pid = fork()) < 0) err_sys("fork error");else if (pid == 0) exit(7); /* child */

if(wait(&status) != pid) err sys("wait error"); /* wait for child */p _ ypr_exit(status);

if((pid = fork()) < 0) err_sys("fork error");else if (pid == 0) /* child */abort(); /* generates SIGABRT */abort(); /* generates SIGABRT */

if(wait(&status) != pid) err_sys("wait error"); /* wait for child */pr_exit(status);

if((pid = fork()) < 0) err_sys("fork error");else if (pid == 0) /* child */status /= 0; /* divide by 0 generates SIGFPE */

if(wait(&status) != pid) err_sys("wait error"); /* wait for child */pr_exit(status);

exit(0);


( );}


실행 결과



프로세스 ID


프로세스 종료 (전체 생략)

레이스 컨디션


기타


Race Conditions (1/2)Race Conditions (1/2)APUE (Process Control)

Multiple processes share some data.

Outcome depends on the order of their execution (i e RACE)Outcome depends on the order of their execution (i.e. RACE)

• (Process A) x = 20;

(Process B) x + 10;• (Process B) x += 10;

After fork(), we cannot predict if the parent or the child runs first!

The order of execution depends on:

• System Load

• Kernel’s Scheduling Algorithm


Race Conditions (2/2)Race Conditions (2/2)APUE (Process Control)

For parent to wait for child,

• Call wait(), waitpid(), wait3(), wait4()Call wait(), waitpid(), wait3(), wait4()

• Use signals or other IPC methods

For child to wait for parentFor child to wait for parent,

• while(getppid() != 1) sleep(1); // Parent가 죽을 때까지 기다림

• Use signals or other IPC methods• Use signals or other IPC methods


예제예제: race.c (1/2): race.c (1/2)APUE (Process Control)

#include <stdio.h> // race.c#include <sys/types.h>

err sys(char *p) { perror(p); exit( 1); }err_sys(char *p) { perror(p); exit(-1); }

int main(void) {pid_t pid;

if ((pid = fork()) < 0) err_sys("fork error");else if (pid == 0) charatatime("output from child\n");else charatatime("output from parent\n");exit(0);exit(0);

}

charatatime(char *str){{

char *ptr;int c;

for (ptr = str; c = *ptr++; ) {for (ptr = str; c = *ptr++; ) {putc(c, stdout);fflush(stdout);usleep(1);

}


}}

예제예제: race.c (2/2): race.c (2/2)APUE (Process Control)

실행 결과


How to Avoid Race Condition?How to Avoid Race Condition?APUE (Process Control)

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

i t i ( id) {int main(void) {pid_t pid;

TELL_WAIT();

if ((pid = fork()) < 0) err_sys("fork error");else if (pid == 0) {WAIT_PARENT(); // parent goes firstcharatatime("output from child\n");

} l {} else {charatatime("output from parent\n");TELL_CHILD(pid);

}exit(0);exit(0);

}

How to implement TELL WAIT() WAIT PARENT() and TELL CHILD()?How to implement TELL_WAIT(), WAIT_PARENT(), and TELL_CHILD()?

• Use signals (강의노트 15)

• Use IPC methods (강의노트 16)


Use IPC methods (강의노트 16)


프로세스 ID


프로세스 종료

레이스 컨디션


기타


Program Execution: exec() (1/2)Program Execution: exec() (1/2)APUE (Process Control)

When a process calls one of the exec() functions

• that process is completely replaced by the new program (새로운 프로그램으로 대체)p p y p y p g (새 운 램 대체)

(text, data, heap, and stack segments)

• and the new program starts at its main function

함수 exec()를 호출하여 완전히 다른 프로그램으로 실행된다.


Program Execution: exec() (2/2)Program Execution: exec() (2/2)APUE (Process Control)

BEFORE exec()pid: 12791 pid: 12791

AFTER exec()

main() {...execl("newpgm", ...);

CODEmain() {

...CODE

p p/*newpgm*/

...}

DATASTACK

}DATASTACK

USER AREA USER AREA


exec() Functionsexec() FunctionsAPUE (Process Control)

#include <unistd.h>

int execl(const char *pathname const char *arg0 (char *)0);int execl(const char pathname, const char arg0, … , (char )0);

int execv(const char *pathname, const char *argv[]);

int execle(const char *pathname, const char *arg0, …

/* (char*) 0, char *const envp[] */);( ) , p[] )

int execve(const char *pathname, const char *argv[],char *const envp[]);

int execlp(const char *filename, const char *arg0, … , (char *)0);

int execvp(const char *filename, const char *argv[]);

All six return: -1 on error, no return on success

exec? (p, l, v, e)exec? (p, l, v, e)

• p: filename (not pathname)

• l: takes a list of arguments (the last argument should be a null pointer)

• v: takes argv[] vector

• e: takes envp[] array i h ‘ ’ h i i bl f h lli i d


without ‘e’, the environment variables of the calling process are copied

Properties inherited to the new programProperties inherited to the new programAPUE (Process Control)

same ID

• process ID, parent process ID, real user ID, real group ID, supplementary group IDs, p , p p , , g p , pp y g p ,process group ID, session ID

controlling terminal

time left until alarm clock

current working directoryg y

root directory

file mode creation maskfile mode creation mask

file locks

process signal mask

pending signals


resource limits, …

예제예제: : nexec.cnexec.c, , echoall.cechoall.c ((1/4)1/4)APUE (Process Control)


예제예제: : nexec.cnexec.c, , echoall.cechoall.c (2/4)(2/4)APUE (Process Control)


예제예제: : nexec.cnexec.c, , echoall.cechoall.c (3/4)(3/4)APUE (Process Control)


APUE (Process Control)예제예제: : nexec.cnexec.c, , echoall.cechoall.c (4/4)(4/4)

실행 결과



프로세스 ID


프로세스 종료

레이스 컨디션


기타


system()system()APUE (Process Control)

#include <stdlib.h>

int system(const char *cmdstring);

주어진 스트링(cmdstring)을 Shell 상에서 수행시킨다.

• e.g.) system(“date > file”);

system() is implemented by calling fork, exec, and waitpid.

Return values:

• -1 with errno: fork or waitpid fails

• 127: exec fails

• Termination status of shell: all 3 functions succeed


예제예제: : system.csystem.c (1/2)(1/2)APUE (Process Control)

#include <sys/types.h> // system.c#include <sys/wait.h>#include <errno.h>#include <unistd.h>int system(const char *cmdstring) /* version without signal handling */{

pid_t pid; int status;

if( d i )if(cmdstring == NULL)return(1); /* always a command processor with Unix */

if((pid = fork()) < 0) {status = 1; /* probably out of processes */status = -1; /* probably out of processes */

} else if (pid == 0) { /* child */execl("/bin/sh", "sh", "-c", cmdstring, (char *) 0);exit(127); /* execl error */_exit(127); / execl error /

} else { /* parent */while (waitpid(pid, &status, 0) < 0)if (errno != EINTR) {status = -1; /* error other than EINTR from waitpid() */break;

}}


return(status);}

예제예제: : system.csystem.c (2/2)(2/2)APUE (Process Control)


예제예제: myls.c (1/2): myls.c (1/2)APUE (Process Control)

#include <stdio.h> // myls.c

main(int ac char *av[])main(int ac, char *av[]){

int i;char cmdstr[1024];[ ];

strcpy(cmdstr, "/bin/ls ");

for(i=1;i < ac;i++) {strcat(cmdstr, av[i]);strcat(cmdstr, " ");

}fprintf(stdout, "cmdstr = \"%s\"\n", cmdstr);

t ( d t )system(cmdstr);

exit(0);}


}

예제예제: myls.c (2/2): myls.c (2/2)APUE (Process Control)

실행 결과


Process Process TimesTimes –– skip skip APUE (Process Control)

#include <sys/times.h>

clock_t times(struct tms *buf);Returns: elapsed wall clock time in clock ticks if OK 1 on errorReturns: elapsed wall clock time in clock ticks if OK, -1 on error

struct tms (clock_t tms_utime; /* user cpu time */clock t tms stime; /* system cpu time */clock_t tms_stime; /* system cpu time */clock_t tms_cutime; /* child user cpu time */clock_t tms_cstime; /* child system cpu time */

}

Wall clock time: the amount of time the process takes to run and depends on the system loads. (실제 수행된 시간)p y

User CPU time: attributed to user instructions(사용자 코드에 의해 CPU를 점유한 시간)

System CPU time: attributed to the kernel, when it executes on behalf of the process (시스템 코드에 의해 CPU를 점유한 시간)


예제예제: : times.ctimes.c (1/3(1/3) ) –– skip skip APUE (Process Control)

#include <stdio.h> // times.c#include <unistd.h>#include <sys/times.h>

err_sys(char *p) { perror(p); exit(-1); }

int main(int argc, char *argv[]) {int i;for (i = 1; i < argc; i++)for (i = 1; i < argc; i++)do_cmd(argv[i]); /* once for each command-line arg */

exit(0);}

do_cmd(char *cmd) /* execute and time the "cmd" */{

int status;clock_t start, end;struct tms tmsstart tmsend;struct tms tmsstart, tmsend;

fprintf(stderr, "\ncommand: %s\n", cmd);if ( (start = times(&tmsstart)) == -1) /* starting values */err sys("times error");_ y ( );

if ( (status = system(cmd)) < 0) /* execute command */err_sys("system() error");

if ( (end = times(&tmsend)) == -1) /* ending values */err_sys("times error");ti ( d t t &t t t &t d)


pr_times(end-start, &tmsstart, &tmsend);}


pr_times(clock_t real, struct tms *tmsstart, struct tms *tmsend){

static long clktck = 0;

if(clktck == 0) clktck = sysconf(_SC_CLK_TCK);

\ /fprintf(stderr, " real: %7.2f\n", real / (double) clktck);fprintf(stderr, " user: %7.2f\n",

(tmsend->tms_utime - tmsstart->tms_utime) / (double) clktck);fprintf(stderr, " sys: %7.2f\n",

/(tmsend->tms_stime - tmsstart->tms_stime) / (double) clktck);fprintf(stderr, " child user: %7.2f\n",

(tmsend->tms_cutime - tmsstart->tms_cutime) / (double) clktck);fprintf(stderr, " child sys: %7.2f\n",

( d i i ) / (d bl ) lk k)(tmsend->tms_cstime - tmsstart->tms_cstime) / (double) clktck);}

sysconf(_SC_CLK_TCK) ticks per second 값을 리턴함



실행 결과


apue (process control)cs.kangwon.ac.kr/~ysmoon/courses/2010_2/us/14.pdf · 2016. 6. 2. · process...

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