Program and OS interac0ons: Excep0ons and Processes
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1 Program and OS interac0ons: Excep0ons and Processes Andrew Case Slides adapted from Jinyang Li, Randy Bryant and Dave O Hallaron 1
2 User- level programs vs. OS safari httpd bomb OS kernel Send/recv packets, Open/close files 2
3 User- level programs vs. OS safari httpd bomb OS kernel Send/recv packets, Open/close files Role of an OS: Implements OS- level services: I/O, network Enforce protechon: no stomping on each other s memory/files/packets Resource sharing: execute mulhple programs simultaneously, 3
4 Ways user- level programs interact with OS Invoke syscalls Send/receive data Create/delete files Execute/kill other programs Generate excep0ons (to be handled by OS) Touch illegal memory Divide by zero Get interrupted by OS OS preempts a program to execute other programs OS does upcalls to user- programs via signals Achieved via h/w s excep0onal control flow 4
5 Topics Excep0onal Control Flow Processes 5
6 Control Flow A CPU core reads and executes a series of instruc0ons, one at a 0me This sequence is the CPU s control flow Physical control flow Time <startup> inst 1 inst 2 inst 3 inst n <shutdown> 6
7 Altering the Control Flow Up to now: two mechanisms for changing control flow: Jumps and branches Call and return Both react to changes in program state excep0onal control flow react to changes in system state data arrives from a disk or a network adapter instruchon divides by zero user hits Ctrl- C at the keyboard System Hmer expires 7
8 Types of Excep0ons Asynchronous (interrupts): caused by events external to CPU hitng Ctrl- C at the keyboard arrival of a packet from a network arrival of data from a disk Synchronous: caused by execu0ng an instruc0on IntenHonal (Traps): e.g. INT 0x80 (syscall) UnintenHonal but recoverable (Faults): e.g. page or protechon faults unintenhonal and unrecoverable (Aborts): e.g. memory error 8
9 Excep0ons An excep:on is a transfer of control to the OS in response to some event (i.e., change in processor state) User Process OS event I_current I_next excep-on excep-on processing by excep-on handler 9
10 Excep0ons An excep:on is a transfer of control to the OS in response to some event (i.e., change in processor state) User Process OS event I_current I_next excep-on excep-on processing by excep-on handler fault: return to I_current trap,interrupt: return to I_next Abort: machine reboot, process termina-on 10
11 Interrupt Vectors Excep0on numbers Excep0on Table n-1... code for excep0on handler 0 code for excep0on handler 1 code for excep0on handler 2... code for excep0on handler n- 1 Each type of excep0on has a unique excep0on number k, used to index into excep0on table (a.k.a. interrupt vector) Handler k is called each 0me excep0on k occurs Handled by the OS, not user- level programs 11
12 Trap Example: Syscall (System call) User- level program calls: open(filename, options) FuncHon open executes system call instruchon int 0804d070 < libc_open>: d082: cd 80 int $0x80 # interrupt d084: 5b pop %ebx... User Process OS int pop excep-on returns open file OS must find or create file, get it ready for reading or wrihng Returns integer file descriptor 12
13 Fault Example: Page Fault User writes to memory locahon That address is not in memory (on disk) int a[1000]; main () { a[500] = 13; 80483b7: c d d movl $0xd,0x8049d10 User Process OS movl excep-on: page fault returns Create page and load into memory OS loads page into physical memory Returns to faulhng instruchon Successful on second try 13
14 Fault Example: Invalid Memory Reference int a[1000]; main () { a[5000] = 13; 80483b7: c e d movl $0xd,0x804e360 User Process OS movl excep-on: page fault signal process detect invalid address OS detects invalid address Sends SIGSEGV signal to user process User process exits with segmentahon fault 14
15 Topics Excep0onal Control Flow Processes 15
16 Processes A process is an instance of a running program. Not the same as program or processor OS and Process provides each program with two key abstrac0ons: Logical control flow Each program seems to have exclusive use of the CPU Private virtual address space Each program seems to have exclusive use of main memory How are these Illusions maintained? Process execuhons interleaved (mulhtasking) or run on separate cores Virtual Memory managed by OS 16
17 Concurrent Processes Two processes run concurrently (are concurrent) if their flows overlap in 0me Otherwise, they are sequen:al Examples (running on single core): Concurrent: A & B, A & C SequenHal: B & C Process A Process B Process C Time 17
18 User View of Concurrent Processes Control flows for concurrent processes are physically disjoint in 0me However, we can think of concurrent processes are running in parallel with each other Process A Process B Process C Time 18
19 Context Switching Processes are managed by a shared chunk of OS code called the kernel Control flow passes from one process to another via a context switch Process A Process B Time user code kernel code user code kernel code user code context switch context switch 19
20 Basic UNIX syscalls for managing processes fork Create a new process exit wait Synchronize among processes execve Load a new process image Syscalls are documented in man pages sec0on 2: man -2 fork Standard C library provides wrapper func0ons for many syscalls 20
21 fork: Crea0ng New Processes int fork(void) creates a new process (child process) that is idenhcal to the calling process (parent process) Fork is called once but returns twice Return 0 to the child process pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); else { printf("hello from parent: child pid is %d\n, pid); Return child s pid to the parent 21
22 Understanding fork Process n Child Process m pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); else { printf("hello from parent\n"); pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); else { printf("hello from parent\n"); pid = m pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); else { printf("hello from parent\n"); pid = 0 pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); else { printf("hello from parent\n"); pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); else { printf("hello from parent\n"); pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); else { printf("hello from parent\n"); hello from parent Which one is first? hello from child 22
23 Fork Example #1 Parent and child both run same code DisHnguish parent from child by return value from fork Start with same state, but each has private copy Including shared output file descriptor RelaHve ordering of their print statements undefined void fork1() { int x = 1; pid_t pid = fork(); if (pid == 0) { printf("child has x = %d\n", ++x); else { printf("parent has x = %d\n", --x); printf("bye from process %d with x = %d\n", getpid(), x); 23
24 Fork Example #2 Both parent and child can con0nue forking void fork2() { printf("l0\n"); fork(); printf("l1\n"); fork(); printf("bye\n"); L1 L0 L1 Bye Bye Bye Bye 24
25 Fork Example #3 Both parent and child can con0nue forking void fork3() { L2 printf("l0\n"); fork(); printf("l1\n"); L1 L2 fork(); printf("l2\n"); L2 fork(); printf("bye\n"); L0 L1 L2 Bye Bye Bye Bye Bye Bye Bye Bye 25
26 Fork Example #4 Just parent can con0nue forking void fork4() { printf("l0\n"); if (fork()!= 0) { printf("l1\n"); if (fork()!= 0) { printf("l2\n"); fork(); printf("bye\n"); L0 L1 L2 Bye Bye Bye Bye 26
27 Fork Example #5 Just child can con0nue forking void fork5() { printf("l0\n"); if (fork() == 0) { printf("l1\n"); if (fork() == 0) { printf("l2\n"); fork(); printf("bye\n"); L0 L2 L1 Bye Bye Bye Bye 27
28 Beware: the fork bomb Both parent and child can con0nue forking indefinitely void fork_bomb() { int cnt = 0; while(1) { printf( L%d\n, cnt++); fork(); L0 L2 L1 L2 L2 L1 L2 CAUTION: DO NOT RUN ON A SYSTEM YOU DO NOT OWN! THIS WILL PROBABLY HOSE YOUR SYSTEM!!! 28
29 exit: Ending a process void exit(int status) exits a process Normally return with status 0 atexit() registers funchons to be executed upon exit void cleanup(void) { printf("cleaning up\n"); void fork6() { atexit(cleanup); fork(); exit(0); 29
30 Zombies Idea When process terminates, shll consumes system resources Why? So that parents can learn of children s exit status Called a zombie - dead, but shll taking up resources Reaping Performed by parent on terminated child Parent is given exit status informahon Kernel discards process What if parent doesn t reap? If any parent terminates without reaping a child, then child will be reaped by init process Ancestor of all 30
31 Zombie Example linux>./forks 7 & [1] 6639 Running Parent, PID = 6639 Terminating Child, PID = 6640 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 forks 6640 ttyp9 00:00:00 forks <defunct> 6641 ttyp9 00:00:00 ps linux> kill 6639 [1] Terminated linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6642 ttyp9 00:00:00 ps void fork7() { if (fork() == 0) { /* Child */ printf("terminating Child, PID = %d\n", getpid()); exit(0); else { printf("running Parent, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ ps shows child process as defunct Killing parent allows child to be reaped by init How do you handle them without killing parent process? 31
32 wait: Synchronizing with Children int wait(int *child_status) Blocks unhl some child exits return value is the pid of terminated child If mulhple children completed, will take in arbitrary order (use waitpid to wait for a specific child) void fork9() { if (fork() == 0) { printf("hc: hello from child\n"); else { printf("hp: hello from parent\n"); wait(null); printf("ct: child has terminated\n"); printf("bye\n"); exit(); HC Bye HP CT Bye 32
33 wait() Example If mulhple children completed, will take in arbitrary order Can use macros WIFEXITED (Wait If Exited) and WEXITSTATUS to get informahon about exit status void fork10() { pid_t pid[n]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) printf("child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("child %d terminate abnormally\n", wpid); 33
34 waitpid(): Wai0ng for a Specific Process waitpid(pid, &status, options) suspends current process unhl specific process terminates various ophons (see textbook) void fork11() { pid_t pid[n]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = N-1; i >= 0; i--) { pid_t wpid = waitpid(pid[i], &child_status, 0); if (WIFEXITED(child_status)) printf("child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("child %d terminated abnormally\n", wpid); 34
35 execve: Execute a new program int execve( char *filename, //executable char *argv[], //arguments char *envp[] //environment ) Kernel virtual memory Stack Does not return (unless error) shared libraries overwrites current process code, data, and stack with new executable keeps pid, open files and signal context Heap Read/write segment (.data,.bss) Read- only segment (.init,.text,.rodata) 0 Unused 35
36 execve Example 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); argv argv[argc] = NULL argv[argc- 1] argv[0] /usr/include -lt ls environ envp[n] = NULL envp[n- 1] envp[0] PWD=/home/cso PATH=/usr/bin:/bin: USER=cso 36
37 execve: Loading and Running Programs int execve( char *filename, char *argv[], char *envp[] ) Loads and runs in current process: Executable filename With argument list argv And environment variable list envp Environment variables: name=value strings getenv() and putenv() Null- terminated env var strings Null- terminated cmd line arg strings unused envp[n] == NULL envp[n- 1] envp[0] argv[argc] == NULL argv[argc- 1] argv[0] Linker vars envp argv argc Stack frame for main Stack bohom Stack top environ 37
38 Summary Excep0ons Used for events that require non- standard control flow Hardware mechanism: excephon control flow Generated externally (interrupts) or internally (traps and faults) OS abstrac0on of Processes At any given Hme, system has mulhple achve processes Each process appears to have total control of CPU + private memory space Programs interface with OS using syscalls/interrupts: fork, exit, wait, exec 38
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