15 213 The course that gives CMU its Zip Exceptional Control Flow Part II October 16 2003 Topics class16 ppt Process Hierarchy Shells Signals Nonlocal jumps 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 15 213 F 03 The World of Multitasking System Runs Many Processes Concurrently Process executing program State consists of memory image register values program counter Continually 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 3 15 213 F 03 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 15 213 F 03 Unix Process Hierarchy 0 init 1 Daemon e g httpd Login shell Child Child Grandchild 5 Child Grandchild 15 213 F 03 The ps command Unix ps aux w forest USER root root root root root root root root root root root root rpc rpcuser daemon root mmdf root root root root root root root root 6 PID 1 2 3 4 5 6 9 12 397 484 533 538 563 578 696 713 721 732 738 739 752 753 744 774 786 TTY STAT S SW SWN SW SW SW SW SW S S S S S S S S S S S S L S L S L S S S output edited to fit slide COMMAND init 3 keventd ksoftirqd CPU0 kswapd bdflush kupdated mdrecoveryd scsi eh 0 sbin pump i eth0 usr local sbin afsd nosettime syslogd m 0 klogd 2 portmap rpc statd usr sbin atd usr local etc nanny init etc nanny conf usr local etc deliver b csmtpcmu usr local sbin named f usr local sbin sshd D usr local etc ntpd n usr local etc ntpd n usr local etc ntpd n usr local sbin zhm n zephyr 1 srv cm gpm t ps 2 m dev mouse crond 15 213 F 03 The ps Command cont USER root agn root agn agn agn agn agn agn agn agn agn agn agn agn agn agn agn agn agn agn agn 7 PID 889 900 921 948 958 966 1184 1212 3346 1191 1204 1207 1208 1209 17814 2469 2483 2484 2485 3042 959 1020 TTY tty1 tty1 tty1 tty1 pts 0 pts 0 pts 0 pts 0 pts 0 8 pts 0 8 pts 0 8 pts 0 8 pts 0 8 pts 0 pts 0 pts 0 pts 0 pts 0 pts 0 tty1 tty1 STAT COMMAND S bin login agn S xinit 0 SL etc X11 X auth usr1 agn Xauthority 0 S bin sh afs cs cmu edu user agn xinitrc S xterm geometry 80x45 1 1 C j ls n S tcsh S usr local bin wish8 0 f usr S usr local bin wish8 0 f S aspell a S S bin sh usr local libexec moz S usr local libexec mozilla S usr local libexec moz S usr local libexec S usr local libexec S usr local libexec S usr local lib Acrobat S java vm S java vm S java vm S java vm S bin sh usr local libexec kde bin sta S kwrapper ksmserver 15 213 F 03 Unix Startup Step 1 1 Pushing reset button loads the PC with the address of a small bootstrap program 2 Bootstrap program loads the boot block disk block 0 3 Boot block program loads kernel binary e g boot vmlinux 4 Boot block program passes control to kernel 5 Kernel handcrafts the data structures for process 0 0 Process 0 handcrafted kernel process Process 0 forks child process 1 init 1 8 Child process 1 execs sbin init 15 213 F 03 Some PC Start up Details Boot Disk CD Floppy Start Execution at 0xfffffff0 LILO or GRUB is loaded from first sector of active partition 9 The Linux kernel is loaded and begins initialization CPU executes LILO CPU Power OK Deassert Reset 0xffffffff 0xffff0000 BIOS ROM BIOS verifies MBR and jumps to 0x00007c00 Copy Master Boot Record into memory 0x00007c00 0x00000000 15 213 F 03 Unix Startup Step 2 0 etc inittab Daemons e g ftpd httpd 10 init 1 getty init forks and execs daemons per etc inittab and forks and execs a getty program for the console 15 213 F 03 Unix Startup Step 3 0 init 1 login 11 The getty process execs a login program 15 213 F 03 Unix Startup Step 4 0 init 1 tcsh login reads login ID and passwd if OK it execs a shell if not OK it execs another getty In case of login on the console xinit may be used instead of a shell to start the window manger 12 15 213 F 03 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 13 Execution is a sequence of read evaluate steps evaluate eval cmdline 15 213 F 03 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 14 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 15 213 F 03 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 15 15 213 F 03 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 …
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