Linux/UNIX Programming APUE (Process Control) [Ch. 8] 최 미 정 강원대학교 컴퓨터과학전공

Linux/UNIX ProgrammingLinux/UNIX Programming

APUE (Process Control) [Ch. APUE (Process Control) [Ch. 8]8]

최 미 정최 미 정강원대학교 컴퓨터과학전공강원대학교 컴퓨터과학전공

UNIX System ProgrammingPage 2

강의 내용강의 내용

프로세스 ID

프로세스 생성

프로세스 종료

레이스 컨디션

프로그램 실행

기타

APUE (Process Control)


Process Identifiers (1/2)Process Identifiers (1/2)

Every process has a unique process ID, a nonnegative integer.

( 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)

Process ID 1: init process

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

• /etc/init or /sbin/init

• 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



System Process System Process 예제예제APUE (Process Control)


PID PID 관련 함수관련 함수

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



프로세스 ID

프로세스 생성

프로세스 종료

레이스 컨디션

프로그램 실행

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 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

Parent and child continue executing instructions

following the fork() call

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



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

DATA

STACK

USER AREA

TEXT

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

DATA

STACK

USER AREA

TEXT

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

DATA

STACK

USER AREA

TEXT

BEFORE fork()

AFTER fork()

pid: 12791

pid: 12791 pid: 12793


예제예제 : fork.c (1/2): fork.c (1/2)

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

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

int tglob = 6; /* external variable in initialized data */

int main(void) { int tvar; /* automatic variable on the stack */ pid_t pid; tvar = 88;

if ((pid = fork()) < 0) err_sys("fork"); else if(pid == 0) { /* child */ tglob++; /* modify variables */ tvar++; } else /* parent */ sleep(2); printf("pid = %d, tglob = %d, tvar = %d\n", getpid(), tglob, tvar); exit(0);}



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 descriptorsfd 0:fd 1:fd 2:

. . . .

fd flags ptr

file tablefd status flagscurrent file offsetv-node ptr

fd status flagscurrent file offsetv-node ptr

v-node tablev-node

information

i-nodeinformation

current file size

file descriptorsfd 0:fd 1:fd 2:

. . . .

fd flags ptr

child process table entry fd status flags

current file offsetv-node ptr

v-nodeinformation

i-nodeinformation

current file size

v-nodeinformation

i-nodeinformation

current file size


Properties Inherited to the Properties Inherited to the ChildChild

real user and group ID, effective user and group ID

supplementary group IDs

process group ID, session ID

set-user-ID and set-group ID flags

current working directory

root 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 Properties NOT Inherited to the ChildChild

the return value from fork()

the process IDs are different

file locks

pending alarms are cleared for the child

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




프로세스 ID

프로세스 생성

프로세스 종료

레이스 컨디션

프로그램 실행

기타



exit() – Process Terminationexit() – Process Termination

A process can terminate in 5 ways:

Normal Termination

• return from main()

• exit() w/ cleanup procedure

• _exit() w/o cleanup procedure

Abnormal Termination

• calling abort() (generates SIGABRT signal)

• process receives signals



exit() – Termination Statusexit() – Termination Status

Exit Status:

• 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.

• by wait() or waitpid()

What if parent terminates before child?

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



exit() – Child Termination exit() – Child Termination skipskip

Suppose child terminates first

• 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()

Zombie 가 되지 않도록 하기 위해서는 Parent 가 wait() 해 주어야 함



wait(), waitpid() (1/2)wait(), waitpid() (1/2)

Child terminates

• Kernel sends SIGCHLD signal to parent.

• an asynchronous event

Default action for signal: ignore it.

Signal handlers can be defined by users

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

• ( 무사히 , 죽을 수 있도록 배려 ~)



wait(), waitpid() (2/2)wait(), waitpid() (2/2)

A process that calls wait or waitpid can

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

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

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

statloc

• a pointer to an integer to store the termination status

options: refer to manual of waitpid()


#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


Macros to examine the termination status Macros to examine the termination status skipskip


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.

WIFSTOPPED(status)

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


예제예제 : nwait.c (1/3) : nwait.c (1/3) skip skip

#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))

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

else if (WIFSTOPPED(status))

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

}



예제예제 : nwait.c (2/3) : nwait.c (2/3) skip skip

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 */ pr_exit(status);

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


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

SIGFPE */


exit(0);}



예제예제 : nwait.c (3/3) : nwait.c (3/3) skip skipAPUE (Process Control)

실행 결과



프로세스 ID

프로세스 생성

프로세스 종료

레이스 컨디션

프로그램 실행

기타



Race Conditions (1/2)Race Conditions (1/2)

Multiple processes share some data.

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

• (Process A) x = 20;

• (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)

For parent to wait for child,

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

• Use signals or other IPC methods

For child to wait for parent,

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

• Use signals or other IPC methods



예제예제 : race.c (1/2): race.c (1/2)

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


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);}

charatatime(char *str){ char *ptr; int c;

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>


TELL_WAIT();

if ((pid = fork()) < 0) err_sys("fork error"); else if (pid == 0) { WAIT_PARENT(); // parent goes first charatatime("output from child\n"); } else { charatatime("output from parent\n"); TELL_CHILD(pid); } exit(0);}

How to implement TELL_WAIT(), WAIT_PARENT(), and TELL_CHILD()?

• Use signals (Ch. 10)

• Use IPC methods (Ch. 14 and 15)



프로세스 ID

프로세스 생성

프로세스 종료

레이스 컨디션

프로그램 실행

기타



Program Execution: exec() Program Execution: exec() (1/2)(1/2)

When a process calls one of the exec functions

• that process is completely replaced by the new program ( 새로운 프로그램으로 대체 )

(text, data, heap, and stack segments)

• and the new program starts at its main function

함수 exec() 를 호출하여 완전히 다른 프로그램으로 실행된다 .


#include <unistd.h> /* exec_echo.c */

int main() { execl("/bin/echo", "echo", "execute /bin/echo", (char*) 0); printf("Can this message be printed? ERROR!\n");}

$ a.outexecute /bin/echo


Program Execution: exec() Program Execution: exec() (2/2)(2/2)


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

DATA

STACK

USER AREA

CODE

main() {

...

}

DATA

STACK

USER AREA

CODE

BEFORE exec()pid: 12791 pid: 12791

AFTER exec()

/*newpgm*/


exec() Functionsexec() FunctionsAPUE (Process Control)

#include <unistd.h>

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[] */);

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)

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


Properties inherited to the new Properties inherited to the new programprogram

same ID

• process ID, parent process ID, real user ID, real group ID, supplementary group IDs, process group ID, session ID

controlling terminal

time left until alarm clock

current working directory

root directory

file mode creation mask

file locks

process signal mask

pending signals

resource limits, …



예제예제 : nexec.c, echoall.c (1/3): nexec.c, echoall.c (1/3)

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


char *env_init[] = { "USER=unknown", "PATH=/tmp", NULL };


if ((pid = fork()) < 0) err_sys("fork error"); else if (pid == 0) { /* specify pathname, specify environment */ if (execle("/home/prof/ysmoon/unix/APUE/echoall", "echoall", "myarg1", "MY ARG2", (char *)0, env_init) < 0) err_sys("execle error"); }

if (waitpid(pid, NULL, 0) < 0) err_sys("wait error");

putchar('\n'); if ((pid = fork()) < 0) err_sys("fork error"); else if (pid == 0) { /* specify filename, inherit environment */ if (execlp("echoall", "echoall", "only 1 arg", (char *) 0) < 0) err_sys("execlp error"); } exit(0);}




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

int main(int argc, char *argv[]){ int i; char **ptr; extern char **environ;

for (i = 0; i < argc; i++) /* echo all command-line args */ printf("argv[%d]: %s\n", i, argv[i]);

for (ptr = environ; *ptr != 0; ptr++) /* and all env strings */ printf("%s\n", *ptr);

exit(0);}




실행 결과




프로세스 ID

프로세스 생성

프로세스 종료

레이스 컨디션

프로그램 실행

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.c: system.c

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

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

} else if (pid == 0) { /* child */ execl("/bin/sh", "sh", "-c", cmdstring, (char *) 0); _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 : myls.c (1/2): myls.c (1/2)

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

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);

system(cmdstr);

exit(0);}



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

실행 결과


Process TimesProcess TimesAPUE (Process Control)

#include <sys/times.h>

clock_t times(struct tms *buf); Returns: 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_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 )

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

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


예제예제 : times.c (1/3): times.c (1/3)

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


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

fprintf(stderr, "\ncommand: %s\n", cmd); if ( (start = times(&tmsstart)) == -1) /* starting values */ err_sys("times error"); if ( (status = system(cmd)) < 0) /* execute command */ err_sys("system() error"); if ( (end = times(&tmsend)) == -1) /* ending values */ err_sys("times error"); pr_times(end-start, &tmsstart, &tmsend);}



예제예제 : times.c (2/3): times.c (2/3)

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", (tmsend->tms_cstime - tmsstart->tms_cstime) / (double) clktck);}


sysconf(_SC_CLK_TCK) ticks per second 값을 리턴함


예제예제 : times.c (3/3): times.c (3/3)APUE (Process Control)

실행 결과

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Linux/UNIX Programming APUE (Process Control) [Ch. 8] 최 미 정 강원대학교 컴퓨터과학전공