PROCESS CONTROL

Topics covered

Introduction

Process identifiers

Fork function

vfork function

Exit function

wait and waitpid function

Wait3 and wait4 functions

Race conditions

Exec conditions

Changing user ID’s and Group ID’s

Interpreter files

System function

User identification

Process times

Process acounting

Introduction

Process control – concerned – creation of new processes , program execution and process termination

Process Identifiers

1. Process ID (pid)

- unique process ID

- Function to retrieve – getpid

prototype

#include<unistd.h>

pid_t getpid(void);

Returns : process ID of calling process

2. Parent Process ID ( ppid) pid retrieved – getppid prototype pid_t getppid(void); Returns : parent process ID of the calling process

3. Real user ID (ruid) is user created – parent process prototype uid_t getuid(void); Returns : real user ID of the calling process

4. Effective user ID (euid) - same as real user id , except file executed – create process – set – uid

turned on prototype : uid_t geteuid(void) Returns : effective user ID of calling process 5. Real group ID (gid) : - id - user – parent process prototype : gid_t getgid(void); Returns : real group ID of calling proces

6. Effective Group ID (egid) :

same as real group id , except file executed – create process – set – gid flag

turned on

function ptototype :

gid_t getegid(void);

Returns : effective ID of the calling process

Fork function

- process created – existing process – calls fork function

#include <unistd.h>

pid_t fork(void);

Returns : 0 in child , process ID of child in parent , -1 on error

- new process created – fork – child process

Program to demonstrate fork function

#include <sys/types.h>

#include <unistd.h>

int main( )

{

fork( );

printf(“\n hello USP”);

}

Properties of parent inherited by child

• Real user ID , real group ID , effective user ID , effective group ID

• Supplementary group ID’s

• Process group ID

• Session ID

• Controlling terminal

• Set -user-id and set-group-id

• Current working directory

• Root directory

• File mode creation mask

• Signal masks and dispositions

• Close – on – exec flag for any open file descriptors

• Environment

• Attached shared memory segments

• Memory mappings

• Resource limits

Differences between parent and child are

• Return value from fork

• Process IDs are different

• The 2 processes have different parent process IDs : parent process ID of the

child is the parent ; the parent process ID of the parent doesn’t change

• The child’s tms_utime , tms_stime , tms_cutime , and tms_cstime values

are set to 0

• File locks set by the parent are not inherited by the child

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

2 reasons for calling fork system call are :

• Process – duplicates itself so that parent and child can each execute

different sections of code at the same time

• Common for network servers

• Process – wants to execute different program

• Common for shells

vfork function

- same calling sequence and same return values as that of fork

- intended – create – new process

Prototype of vfork is

#include<unistd.h>

#include<sys/types.h>

pid_t vfork(void);

Returns : 0 in child , process ID of child in parent , -1 on error

Example of vfork#include<unistd.h>#include<sys/types.h>int glob = 6;int main(void){ int var; pid_t pid; var = 88; printf(“before vfork\n”); if((pid=vfork( ))<0){

err_sys(“vfork error”);}else if (pid= = 0){glob++;var++;_exit(0);}

Exit functions

Process can be terminated in 5 ways

• Executing return from main function

• Calling exit function

• Calling the _exit or _Exit

• Executing a return from the start routine of the last thread in the process

• Calling the pthread_exit function from the last thread in the process

3 forms of abnormal termination

• Calling abort

• When the process receives certain signals

• The last thread responds to a cancellation request

Wait and waitpid functions

Parent process – use – wait – waitpid – wait – child process – terminate and

retrieve the child exit status

prototype

#include<sys/wait.h>

pid_t wait(int *statloc);

#include<sys/wait.h>

pid_t wait(int *statloc);

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

both return : process ID if ok and -1 on error

Differences between wait and waitpid

Wait – block – caller , waitpid – option – prevents – blocking

Waitpid – doesn’t wait – child – terminates

Wait – child terminated

Parent can check the exit status code with the following macro’s defined in <sys/wait.h>

• WIFEXITED

• WEXITSTATUS

• WIFSIGNALLED

• WTERMSIG

• WIFSTOPPED

• WSTOPSIG

Wait3 and wait4 functions - provide additional arguments – allows – kernel – return – summary –

resources – used – terminated process prototype #include < sys/types.h> #include <sys/wait.h> #include <sys/time.h> #include<sys/resource.h> pid_t wait3(int *statloc , int options , struct rusage *rusage); pid_t wait4(pid_t pid , int *statloc , int options , struct rusage *rusage); both return : process ID if ok , -1 on error

Race conditions• Occurs – multiple processes – trying – something – shared data – final

outcome – depends – order – processes runs• To avoid race condition and avoid poling – signaling required – multiple

processes• Program that contains a race condition : #include<stdlib.h> #include<sys/types.h> static void charatatime(char *); int main (void) { pid_t pid;

if((pid=fork( ))<0){ printf(“fork error”);}else if(pid = = 0){ charatatime(“output from child\n”);}else{ charatatime(“output from parent\n”); }

exit(0);}

static void charatatime(char *str){ char *ptr; int c; stdbuf(stdout , NULL); for (ptr = str; (c = *ptr++) ! = 0) putc(c, stdout);}• To avoid race condition TELL and WAIT functions can be used in above

program

Exec functions• Causes – calling process – change – context – execute – different program• 6 versions of exec system calls• Same function – differ – argument lists• Process ID does not change – new process – not createdPrototype of these functions#include <unistd.h>int execl(const char *pathname , const char *arg0 , …/* (char *) 0 */ );int execv(const char *pathname , char *const argv [ ]);int execle(const char *pathname , const char *arg0,…/*(char *)0, char *constenvp */ );

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

int execlp(const char *filename, const char *ag0,…/* (char *)0 */ );

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

Return : -1 on error , no return on success

- 1st difference – 1st 4 – pathname – argument , last 2 – filename argument

- Filename - / - pathname

- Executable file – searched – directories specified – PATH environment

variable

Additional properties inherited from calling process

• Process ID and parent process ID

• Real user ID and real group ID

• Supplementary group ID’s

• 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

Changing user ID’s and group ID’s

1.Setuid and setgid functions

Real group ID and effective group ID - set – setgid

Prototype :

#include<unistd.h>

int setuid(uid_t uid);

int setgid(gid_t gid);

Return : 0 if ok , 1 on error

Rules to change these ID’s

• Process – superuser privileges – setuid function – sets – real user ID ,

effective user ID , saved user ID – uid

• Process – no – superuser privileges – uid – equals – either – real user ID –

saved set-user-ID

• Errno – EPERM – 1 RETURNED

• Superuser process – change – real user ID• Effective user ID – set – exec functions – set user ID set – program file• Saved set user ID – copied – effecctive user ID – exec

Setreuid and setregid functions

• Swapping – real user ID , effective user ID – setreuid function

#include <unistd.h>

int setreuid(uid_t ruid , uid_t euid);

int setregid(gid_t rgid , gid_t egid);

Return : 0 if ok , -1 on error

Seteuid and setegid functions

- Similar – setuid and setgid , effective user ID and effcetive group ID

changed

#include <unistd.h>

int seteuid(uid_t uid);

int setegid(gid_t gid);

Return : 0 if ok , 1 on error

Interpreter files

- Text files – begin – line of form

# ! pathname [ optional – argument]

- Space – exclamation – pathname- optional

- Most common

# !/bin/sh

Program

#include < sys/types.h>

#include <sys/wait.h>

int main (void)

{

pid_t pid;

if ((pid = fork( )) < 0 ){ printf(“fork error”);}else if( pid = = 0){ /* child*/if (execl(“/home/vtu/testinterp” , “testinterp” , “myarg1 “ ,”MY ARG2” , (char * )

0) < 0 )printf(“execl error”);}if(waitpid(pid , NULL , 0) < 0 ) /*parent*/printf(“waitpid error”);exit(0);}

System function

- Used – execute – command string – program

- Can be called as

system(“date > file “);

Prototype :

#include <stdlib.h>

int system(const char *cmdstring);

- System function – implemented – calling fork , exec and waitpid

- Calls – fork – create – child process

- Inturn – calls – execlp – execute – bourne shell with –c option

3 types of return values

• either – fork – fails – waitpid – returns – error – system returns 1 – errno –

set – indicate – error

• Exec – fails – return value – shell – executed – exit

• All 3 succeed – return value – termination status

User identification

System – keeps – track – name – logged in

getlogin – way – fetch – login name

Prototype :

#include <unistd.h>

char *getlogin(void);

Return : pointer to string giving login name if ok , NULL on error

Process times

3 values of a process maintained

• Wall clock time

• System CPU time

• User CPU time

Prototype :

#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;

clock_t tms_stime;

clock_t tms_cutime;

clock_t tms_cstime;

};

Process accounting

- When enabled – kernel – writes – acounting record – process terminates

- Accounting records – small – amount – binary data – command name , amount

of CPU time used , user ID , group ID

- Superuser – executes – accton – pathname argument – enable accounting

- Structure – accounting records – defined - <sys/acct.h>

struct acct

{

char ac_flag;char ac_stat;uid_t ac_uid;gid_t ac_gid;dev_t ac_tty;time_t ac_btime;comp_t ac_utime;comp_t ac_stime;comp_t ac_etime;comp_t ac_mem;comp_t ac_io;comp_t ac_rw;};

ac_flag description

AFORK process is the result of fork , but never called exec

ASU process used superuser privileges

ACOMPAT process used compatibility mode

ACORE process dumped core

AXSIG process was killed by a signal

AEXPND expanded accounting entry