Recitation 8 (Nov. 1) Outline Process & job control Lab 5 Reminder Lab 5: Due Thursday Minglong Shao Office hours: Thursdays 5-6PM Wean 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 1 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 "); } 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 “) “c “ “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- ground job #1 Back- ground job #2 Shell Child pid=10 pgid=10 Foreground process group 20 Background process group 32 Backgroud process group 40 pid=20 pgid=20 pid=32 pgid=32 pid=40 pgid=40 pid=21 pgid=20 pid=22 pgid=20 Each job has a unique process group id int setpgid(pid_t pid, pid_t pgid); setpgid(0, 0);

Process tree for tsh Fore- ground job Back- ground job #1 Back- ground job #2 tsh Child pid=10 pgid=10 Foreground process group 20 Background process group 32 Backgroud process group 40 pid=20 pgid=20 pid=32 pgid=32 pid=40 pgid=40 pid=21 pgid=20 pid=22 pgid=20 UNIX shell pid=5 pgid=5 Foreground job receives SIGINT, SIGTSTP, when you type ctrl-c, ctrl-z Forward signals int kill(pid_t pid, int sig) pid > 0: send sig to specified process pid = 0: send sig to all processes in same group pid < -1: send sig to group -pid

Execute program int execve(const char *fname, char *const argv[], char *const envp[]); Examples: execve(“/bin/ls”, NULL, NULL); execve(“./mytest”, argv, envp); “front-end” functions for execve int 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 process waitpid(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 zombied WUNTRACED: report status of terminated or stopped children In Lab 5, use waitpid(-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 HandlerChild fork() addjob() execve() exit() sigchld_handler() deletejobs() time

A problematic schedule Shell Signal HandlerChild fork() execve() exit() sigchld_handler() deletejobs() time addjob() Job added to job list after the signal handler tried to delete it!

Solution: blocking signals sigchld_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 process eval() { 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