CS 3214 Computer Systems

1. CS 3214Computer Systems Godmar Back Lecture 13

2. Announcements • Project 3 milestone due Oct 8 • Have still not heard from everybody regarding SVN • Exercise 6 handed out CS 3214 Fall 2010

3. Part 1 Threads and Processes CS 3214 Fall 2010

4. Timer interrupt: P1 is preempted, context switch to P2 I/O device interrupt:P2’s I/O completeswitch back to P2 user mode kernel mode System call: (trap): P2 starts I/O operation, blocks context switch to process 1 Timer interrupt: P2 still has time left, no context switch A Context Switch Scenario Process 1 Process 2 Kernel CS 3214 Fall 2010

5. Bottom Up View: Exceptions • An exception is a transfer of control to the OS in response to some event (i.e., change in processor state) User Process OS exception current event next exception processing by exception handler exception return (optional) CS 3214 Fall 2010

6. RUNNING Scheduler picks process Process must wait for event Process preempted BLOCKED READY Event arrived Reasoning about Processes:Process States • Only 1 process (per CPU) can be in RUNNING state CS 3214 Fall 2010

7. User View • If process’s lifetimes overlap, they are said to execute concurrently • Else they are sequential • Default assumption is concurrently • Exact execution order is unpredictable • Programmer should never make any assumptions about it • Any interaction between processes must be carefully synchronized CS 3214 Fall 2010

8. #include <unistd.h> #include <stdio.h> int main() { int x = 1; if (fork() == 0) { // only child executes this printf("Child, x = %d\n", ++x); } else { // only parent executes this printf("Parent, x = %d\n", --x); } // parent and child execute this printf("Exiting with x = %d\n", x); return 0; } fork() Child, x = 2 Exiting with x = 2 Parent, x = 0 Exiting with x = 0 CS 3214 Fall 2010

9. The fork()/join() paradigm • After fork(), parent & child execute in parallel • Unlike a fork in the road, here we take both roads • Used in many contexts • In Unix, ‘join()’ is called wait() • Purpose: • Launch activity that can be done in parallel & wait for its completion • Or simply: launch another program and wait for its completion (shell does that) Parent: fork() Parent process executes Child process executes Child process exits Parent:join() OS notifies CS 3214 Fall 2010

10. fork() #include <sys/types.h> #include <unistd.h> #include <stdio.h> int main(int ac, char *av[]) { pid_t child = fork(); if (child < 0) perror(“fork”), exit(-1); if (child != 0) { printf ("I'm the parent %d, my child is %d\n", getpid(), child); wait(NULL); /* wait for child (“join”) */ } else { printf ("I'm the child %d, my parent is %d\n", getpid(), getppid()); execl("/bin/echo", "echo", "Hello, World", NULL); } } CS 3214 Fall 2010

11. fork() vs. exec() • fork(): • Clone most state of parent, including memory • Inherit some state, e.g. file descriptors • Keeps program, changes process • Called once, returns twice • exec(): • Overlays current process with new executable • Keeps process, changes program • Called once, does not return (if successful) CS 3214 Fall 2010

12. exit(3) vs. _exit(2) • exit(3) destroys current processes • OS will free resources associated with it • E.g., closes file descriptors, etc. etc. • Can have atexit() handlers • _exit(2) skips them • Exit status is stored and can be retrieved by parent • Single integer • Convention: exit(EXIT_SUCCESS) signals successful execution, where EXIT_SUCCESS is 0 CS 3214 Fall 2010

13. wait() vs waitpid() • int wait(int *status) • Blocks until any child exits • If status != NULL, will contain value child passed to exit() • Return value is the child pid • Can also tell if child was abnormally terminated • intwaitpid(pid_tpid, int *status, int options) • Can say which child to wait for CS 3214 Fall 2010

14. Wait Example 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); } } If multiple children completed, wait() returns them in arbitrary order • Can use macros WIFEXITED and WEXITSTATUS to get information about exit status CS 3214 Fall 2010

15. Observations on fork/exit/wait • Process can have many children at any point in time • Establishes a parent/child relationship • Resulting in a process tree • Zombies: processes that have exited, but their parent hasn’t waited for them • “Reaping a child process” – call wait() so that zombie’s resources can be destroyed • Orphans: processes that are still alive, but whose parent has already exited (without waiting for them) • Become the child of a dedicated process (“init”) who will reap them when they exit • “Run Away” processes: processes that (unintentionally) execute an infinite loop and thus don’t call exit() or wait() CS 3214 Fall 2010

16. Unix File Descriptors • Unix provides a file descriptor abstraction • File descriptors are • Small integers that have a local meaning within one process • Can be obtained from kernel • Several functions create them, e.g. open() • Can refer to various kernel objects (not just files) • Can be passed to a standard set of functions: • read, write, close, lseek, (and more) • Can be inherited when a process forks a child CS 3214 Fall 2010

17. Examples • 0-2 are initially assigned • 0 – stdin • 1 – stdout • 2 – stderr • But this assignment is not fixed – process can change it via syscalls • int fd = open(“file”, O_RDONLY); • int fd = creat(“file”, 0600); CS 3214 Fall 2010

18. Implementing I/O Redirection • dup and dup2() system call • pipes: pipe(2) CS 3214 Fall 2010

19. dup2 #include <stdio.h> #include <stdlib.h> // redirect stdout to a file int main(int ac, char *av[]) { int c; intfd = creat(av[1], 0600); if (fd == -1) perror("creat"), exit(-1); if (dup2(fd, 1) == -1) perror("dup2"), exit(-1); while ((c = fgetc(stdin)) != EOF) fputc(c, stdout); } CS 3214 Fall 2010

20. user view kernel view The Big Picture Process 1 0 1 2 Terminal Device 3 open(“x”) 4 File Descriptor Open File x open(“x”) close(4) Process 2 0 File Descriptor 1 2 3 dup2(3,0) CS 3214 Fall 2010

21. Reference Counting • Multiple file descriptors may refer to same open file • Within the same process: • fd = open(“file”); fd2 = dup(fd); • Across anchestor processes: • fd = open(“file”); fork(); • But can also open a file multiple times: • fd = open(“file”); fd2 = open(“file”); • In this case, fd and fd2 have different read/write offsets • In both cases, closing fd does not affect fd2 • Reference Counting at 2 Levels: • Kernel keeps track of how many processes refer to a file descriptor –fork() and dup() may add refs • And keeps track of how many file descriptors refer to open file • close(fd) removes reference in current process CS 3214 Fall 2010

22. Practical Implications • Number of simultaneously open file descriptors per process is limited • 1024 on current Linux, for instance • Must make sure fd’s are closed • Else ‘open()’ may fail • Number space is reused • “double-close” error may inadvertently close a new file descriptor assigned the same number CS 3214 Fall 2010

23. IPC via “pipes” • A bounded buffer providing a stream of bytes flowing through • Properties • Writer() can put data in pipe as long as there is space • If pipe() is full, writer blocks until reader reads() • Reader() drains pipe() • If pipe() is empty, readers blocks until writer writes • Classic abstraction • Decouples reader & writer • Safe – no race conditions • Automatically controls relative progress – if writer produces data faster than reader can read it, it blocks – and OS will likely make CPU time available to reader() to catch up. And vice versa. write() read() Fixed Capacity Buffer CS 3214 Fall 2010

24. int main() { intpipe_ends[2]; if (pipe(pipe_ends) == -1) perror("pipe"), exit(-1); int child = fork(); if (child == -1) perror("fork"), exit(-1); if (child == 0) { char msg[] = { "Hi" }; close(pipe_ends[0]); write(pipe_ends[1], msg, sizeofmsg); } else { char bread, pipe_buf[128]; close(pipe_ends[1]); printf("Child said "); fflush(stdout); while ((bread = read(pipe_ends[0], pipe_buf, sizeofpipe_buf)) > 0) write(1, pipe_buf, bread); } } pipe Note: there is no race condition in this code. No matter what the scheduling order is, the message sent by the child will reach the parent. CS 3214 Fall 2010

25. esh – extensible shell • Open-ended assignment • Encourage collaborative learning • Run each other’s plug-ins • Does not mean collaboration on your implementation • Secondary goals: • Exposure to yacc/lex and exposure to OO-style programming in C CS 3214 Fall 2010

26. Using the list implementation list_entry(e, structesh_command, elem) structesh_pipeline: …. struct list commands structesh_command: …. structlist_elemelem; …. • Key features: “list cell” – here call ‘list_elem’ is embedded in each object being kept in list • Means you need 1 list_elem per list you want to keep an object in structesh_command: …. structlist_elemelem; …. structlist_elem *next; structlist_elem *prev; structlist_elem head structlist_elem tail structlist_elem *next; structlist_elem *prev; structlist_elem *next; structlist_elem *prev; structlist_elem *next; structlist_elem *prev; CS 3214 Fall 2010