Carnegie Mellon Introduction to Computer Systems 15 213 fall 2009 14th Lecture Oct 14th Instructors Majd Sakr and Khaled Harras 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 Unix Process Hierarchy 0 init 1 Daemon e g httpd Login shell Child Child Grandchild Child Grandchild Carnegie Mellon 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 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 Carnegie Mellon 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 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 Carnegie Mellon 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 Child process 1 execs sbin init Carnegie Mellon Some PC Start up Details Boot Disk CD Floppy CPU 0xffffffff 0xffff0000 0x00000000 BIOS ROM Carnegie Mellon Some PC Start up Details Boot Disk CD Floppy CPU Power on Reset 0xffffffff 0xffff0000 0x00000000 BIOS ROM Carnegie Mellon Some PC Start up Details Boot Disk CD Floppy Start Execution at 0xfffffff0 CPU 0xffffffff 0xffff0000 0x00000000 BIOS ROM Carnegie Mellon Some PC Start up Details Boot Disk CD Floppy CPU 0xffffffff 0xffff0000 Copy Master Boot Record MBR into memory 0x00007c00 0x00000000 BIOS ROM Carnegie Mellon Some PC Start up Details Boot Disk CD Floppy CPU 0xffffffff 0xffff0000 BIOS verifies MBR and jumps to 0x00007c00 0x00007c00 0x00000000 BIOS ROM Carnegie Mellon Some PC Start up Details Boot Disk CD Floppy CPU 0xffffffff 0xffff0000 LILO or GRUB is loaded from first sector of active partition 0x00007c00 0x00000000 BIOS ROM Carnegie Mellon Some PC Start up Details Boot Disk CD Floppy CPU 0xffffffff 0xffff0000 CPU executes LILO 0x00007c00 0x00000000 BIOS ROM Carnegie Mellon Some PC Start up Details Boot Disk CD Floppy CPU 0xffffffff 0xffff0000 The Linux kernel is loaded and begins initialization 0x00007c00 0x00000000 BIOS ROM Carnegie Mellon Unix Startup Step 2 0 etc inittab Daemons e g ftpd httpd init 1 getty init forks and execs daemons per etc inittab and forks and execs a getty program for the console Carnegie Mellon Unix Startup Step 3 0 init 1 login The getty process execs a login program Carnegie Mellon 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 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 …
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