CS 105 Tour of the Black Holes of Computing Exceptional Control Flow Part II Topics except2 ppt Process Hierarchy Shells Signals ECF Exists at All Levels of a System Exceptions Hardware and operating system kernel software Previous Lecture Concurrent processes Hardware timer and kernel software Signals Kernel software This Lecture Non local jumps 2 Application code CS 105 Programmer s Model of Multitasking Basic Functions fork spawns new process Called once returns twice exit terminates own process Called once never returns Puts it into zombie status wait and waitpid wait for and reap terminated children execl and execve run a new program in an existing process Called once normally never returns Programming Challenge Understanding the nonstandard semantics of the functions Avoiding improper use of system resources E g Fork bombs can disable a system 4 CS 105 Shell Programs A shell is an application program that runs programs on behalf of the user sh Original Unix Bourne Shell csh BSD Unix C Shell tcsh Enhanced C Shell bash Bourne Again Shell int main char cmdline MAXLINE while 1 read printf Fgets cmdline MAXLINE stdin if feof stdin exit 0 10 Execution is a sequence of read evaluate steps evaluate eval cmdline CS 105 Simple Shell eval Function void eval char cmdline char argv MAXARGS argv for execve int bg should the job run in bg or fg pid t pid process id bg parseline cmdline argv if builtin command argv if pid Fork 0 child runs user job execve argv 0 argv environ printf s Command not found n argv 0 exit 0 11 if bg parent waits for fg job to terminate int status if waitpid pid status 0 0 unix error waitfg waitpid error else otherwise don t wait for bg job printf d s pid cmdline CS 105 Problem with Simple Shell Example Shell correctly waits for and reaps foreground jobs But what about background jobs Will become zombies when they terminate Will never be reaped because shell typically will not terminate Creates a memory leak that will eventually crash the kernel when it runs out of memory Solution Reaping background jobs requires a mechanism called a signal 12 CS 105 Signals A signal is a small message that notifies a process that an event of some type has occurred in the system ID Kernel abstraction for exceptions and interrupts Sent from the kernel sometimes at the request of another process to a process Different signals are identified by small integer ID s The only information in a signal is its ID and the fact that it arrived Name 2 SIGINT 9 SIGKILL Default Action Corresponding Event Terminate Interrupt from keyboard ctl c Terminate Kill program cannot override or ignore 11 SIGSEGV 14 SIGALRM Terminate Dump Segmentation violation Terminate Timer signal 17 SIGCHLD Ignore Child stopped or terminated 13 CS 105 Signal Concepts Sending a signal Kernel sends delivers a signal to a destination process by updating some state in the context of the destination process Kernel sends a signal for one of the following reasons Kernel has detected a system event such as divide by zero SIGFPE or the termination of a child process SIGCHLD Another process has invoked the kill system call to explicitly request that the kernel send a signal to the destination process 14 CS 105 Signal Concepts cont Receiving a signal A destination process receives a signal when it is forced by the kernel to react in some way to the delivery of the signal Five possible ways to react Ignore the signal do nothing Terminate the process Stop the process from running Continue a stopped process let it run again Catch the signal by executing a user level function called a signal handler Akin to a hardware exception handler being called in response to an asynchronous interrupt 15 CS 105 Signal Concepts cont A signal is pending if it has been sent but not yet received There can be at most one pending signal of any particular type Important Signals are not queued If a process has a pending signal of type k then subsequent signals of type k that are sent to that process are discarded A process can block the receipt of certain signals Blocked signals can be delivered but will not be received until the signal is unblocked A pending signal is received at most once 16 CS 105 Signal Concepts Kernel maintains pending and blocked bit vectors in the context of each process pending represents the set of pending signals Kernel sets bit k in pending whenever a signal of type k is delivered Kernel clears bit k in pending whenever a signal of type k is received blocked represents the set of blocked signals Can be set and cleared by the application using the sigprocmask function 17 CS 105 Process Groups Every process belongs to exactly one process group pid 10 pgid 10 pid 20 pgid 20 Foreground job Child Child pid 21 pgid 20 pid 22 pgid 20 Foreground process group 20 18 Shell Background job 1 pid 32 pgid 32 Background process group 32 Background job 2 pid 40 pgid 40 Background process group 40 getpgrp Return process group of current process setpgid Change process group of a process CS 105 Sending Signals with kill kill sends arbitrary signal to a process or process group Examples kill 9 24818 Send SIGKILL to process 24818 kill 9 24817 Send SIGKILL to every process in process group 24817 19 linux forks 16 linux Child1 pid 24818 pgrp 24817 Child2 pid 24819 pgrp 24817 linux ps PID TTY TIME CMD 24788 pts 2 00 00 00 tcsh 24818 pts 2 00 00 02 forks 24819 pts 2 00 00 02 forks 24820 pts 2 00 00 00 ps linux kill 9 24817 linux ps PID TTY TIME CMD 24788 pts 2 00 00 00 tcsh 24823 pts 2 00 00 00 ps linux CS 105 Sending Signals from the Keyboard Typing ctrl c ctrl z sends a SIGINT SIGTSTP to every job in the foreground process group SIGINT default action is to terminate each process SIGTSTP default action is to stop suspend each process pid 10 pgid 10 Shell Forepid 20 pgid 20 ground Background job 1 Child Child Background process group 32 pid 21 pgid 20 pid 22 pgid 20 job pid 32 pgid 32 Background job 2 pid 40 pgid 40 Background process group 40 Foreground process group 20 20 CS 105 Example of ctrl c and ctrl z linux forks 17 Child pid 24868 pgrp 24867 Parent pid 24867 pgrp 24867 typed ctrl z Suspended linux ps a PID TTY STAT TIME COMMAND 24788 pts 2 S 0 00 usr local bin tcsh i 24867 pts 2 T 0 01 forks 17 24868 pts 2 T 0 01 forks 17 24869 pts 2 R 0 00 ps a linux fg forks 17 typed ctrl c linux ps a PID TTY STAT TIME COMMAND 24788 pts 2 S 0 00 usr local bin tcsh i 24870 pts 2 R 0 00 ps a 21 CS 105 Sending Signals with kill Function void fork12 pid t pid
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