[unix programming] process3

[Unix Programming][Unix Programming]Process3Process3

Young-Ju, HanYoung-Ju, Han

Email: Email: yjhan@imtl.skku.ac.kr

2007 UNIX Programming2007 UNIX Programming 22

ContentsContents

inherited data and file descriptors synchronizing processes process attributes

Process id Process ownership Changing of Process ownership Process Time

2007 UNIX Programming2007 UNIX Programming 33

inherited data and file descriptorsinherited data and file descriptors

#include <unistd.h> #include <fcntl.h> int fatal(char *s) {perror(s); exit(1);} void printfos(char *s, int fd){ printf(“%s : %d\n”, lseek(fd,0, SEEK_CUR)); main(){ int fd, pid_t pid; char buf[10];

fd = open(“data”, O_RDONLY); read(fd, buf, 10); printpos(“Before fork”, fd); switch(pid = fork()) { case -1 : fatal(); break; case 0 : printpos(“Child before read”, fd); read(fd, buf, 10); printpos(“Child after read”, fd); break; default : wait((int *)0); printpos(“Parent after wait”, fd);} }

ex) pro_fileBefore fork : 10Child before read : 10Child after read : 20Parent after wait : 20

2007 UNIX Programming2007 UNIX Programming 44

inherited data and file descriptorsinherited data and file descriptors

#include <fcntl.h> main(){ int fd; . fd = open(“data”, O_RDONLY); . fcntl(fd, F_SETFD, 1);

execl(“/bin/ls”,”ls”, (char*) 0); . }

exec and open files close-on-exec flag (FD_CLOEXEC- default : off(0)) If FD_CLOEXEC == 1, exec 를 호출할 때 개방된 file descriptors

를 close 시킴 If FD_CLOEXEC == 0, exec 를 호출할 때 개방된 file descriptors

계속 유지

res = fcntl(fd, F_GETFD, 0);if res == 1 then on else off

2007 UNIX Programming2007 UNIX Programming 55

synchronizing processessynchronizing processes

Child terminates Kernel sends SIGCHLD signal to parent an asynchronous event

Default action for SIGCHLD signal: ignore it

Signal handlers can be defined by users call wait() to fetch the termination status of child

2007 UNIX Programming2007 UNIX Programming 66

synchronizing processessynchronizing processes

wait block until any child has finished return immediately with the termination status of a child, or return immediately with an error (if it doesn’t have any child

processes) often called just after a fork call

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

pid_t wait (int *status);

PID of the exiting child-1 = no child exists (ECHILD)

WIFEXITED(status) = 1 : normally exit 0 : stopped by signalWEXITSTATUS(status) = value stored in the high-order bits of status

NULL : ignorevalid pointer : status info. when wait return

2007 UNIX Programming2007 UNIX Programming 77

synchronizing processessynchronizing processes

waitpid waiting for a particular child, supporting for non-blocking

pid == -1; waits for any child process ( =wait) pid > 0; waits for the child whose process ID equals pid pid == 0; waits for any child whose process group ID equal that of

the calling process pid < -1 waits for any child whose process group ID equals the

absolute value of pid

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

pid_t waitpid (pid_t pid, int *status, int options);

child PID0 = if WNOHANG is setting and no use child

-1 = error

0 : no useWNOHANG : return 0 if the child has runningWUNTRACED : the status of any stopped child <sys/wait.h>

2007 UNIX Programming2007 UNIX Programming 88

synchronizing processessynchronizing processes

Macro Description

WIFEXITED(status) True if child is terminated normally (exit or _exit)

WEXITSTATUS(status) : to get exit status (high-order 8 bits)

WIFSIGNALED(status) True if child is terminated abnormally (by receipt of a signal)

WTERMSIG(status) : to fetch the signal number that caused the termination

WCOREDUMP(status) : true if a core file was generated

WIFSTOPPED(status) True if child is currently stopped,

WSTOPSIG(status) : to fetch the signal number that caused the stop

o Macros to examine the termination status

2007 UNIX Programming2007 UNIX Programming 99

synchronizing processessynchronizing processes

ex) status#include <sys/wait.h>#include <unistd.h>#include <stdlib.h>

main(){ pid_t child_pid; int status;

if ( (child_pid = fork()) < 0 ) exit(1);

if (child_pid == 0) {sleep(4); exit(5);}

if ( (child_pid = wait(&status)) == -1 ) exit(2);

if (WIFEXITED(status))

printf(“%d child: Nomal termination, exit status = %d\n”

, child_pid, WEXITSTATUS(status)); else if (WIFSIGNALED(status))

printf(“%d child: Abnomal Termination, signal number = %d\n” ,child_pid, WTERMSIG(status)); else if (WIFSTOPPED(status)

printf(“%d child: stopped,signal number = %d\n”, child_pid, WSTOPSIG(status));

exit(0);}

2007 UNIX Programming2007 UNIX Programming 1010

process attributesprocess attributes

Process-id a non-negative number unique number

getpid(): to get own process id getppid(): to get parent process id

reserved PID PID 0-sched, PID 1-init, PID 2-pageout

Process ownership Real user id & group id for calling process

uid_t getuid() : to get real user id uid_t getgid() : to get real group id

Effective user id & group id for calling process uid_t geteuid(): to get effective user id uid_t getegid(): to get effective group id

2007 UNIX Programming2007 UNIX Programming 1111

process attributesprocess attributes

Change ownership

#include <sys/types.h>#include <unistd.h>// for real user id and real group idint setuid (uid_t uid); Int setgid(gid_t gid);

// for effective user id and group idint seteuid(uid_t uid);int setegid(gid_t gid);

// for both real user id(group id) and effective user id(groupid)int setreuid(uid_t ruid, uid_t euid); // if don’t want to change,int setregid(gid_t rgid, gid_t egid); // ruid or egid = -1

0 : ok-1 : error

2007 UNIX Programming2007 UNIX Programming 1212

process attributesprocess attributes

A process has three user ID that the kernel maintains Real User ID(ruid)

Only superuser process can change ruid Normally ruid is set login program (superuser process :

using setuid)

Effective User ID (euid) Set by the exec function only if the set-user-ID bit is set for

the program file If the set-user-ID is not set, the exec function leave the euid

as its current value By calling setuid, euid is set to ether the ruid or saved set-

user-ID

2007 UNIX Programming2007 UNIX Programming 1313

process attributesprocess attributes

Saved set-user-ID Is copied from the euid by exec This is saved after exec stores the euid from the file’s user id (if the

file’s set_user_ID bit is set) But, 4.3BSD is not supported

ID

exec setuid(uid) seteuid(uid)

Set-user-id bit off

Set-user-id bit on superuserUnprivilege

d usersuperuer

Unprivileged user

ruid Unchanged UnchangedSet to uid

( 임의의 uid)Unchanged Unchanged Unchaged

euid UnchangedSet from user ID of program file

Set to uid

( 임의의 uid)

Set to uid

(ruid or saved-set-user-id 만 )

Set to uid

( 임의의 uid)

Set to uid

(ruid or saved-set-user-id 만 )

Saved

set-user-id

Copied by effective user

id

Copied from effective user id

Set to uid

( 임의의 uid)Unchanged unchanged unchagned

2007 UNIX Programming2007 UNIX Programming 1414

process attributesprocess attributes

Example of setuid Caber-toss game program Game user 에 의해서만 game 의 점수 파일 접근 가능

-rwsr-xr-x 1 games games 184422 Jul 30 15:17 caber-toss-rw------- games games 1435 Jul 30 15:17 /tmp/score

#include <stdio.h>#include <sys/types.h>#include <unistd.h>#include <stdlib.h> /* euid 와 ruid 를 저장하라 . */ static uid_t euid, ruid;

/* 원래의 값으로 euid 를 반환하라 . */ void do_setuid() {

int status; #ifdef _POSIX_SAVED_IDS

status = setuid (euid); // [myid, games, games]#else

status = setreuid (ruid, euid); // [myid, games]#endif

if(status < 0){ fprintf (stderr,"Couldn't set uid.\n"); exit (status); }}

2007 UNIX Programming2007 UNIX Programming 1515

process attributesprocess attributes

Example of setuid [real uid, effective uid, saved-set-uid]/* ruid 로 euid 를 설정하라 . */ void undo_setuid(){

int status; #ifdef _POSIX_SAVED_IDS status = setuid (ruid); // [myid, myid, games]

#else status = setreuid (euid, ruid); // [games, myid]#endif

if(status < 0) { fprintf(stderr,"Couldn't set uid.\n"); exit(status);}}

/* main 프로그램 */ int main(void){

/* ruid 와 euid 들을 저장하라 . */ruid = getuid (); // [myid, games]

euid = geteuid (); // [myid, games]undo_setuid();

/* 게임을 하고 점수를 기록하라 . */ record_score(300);}

2007 UNIX Programming2007 UNIX Programming 1616

process attributesprocess attributes

Example of setuid

#define SCORES_FILE “/tmp/score”

/* 점수를 기록하라 . */ int record_score (int score){

FILE *stream; char *myname = “myname”;

/* 점수파일을 개방하라 . */

do_setuid (); // [myid, games] stream = fopen (SCORES_FILE, "a"); undo_setuid ();

/* 파일에 그 점수를 기록하라 . */ if(stream){

fprintf (stream, "%10s: %d feet.\n", myname, score); fclose (stream);

return 0; } else return -1;}

2007 UNIX Programming2007 UNIX Programming 1717

process attributesprocess attributes

Summary of all the functions that set the different usr IDs

realuser ID

effectiveuser ID

savedset-user-ID

superusersetreuid(ruid, euid)

superusersetuid(uid)

superuserseteuid(uid)

exec ofset-user-ID

unpriviligedsetuid or seteuid unpriviliged

setuid or seteuid

unpriviligedsetreuid

unpriviligedsetreuid

ruideuid uid uid

uid

uid

2007 UNIX Programming2007 UNIX Programming 1818

process attributesprocess attributes

I want to know how much time has been taken by my process and/or my child process Use command time Use times() function in your program

times() functions

Returns : elapsed wall clock time in clock ticks if OK, -1 on error

Function sysconf(_SC_CLK_TCK) returns # of clock ticks per second

#include <sys/times.h>clock_t times(struct tms *buf);

2007 UNIX Programming2007 UNIX Programming 1919

process attributesprocess attributes

Parameters struct tms *buf : 현재 process와 그것의 child process가

사용한 user CPU time, system CPU time 값을 가져올 구조체

tms_utime: times() 가 호출될때까지 process 가 사용한 user CPU time (the time spent for user instructions)

tms_stime: times() 가 호출될때까지 process 가 사용한 system CPU time (the time spent for system calls)

tms_cutime: times() 가 호출되기전에 , 수행을 마친 child process 들의 (tms_utime + tms_cutime) 값

tms_cstime: times() 가 호출되기전에 , 수행을 마친 child process 들의 (tms_stime + tms_cstime) 값

struct tms {clock_t tms_utime; /* user CPU time */clock_t tms_stime; /* system CPU time */clock_t tms_cutime; /* user CPU time, terminated child*/clock_t tms_cstime; /* system CPU time, terminated child */

}