Recitation 8 Signals Shells Andrew Faulring 15213 Section A 28 October 2002 Andrew Faulring faulring cs cmu edu Office hours NSH 2504 lab 2507 conference room Thursday 5 6 Lab 5 due Thursday 31 Oct 11 59pm Halloween Night happy reaping Today s Plan Process IDs Process Groups Process Control Signals Preemptive Scheduler Race hazards Reaping Child Processes Lab 5 Shell tshref Use as a guide for output You shell should have same behavior How Programmers Play with Processes Process executing copy of program Basic functions fork spawns new process exit terminates calling process wait and waitpid wait for and reap terminated children execl and execve run a new program in an existing process Process IDs Process Groups Each process has its own unique process ID pid t getpid Each process belongs to exactly one process group pid t getpgid To which process group does a new process initially belong Its parent s process group A process can make a process group for itself and its children setpgid 0 0 pid 10 pgid 10 pid 20 pgid 20 Shell Background job 1 Foreground job Child Child pid 21 pgid 20 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 Signals Section 8 5 in text Read at least twice really A signal tells our program that some event has occurred For instance a child process has terminated Can we use signals to count events No Important Signals SIGINT Interrupt signal from keyboard ctrl c SIGTSTP Stop signal from keyboard ctrl z SIGCHLD A child process has stopped or terminated Look at Figure 8 23 for a complete list of Linux signals Sending a Signal Send a signal Sent by either the kernel Or another process Why is a signal sent The kernel detects a system event Divide by zero SIGFPE Termination of a child process SIGCHLD Another process invokes a system call kill pid t pid int SIGINT kill 1500 SIGINT Send SIGINT to process 1500 kill 1500 SIGINT Send SIGINT to progress group 1500 alarm unsigned int secs Receiving a Signal Default action The process terminates and dumps core The process stops until restarted by a SIGCONT signal The process ignore the signal Can modify the default action with the signal function Additional action Handle the signal void sigint handler int sig signal SIGINT sigint handler Cannot modify action for SIGSTOP and SIGKILL Receiving a Signal pending bit vector bit k is set when signal type k is delivered clear when signal received blocked bit vector of signals that should not be received Only receive non blocked pending signals pending blocked Synchronizing Processes Preemptive scheduler run multiple programs concurrently by time slicing How does time slicing work The scheduler can stop a program at any point Signal handler code can run at any point too Program behaviors depend on how the scheduler interleaves the execution of processes Racing condition between parent and child Why Race Hazard Different behaviors of program depending upon how the schedule interleaves the execution of code Parent Child Race Hazard sigchld handler pid waitpid deletejob pid eval pid fork if pid 0 child execve parent signal handler might run BEFORE addjob addjob An Okay Schedule time Shell Signal Handler Child fork addjob execve exit sigchld handler deletejobs A Problematic Schedule time Shell Signal Handler Child fork execve exit sigchld handler deletejobs addjob Job added to job list after the signal handler tried to delete it Solution to Race Hazard 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 More details 8 5 6 page 633 Reaping Child Process Child process becomes zombie when terminates Still consume system resources Parent performs reaping on terminated child Using either wait or waitpid syscall Where to wait children processes to terminate Two waits sigchld handler eval for foreground processes One wait sigchld handler But what about foreground processes Busy Wait void eval parent addjob while fg process still alive sigchld handler pid waitpid deletejob pid Pause void eval parent addjob while fg process still alive pause If signal handled SIGCHLD before call to pause then pause will not return sigchld handler pid waitpid deletejob pid Sleep void eval parent addjob while fg process still alive sleep 1 sigchld handler pid waitpid deletejob pid waitpid Used for reaping zombied child processes pid t waitpid pid t pid int status int options pid wait until child process with pid has terminated 1 wait for any child process status tells why child terminated options WNOHANG return immediately if no children have exited zombied waitpid returns 1 WUNTRACED report status of stopped children too waitpid s status int status waitpid pid status NULL WIFEXITED status child exited normally WEXITSTATUS status return code when child exits WIFSIGNALED status child exited because a signal was not caught WTERMSIG status gives the number of the terminating signal WIFSTOPPED status child is stopped WSTOPSIG status gives the number of the stop signal Summary Process provides applications with the illusions of Exclusively use of the processor and the main memory At the interface with OS applications can Creating child processes Run new programs Catch signals from other processes Use man if anything is not clear
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