Recitation 8 (Nov. 1)

Outline Process & job control Lab 5

Reminder Lab 5:

Due Thursday

Minglong Shaoshaoml+213@cs.cmu.edu

Office hours:Thursdays 5-6PMWean Hall 1315

Process & concurrency

int fork(void) Create an identical copy of the parent process Return values differentiate parent from child

Any scheduling order of processes are possible Unless code explicitly imposes some order

(synchronization) Context switching can happen at any point

Example 1int main(){ pid_t pid;

if((pid = fork()) == 0) { printf("a "); } else { printf("b "); if(fork() == 0) { printf("c "); } } if(waitpid(pid, NULL, 0) > 0) printf("d ");

printf("e ");}

Possible outputs

1. a b c d e e e ?2. b a e c d e e ?3. a b c e e d e ?4. b e c e a d e ?

Example 1 (cont.)int main(){ pid_t pid;

if((pid = fork()) == 0) { printf("a "); } else { printf("b "); if(fork() == 0) { printf("c "); } } if(waitpid(pid, NULL, 0) > 0) printf("d ");

printf("e ");}

L0

L1

“b “

L2

“a “ “e “

(“d “)

(“d “)“c “

“e “

“e “

In L1, pid = 0 wait for any child process

In L0 and L2, pid is the process id of L1 wait for process L1

1. a b c d e e e N 2. b a e c d e e Y

3. a b c e e d e Y 4. b e c e a d e N

Lab 5

A tiny shell (tsh) with job control & I/O redirection Key points:

Reap all child processes Handle SIGCHLD, SIGTSTP, SIGINT Avoid race hazard

Process tree for shell

Fore-ground

job

Back-groundjob #1

Back-groundjob #2

Shell

Child Child

pid=10pgid=10

Foregroundprocess group 20

Backgroundprocess group 32

Backgroudprocess group 40

pid=20pgid=20

pid=32pgid=32

pid=40pgid=40

pid=21pgid=20

pid=22pgid=20

Each job has a unique process group idint setpgid(pid_t pid, pid_t pgid);setpgid(0, 0);

Process tree for tsh

Fore-ground

job

Back-groundjob #1

Back-groundjob #2

tsh

Child Child

pid=10pgid=10

Foregroundprocess group 20

Backgroundprocess group 32

Backgroudprocess group 40

pid=20pgid=20

pid=32pgid=32

pid=40pgid=40

pid=21pgid=20

pid=22pgid=20

UNIXshell

pid=5pgid=5

Foreground jobreceives SIGINT, SIGTSTP, when you type ctrl-c, ctrl-z

Forward signals

int kill(pid_t pid, int sig)pid > 0: send sig to specified processpid = 0: send sig to all processes in same grouppid < -1: send sig to group -pid

Execute programint execve(const char *fname, char *const argv[], char *const envp[]);Examples:execve(“/bin/ls”, NULL, NULL);execve(“./mytest”, argv, envp);

“front-end” functions for execveint execl(const char* path, const char *arg, …);int execlp(const char* file, const char *arg, …);int execle(const char*path, const char *arg, … char

* const envp[]);int execv(const char *path, char *const argv[]);int execvp(const char *file, char *const argv[]);

Reaping child processwaitpid(pid_t pid, int *status, int options)

pid: wait for child process with pid (process id or group id)-1: wait for any child process

status: tell why child terminated

options:WNOHANG: return immediately if no children zombiedWUNTRACED: report status of terminated or stopped children

In Lab 5, usewaitpid(-1, &status, WNOHANG|WUNTRACED)

In sigchld_handler():while ((c_pid = waitpid(…)) > 0) { … }

Check status int status;waitpid(pid, &status, WNOHANG|WUNTRACED)

Macros to evaluate status: WIFEXITED(status): child exited normally WEXITSTATUS(status): returns code when child exits

WIFSIGNALED(status): child exited because a signal is not caught

WTERMSIG(status): gives the terminating signal number

WIFSTOPPED(status): child is currently stopped WSTOPSIG(status): gives the stop signal number

Reaping child process in tsh

Where to put waitpid(…)

Two waits In sigchld_handler(): for bg processes In eval(): for fg processes

One wait In sigchld_handler(): for both tsh should still wait for fg job to complete, how?

Busy wait for foreground job

In eval():

if(fork() != 0) { /* parent */

addjob(…);

while(fg process still alive){

/* do nothing */

}

}

Sleep

In eval():

if(fork() != 0) { /* parent */

addjob(…);

while(fg process still alive){

sleep(1);

}

}

Race hazard

A data structure is shared by two pieces of code that can run concurrently

Different behaviors of program depending upon how the schedule interleaves the execution of code.

An example of race hazard

sigchld_handler() { while ((pid = waitpid(…)) > 0){ deletejob(pid); }}eval() { pid = fork(); if(pid == 0) { /* child */ execve(…); } /* parent */ /* signal handler may run BEFORE addjob()*/ addjob(…);}

An okay schedule

Shell Signal Handler Child

fork()

addjob()

execve()

exit()

sigchld_handler()

deletejobs()

time

A problematic schedule

Shell Signal Handler Child

fork()

execve()

exit()

sigchld_handler()

deletejobs()

time

addjob()

Job added to job list after the signal handler tried to delete it!

Solution: blocking signalssigchld_handler() { pid = waitpid(…); deletejob(pid);}eval() { sigprocmask(SIG_BLOCK, …) pid = fork(); if(pid == 0) { /* child */ sigprocmask(SIG_UNBLOCK, …) execve(…); } /* parent */ /* signal handler might run BEFORE addjob() */ addjob(…); sigprocmask(SIG_UNBLOCK, …)}

I/O redirection Do it before call execve() in child processeval() { pid = fork(); if(pid == 0) { /* child */ /* Redirect I/O */ if (redirect_input) dup2(…);

if (redirect_output) dup2(…); execve(…); } addjob(…);}

dup2(int oldfd, int newfd) Covered in Chapter 11

oldfd: old file descriptor

newfd: new file descriptor

Some examples:

Get input from my_infd instead of standard input

dup2(my_infd, STDIN_FILENO);

Print output to my_outfd instead of standard output

dup2(my_outfd, STDOUT_FILENO);

Reminders

Some important system calls: fork(), execve(), waitpid(), sigprocmask(),

setpgid(), kill() … Check man pages for details about system calls

man 2 kill

Check return values of all system calls

STEP by STEP Test your shell by typing commands first I/O redirection: last step (15 pts out of 135 pts)

Start now!

Virtual memory

One of the most important concepts in CS Why use virtual memory (VM)

Use RAM as a cache for disk Easier memory management Access protection Enable “partial swapping” Share memory efficiently