Linux/UNIX Programming APUE (Process Control) 문양세 강원대학교 IT 대학 컴퓨터과학전공

Linux/UNIX Programming

APUE (Process Control)

문양세강원대학교 IT 대학 컴퓨터과학전공

UNIX System Program-mingby Yang-Sae Moon

Page 2

강의 내용

프로세스 ID프로세스 생성

프로세스 종료

레이스 컨디션

프로그램 실행

기타



Page 3

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



Page 4

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



Page 5

System Process 예제APUE (Process Control)


Page 6

PID 관련 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


Page 7

강의 내용


프로세스 종료

레이스 컨디션

프로그램 실행

기타



Page 8

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.


Page 9


Child gets a copy of parent’s data space, heap, and stack • Often, read-only text segment is shared

Parent and child continue executing instructionsfollowing the fork() call

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


Page 10


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

DATASTACK

USER AREA

TEXT

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

DATASTACK

USER AREA

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

DATASTACK

USER AREA

TEXT

BEFORE fork()

AFTER fork()

pid: 12791

pid: 12791 pid: 12793


Page 11

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


Page 12

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

실행 결과


Page 13

Properties Inherited to the Child

real user and group ID, effective user and group IDsupplementary group IDsprocess group ID, session IDset-user-ID and set-group ID flagscurrent working directoryroot directoryfile mode creation masksignal mask and dispositionsthe close-on-exec flag for any open file descriptorsenvironmentattached shared memory segmentsresource limits



Page 14

Properties NOT Inherited to the Child

the return value from fork()the process IDs are differentfile lockspending alarms are cleared for the childthe set of pending signals for the child is set to the empty set



Page 15

강의 내용


프로세스 종료

레이스 컨디션

프로그램 실행

기타



Page 16

exit() – 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



Page 17

wait(), waitpid() – skip

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 ~)



Page 18

강의 내용


프로세스 종료

레이스 컨디션

프로그램 실행

기타



Page 19

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



Page 20

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



Page 21

예제 : race.c (1/2)#include <stdio.h> // race.c#include <sys/types.h>

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);}

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

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



Page 22

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

실행 결과


Page 23

How to Avoid Race Condition?APUE (Process Control)

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

int main(void) { pid_t pid;

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 ( 강의노트 15)• Use IPC methods ( 강의노트 16)


Page 24

강의 내용


프로세스 종료

레이스 컨디션

프로그램 실행

기타



Page 25

Program Execution: exec() (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 .



Page 26

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

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

DATASTACK

USER AREA

CODEmain() {

...

}DATASTACK

USER AREA

CODE

BEFORE exec()pid: 12791 pid: 12791

AFTER exec()

/*newpgm*/


Page 27

exec() 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 process are copied


Page 28

Properties inherited to the new pro-gram

same ID• process ID, parent process ID, real user ID, real group ID, supplementary

group IDs, process group ID, session ID

controlling terminaltime left until alarm clockcurrent working directoryroot directoryfile mode creation maskfile locksprocess signal maskpending signalsresource limits, …



Page 29

예제 : nexec.c, echoall.c (1/4)APUE (Process Control)


Page 30



Page 31



Page 32


실행 결과

예제 : nexec.c, echoall.c (4/4)


Page 33

강의 내용


프로세스 종료

레이스 컨디션

프로그램 실행

기타



Page 34

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


Page 35

예제 : 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);}



Page 36

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

실행 결과


Page 37

Process Times – 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 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 )

Documents

Linux/UNIX Programming APUE (Process Control) 문양세 강원대학교 IT 대학 컴퓨터과학전공