UNIX System Programming - UCY · UNIX System Programming • Objectives look at how to program UNIX processes fork( ), wait( ) Processes

UNIX System Programming

• Objectives ▫ look at how to program UNIX processes ▫ fork( ), wait( )

Processes

Overview

1. What is a Process? 2. fork() 3. wait() 4. Process Data

1. What is a Process?

A process is the context (the information/data) maintained for an executing program. Intuitively, a process is the abstraction of a physical

processor. Exists because it is difficult for the OS to otherwise coordinate many concurrent

activities, such as incoming network data, multiple users, etc.

IMPORTANT: A process is sequential

What makes up a Process?

program code machine registers global data stack open files (file descriptors) an environment (environment variables;

credentials for security)

Some of the Context Information

▫ Process ID (pid) unique integer ▫ Parent process ID (ppid) ▫ Real User ID ID of user/process

which started this process

▫ Effective User ID ID of user who wrote the process’ program

continued

Important System Processes

init – Mother of all processes. init is started at boot time and is responsible for starting other processes. init uses file inittab & directories: /etc/rc?.d getty – login process that manages login

sessions.

Unix Start Up Processes Diagram OS kernel

Process 0 (sched)

Process 1 (init)

getty getty getty

login

csh

login

bash

Pid and Parentage

A process ID or pid is a positive integer that uniquely identifies a running process, and is stored in a variable of type pid_t. You can get the process pid or parent’s pid #include <sys/types> main() { pid_t pid, ppid; printf( "My PID is:%d\n\n",(pid = getpid()) ); printf( "Par PID is:%d\n\n",(ppid = getppid()) ); }

2. fork()

#include <sys/types.h> #include <unistd.h> pid_t fork( void );

Creates a child process by making a copy of the

parent process --- an exact duplicate. Implicitly specifies code, registers, stack, data,

files Both the child and the parent continue running.

fork() as a diagram

Parent

pid = fork()

Returns a new PID: e.g. pid == 5

Data

Shared Program Data

Copied

Child

pid == 0

Process IDs (pids revisited)

pid = fork(); In the child: pid == 0;

In the parent: pid == the process ID of the child. A program almost always uses this pid

difference to do different things in the parent and child.

fork() Example (parchld.c)

#include <stdio.h> #include <sys/types.h> #include <unistd.h> int main() {

pid_t pid; /* could be int */ int i; pid = fork(); if( pid > 0 )

{ /* parent */ for( i=0; i < 1000; i++ ) printf(“\t\t\tPARENT %d\n”, i); }

else { /* child */ for( i=0; I < 1000; i++ )

printf( “CHILD %d\n”, i ); } return 0; }

Possible Output

CHILD 0 CHILD 1 CHILD 2 PARENT 0 PARENT 1 PARENT 2 PARENT 3 CHILD 3 CHILD 4 PARENT 4 :

Things to Note

i is copied between parent and child. The switching between the parent and child

depends on many factors: machine load, system process scheduling

I/O buffering effects amount of output shown.

Output interleaving is nondeterministic cannot determine output by looking at code

3. wait() #include <sys/types.h> #include <sys/wait.h> pid_t wait(int *statloc); Suspends the calling process until one of its child processes

ends. If the system already has information about a terminated child process when wait is called, the return from wait occurs immediately.

Status information for the terminating child process is usually stored at the location pointed to by *statloc. if wait is called with NULL as the *statloc value, no status information is returned.

If successful, wait returns the process ID of the child process. If unsuccessful, a -1 is returned.

wait() Actions

A process that calls wait() can: suspend (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 there are no

child processes.

4. Process Data

Since a child process is a copy of the parent, it has copies of the parent’s data. A change to a variable in the child will not

change that variable in the parent.

PID=981

Δεδομένα :p = ? mypid = ?

Κείμενο:

pid_t p, mypid;…p = fork();if (p < 0) {

perror(“fork”);exit(1);

} else if (p == 0) {mypid = getpid();child();

} else {father();

}

ΔημιουργίαΔημιουργία στοστο μοντέλομοντέλο τουτου UNIX: fork()UNIX: fork()

PID=981


Κείμενο:




} else {father();

}


PID=981


Κείμενο:




} else {father();

}


PID=981


Κείμενο:




} else {father();

}


p = -1, errno = ENOMEM

-1

PID=981


Κείμενο:




} else {father();

}



-1

PID=981


Κείμενο:




} else {father();

}



-1

PID=981


Κείμενο:




} else {father();

}



-1


PID=981


Κείμενο:




} else {father();

}


PID=981


Κείμενο:




} else {father();

}


PID=981


Κείμενο:




} else {father();

}


PID=987


Κείμενο:




} else {father();

}

PID=981


Κείμενο:




} else {father();

}

987


p = 987

PID=987


Κείμενο:




} else {father();

}

p = 0

0

PID=981


Κείμενο:




} else {father();

}

987


p = 987

PID=987


Κείμενο:




} else {father();

}

p = 0

0

PID=981


Κείμενο:




} else {father();

}

987


p = 987

PID=987


Κείμενο:




} else {father();

}

p = 0

0

PID=981


Κείμενο:




} else {father();

}

987


p = 987

PID=987


Κείμενο:




} else {father();

}

p = 0

0

PID=981


Κείμενο:




} else {father();

}

987


p = 987

PID=987


Κείμενο:




} else {father();

}

p = 0

9870

PID=981


Κείμενο:




} else {father();

}

987


p = 987

PID=987


Κείμενο:




} else {father();

}

p = 0

9870

PID=981


Κείμενο:




} else {father();

}

987


p = 987

PID=987


Κείμενο:




} else {father();

}

p = 0

9870


� Όλες οι διεργασίες προκύπτουν με fork() [σχεδόν όλες]� Ίδιο πρόγραμμα με γονική διεργασία, αντίγραφο χώρου μνήμης,

κληρονομεί ανοιχτά αρχεία, συνδέσεις, δικαιώματα πρόσβασης� Αντικατάσταση προγράμματος διεργασίας: execve()� Η γονική διεργασία ενημερώνεται για το θάνατο του

παιδιού με wait() → συλλογή τιμής τερματισμού (exit status)� Μέχρι τότε, παιδί που έχει καλέσει την exit() είναι zombie� Αν ο γονέας πεθάνει πρώτα, η διεργασία γίνεται παιδί της

init (PID = 1), που κάνει συνεχώς wait()

prog-A981

Prog-A981

fork

Prog-A 987

execveProg-B

987 exit

Prog-B 987 (zombie)

wait

wait() / wait() / waitpidwaitpid()()

� Για κάθε fork() πρέπει να γίνει ένα wait()� wait(&status)

� Μπλοκάρει έως οποιοδήποτε παιδί πεθάνει� Το status κωδικοποιεί πώς πέθανε η διεργασία

� Κανονικά (exit()), λόγω κάποιου σήματος (SIGTERM, SIGKILL)� Χρήσιμες μακροεντολές για την ερμηνεία του status

� WIFEXITED(), WEXITSTATUS(), WIFSIGNALED(), WTERMSIG()� σας δίνεται η explain_wait_status()

� Μια πιο ευέλικτη wait(): waitpid()� Περιμένει για αλλαγή κατάστασης συγκεκριμένου ή

οποιουδήποτε PID διεργασίας-παιδιού� Συμπεριφορά ελεγχόμενη από flags (WNOHANG, WUNTRACED)

explain_wait_statusexplain_wait_status()()

void explain_wait_status(pid_t pid, int status){if (WIFEXITED(status))

fprintf(stderr, "Child with PID = %ld terminated normally, exit status = %d\n",(long)pid, WEXITSTATUS(status));

else if (WIFSIGNALED(status))fprintf(stderr, "Child with PID = %ld was terminated by a signal, signo = %d\n",

(long)pid, WTERMSIG(status));else if (WIFSTOPPED(status))

fprintf(stderr, "Child with PID = %ld has been stopped by a signal, signo = %d\n",(long)pid, WSTOPSIG(status));

else {fprintf(stderr, "%s: Internal error: Unhandled case, PID = %ld, status = %d\n",

__func__, (long)pid, status);exit(1);

}fflush(stderr);

}

explain_wait_statusexplain_wait_status()()

void explain_wait_status(pid_t pid, int status){if (WIFEXITED(status))

fprintf(stderr, "Child with PID = %ld terminated normally, exit status = %d\n",(long)pid, WEXITSTATUS(status));

else if (WIFSIGNALED(status))fprintf(stderr, "Child with PID = %ld was terminated by a signal, signo = %d\n",

(long)pid, WTERMSIG(status));else if (WIFSTOPPED(status))

fprintf(stderr, "Child with PID = %ld has been stopped by a signal, signo = %d\n",(long)pid, WSTOPSIG(status));

else {fprintf(stderr, "%s: Internal error: Unhandled case, PID = %ld, status = %d\n",

__func__, (long)pid, status);exit(1);

}fflush(stderr);

}

Παράδειγμα:

pid = wait(&status);explain_wait_status(pid, status);if (WIFEXITED(status) || WIFSIGNALED(status))

--processes_alive;

ΚώδικαςΚώδικας: : παράδειγμαπαράδειγμα fork() / wait()fork() / wait()

void child(void){

compute(10000);exit(7);

}

int main(void){

pid_t p;int status;

p = fork();if (p < 0) {

perror("fork");exit(1);

}if (p == 0) {

child(); exit(1);}

p = wait(&status);explain_wait_status(p, status);return 0;

}

ΣωληνώσειςΣωληνώσεις στοστο UNIXUNIX (1)(1)

� Ένας από τους βασικότερους μηχανισμούς στο UNIX� Μονόδρομη μεταφορά δεδομένων� Από το άκρο εγγραφής στο άκρο ανάγνωσης

� Δημιουργία με pipe(), επικοινωνία με write() και read()� Αν η σωλήνωση είναι άδεια; → η read() μπλοκάρει

M

write fd[1]

readfd[0]

Χώρος Χρήστη

Χώρος Πυρήνα

Διεργασία

άκροεγγραφής

άκροανάγνωσης


int fd[2];

int num1, num2;

pipe(fd);

write(fd[1], &num1, sizeof(num1));

read(fd[0], &num2, sizeof(num2));



Διεργασία


int fd[2];

int num1, num2;

pipe(fd);





Διεργασία




int fd[2];

int num1, num2;

pipe(fd);



M

write fd[1]



Διεργασία




int fd[2];

int num1, num2;

pipe(fd);



M

write fd[1]

readfd[0]



Διεργασία




int fd[2];

int num1, num2;

pipe(fd);



write fd[1]

readfd[0]



Διεργασία




pipe(fd);

fork();

… ο πατέρας κλείνει το άκρο ανάγνωσης

…το παιδί κλείνει το άκρο εγγραφής



Διεργασίαπατέρας


pipe(fd);

fork();



write fd[1]






readfd[0]


pipe(fd);

fork();



write fd[1]






readfd[0]


pipe(fd);

fork();



write fd[1]






Διεργασίαπαιδί

readfd[0]

write fd[1]

readfd[0]


pipe(fd);

fork();



write fd[1]







write fd[1]

readfd[0]


pipe(fd);

fork();



write fd[1]







readfd[0]


pipe(fd);

fork();



M

write fd[1]







readfd[0]


pipe(fd);

fork();



write fd[1]







readfd[0]

ΚώδικαςΚώδικας: : παράδειγμαπαράδειγμα IPCIPC μεμε UNIX pipesUNIX pipes

ΚώδικαςΚώδικας: : παράδειγμαπαράδειγμα IPCIPC μεμε UNIX pipesUNIX pipesdouble value;int pfd[2];pid_t p;

if (pipe(pfd) < 0) {perror(“pipe”);exit(1);

}

p = fork();if (p < 0) {


} else if (p == 0) {if (read(pfd[0], &value, sizeof(value)) != sizeof(value)) {

perror(“read from pipe”);exit(1);

}printf(“child received value: value = %f\n”, value);exit(0);

} else {compute_value(&value);if (write(pfd[1], &value, sizeof(value)) != sizeof(value)) {

perror(“write to pipe”);exit(1);

}exit(0);

}

ΚώδικαςΚώδικας: : παράδειγμαπαράδειγμα IPCIPC μεμε UNIX pipesUNIX pipesdouble value;int pfd[2];pid_t p;

if (pipe(pfd) < 0) {perror(“pipe”);exit(1);

}

p = fork();if (p < 0) {


} else if (p == 0) {if (read(pfd[0], &value, sizeof(value)) != sizeof(value)) {

perror(“read from pipe”);exit(1);

}printf(“child received value: value = %f\n”, value);exit(0);

} else {compute_value(&value);if (write(pfd[1], &value, sizeof(value)) != sizeof(value)) {

perror(“write to pipe”);exit(1);

}exit(0);

}

UNIX System Programming - UCY · UNIX System Programming • Objectives look at how to program UNIX processes fork( ), wait( ) Processes

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