15 213 Fall 06 Outline Shell Lab Processes Signals 1 Process IDs process groups A process has its own unique process ID pid t getpid A process belongs to exactly one process group pid t getpgrp A new process belongs to which process group Its parent s process group Make a new process group for myself and my children setpgid 0 0 2 Process concurrency int fork void Create a new process identical to parent process Return 0 to child process Return child s pid to the parent process Any scheduling order of processes is possible Unless code uses explicit synchronization Context switching can happen at any point 3 Lab 5 A tiny shell with job control I O redirection Key points Reap all child processes Handle SIGCHLD SIGTSTP SIGINT Avoid race hazards 4 How to Send Signals To a single process int kill pid t pid int sig To every process in group abs gid int kill pid t gid int sig where gid is negative Can we use signals to count events No Why Signals not queued 5 Signals sending Process 1 Process 2 kill pid SIGINT 1 blocked pending OS signal manager divide by zero SIGFPE ctrl c SIGINT child process exit SIGCHLD other events OS Kernel 6 Signals receiving OS delivers the pending nonblocked signals Process 2 0 1 blocked pending OS signal manager OS Kernel 7 Key signals in the shell lab SIGINT Triggered by Interrupt signal ctrl c Default action the process terminates SIGTSTP Triggered by Stop signal from terminal ctrl z Default action the process stops Can be restarted later by sending SIGCONT to it SIGCHLD Triggered by a child process has stopped or terminated 8 Shell the process tree pid 10 pgid 10 Background job 1 pid 20 Forepgid 20 ground job Child pid 21 pgid 20 Shell Child pid 22 pgid 20 Foreground process group 20 pid 32 pgid 32 Background process group 32 Background job 2 pid 40 pgid 40 Backgroud process group 40 Each job has a unique process group id int setpgid pid t pid pid t pgid setpgid 0 0 9 Process tree for tsh pid 5 pgid 5 pid 10 pgid 10 rd a rw Fo pid 21 pgid 20 Child pid 22 pgid 20 Foreground process group 20 Foreground job receives SIGINT SIGTSTP when you type ctrl c ctrl z tsh Background job 1 pid 20 Forepgid 20 ground job Child als n sig UNIX shell pid 32 pgid 32 Background process group 32 Background job 2 pid 40 pgid 40 Backgroud process group 40 int kill pid t pid int sig pid 0 send sig to process with PID pid pid 0 send sig to all processes in my group pid 1 send sig to all processes with PID 1 10 pid 1 send sig to group abs pid Execute program int execve const char fname char const argv char const envp Examples execve bin ls NULL NULL execve mytest argv envp What happens to the caller process It effectively terminates The new program overwrites its state and takes its PID Any signal handlers installed by the caller are reset 11 Reaping terminated child processes pid t waitpid pid t pid int status int options pid 0 wait for process with PID pid 1 wait for any process pid 1 wait for any process from group abs pid By default waitpid blocks until at least one zombie process becomes available options WNOHANG return immediately if no zombies available WUNTRACED also return if some process has been stopped WNOHANG WUNTRACED combination is very useful in the shell lab it detects all the necessary events and doesn t block if no events 12 Reaping terminated child processes pid t waitpid pid t pid int status int options pid 0 wait for process with PID pid 1 wait for any process pid 1 wait for any process from group abs pid Return value pid of the process that exited or zero if WNOHANG was used and no zombie process available or 1 on error then see errno status gives info on why the process terminated or stopped if WUNTRACED used 13 Status int status waitpid pid status WNOHANG WUNTRACED Macros to evaluate status WIFEXITED status process exited normally WEXITSTATUS status exit code of the process WIFSIGNALED status process exited because a signal was not caught SIGINT SIGKILL etc WTERMSIG status identifies the signal that was not caught WIFSTOPPED status process was stopped WSTOPSIG status identifies the stopping signal 14 Reaping child process in tsh Where to put waitpid As the handout suggests One centralized reaping for both fg and bg In sigchld handler 15 Busy wait for foreground job tsh should still wait for fg job to complete how At an appropriate place in eval while fg process still alive do nothing 16 Better than simple busy waiting Sleep At an appropriate place in eval while fg process still alive sleep 1 17 Race hazards 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 18 An example of a race hazard sigchld handler waitpid deletejob pid eval pid fork if pid 0 child execve parent signal handler may run BEFORE addjob addjob 19 Solution blocking signals 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 20 I O Redirection Covered in Chapter 11 Make file descriptor newfd a copy of oldfd dup2 int oldfd int newfd Get input from my infd instead of standard input dup2 my infd STDIN FILENO Make a copy of a file descriptor standard output in this case int my outfd dup STDOUT FILENO 21 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 Flush output buffers fflush stdout STEP by STEP Test your shell by typing commands first Start now 22 What are the possible program outputs include unistd h include stdio h int cnt 0 int main void if fork 0 cnt in child fork cnt cnt printf d cnt return 0 Possible outputs 133 313 331 23
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