Carnegie Mellon Introduction to Computer Systems 15 213 18 243 spring 2009 15th Lecture Mar 5th Instructors Gregory Kesden and Markus P schel Carnegie Mellon ECF Exists at All Levels of a System Exceptions Hardware and operating system kernel Previous Lecture software Signals Kernel software Non local jumps Application code This Lecture Carnegie Mellon Today Multitasking shells Signals Long jumps More on signals Carnegie Mellon The World of Multitasking System runs many processes concurrently Process executing program State includes memory image register values program counter Regularly switches from one process to another Suspend process when it needs I O resource or timer event occurs Resume process when I O available or given scheduling priority Appears to user s as if all processes executing simultaneously Even though most systems can only execute one process at a time Except possibly with lower performance than if running alone Carnegie Mellon 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 new program in 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 Carnegie Mellon Shell Programs A shell is an application program that runs programs on behalf of the user sh Original Unix shell Stephen Bourne AT T Bell Labs 1977 csh BSD Unix C shell tcsh csh enhanced at CMU and elsewhere bash Bourne Again Shell Execution is a sequence of int main read evaluate steps char cmdline MAXLINE while 1 read printf Fgets cmdline MAXLINE stdin if feof stdin exit 0 evaluate eval cmdline Carnegie Mellon 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 if execve argv 0 argv environ 0 printf s Command not found n argv 0 exit 0 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 Carnegie Mellon What Is a Background Job Users generally run one command at a time Type command read output type another command Some programs run for a long time Example delete this file in two hours sleep 7200 rm tmp junk shell stuck for 2 hours A background job is a process we don t want to wait for sleep 7200 rm tmp junk 1 907 ready for next command Carnegie Mellon 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 Will create a memory leak that could theoretically run the kernel out of memory Modern Unix once you exceed your process quota your shell can t run any new commands for you fork returns 1 limit maxproc maxproc 3574 ulimit u 3574 csh syntax bash syntax Carnegie Mellon ECF to the Rescue Problem The shell doesn t know when a background job will finish By nature it could happen at any time The shell s regular control flow can t reap exited background processes in a timely fashion Regular control flow is wait until running job completes then reap it Solution Exceptional control flow The kernel will interrupt regular processing to alert us when a background process completes In Unix the alert mechanism is called a signal Carnegie Mellon Today Multitasking shells Signals Long jumps More on signals Carnegie Mellon Signals A signal is a small message that notifies a process that an event of some type has occurred in the system akin to exceptions and interrupts sent from the kernel sometimes at the request of another process to a process signal type is identified by small integer ID s 1 30 only information in a signal is its ID and the fact that it arrived ID Name Default Action Corresponding Event 2 SIGINT Terminate Interrupt e g ctl c from keyboard 9 SIGKILL Terminate Kill program cannot override or ignore 11 SIGSEGV Terminate Dump Segmentation violation 14 SIGALRM Terminate Timer signal 17 SIGCHLD Ignore Child stopped or terminated Carnegie Mellon 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 the kernel to send a signal to the destination process Carnegie Mellon 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 Three possible ways to react Ignore the signal do nothing Terminate the process with optional core dump Catch the signal by executing a user level function called signal handler Akin to a hardware exception handler being called in response to an asynchronous interrupt Carnegie Mellon Signal Concepts continued A signal is pending if 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 Carnegie Mellon 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 when a signal of type k is delivered Kernel clears bit k in pending when a signal of type k is received blocked represents the set of blocked signals Can be set and cleared by using the sigprocmask function Carnegie Mellon Process Groups Every process belongs to exactly one process group pid 10 pgid 10 pid 20 pgid 20 Background job 1 Foreground job Child Child pid 21 pgid 20 pid 22 pgid 20 Foreground process group 20 Shell pid 32 pgid 32 Background process group 32 Background job 2 pid 40 pgid 40 Background process group 40 getpgrp Return process
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