From 01a6c054d548d9fff8bbdfac4d3f3de4ae8677a1 Mon Sep 17 00:00:00 2001 From: Austin Clements Date: Wed, 7 Sep 2011 11:49:14 -0400 Subject: Remove web directory; all cruft or moved to 6.828 repo --- web/l1.html | 288 ------------------------------------------------------------ 1 file changed, 288 deletions(-) delete mode 100644 web/l1.html (limited to 'web/l1.html') diff --git a/web/l1.html b/web/l1.html deleted file mode 100644 index 9865601..0000000 --- a/web/l1.html +++ /dev/null @@ -1,288 +0,0 @@ -L1 - - - - - -

OS overview

- -

Overview

- -

Goal of course: - -
- Understand operating systems in detail by designing and -implementing miminal OS -
- Hands-on experience with building systems ("Applying 6.033") -
- -
What is an operating system? -
- a piece of software that turns the hardware into something useful -
- layered picture: hardware, OS, applications -
- Three main functions: fault isolate applications, abstract hardware, -manage hardware -
- -
Examples: -
- OS-X, Windows, Linux, *BSD, ... (desktop, server) -
- PalmOS Windows/CE (PDA) -
- Symbian, JavaOS (Cell phones) -
- VxWorks, pSOS (real-time) -
- ... -
- -
OS Abstractions -
- processes: fork, wait, exec, exit, kill, getpid, brk, nice, sleep, -trace -
- files: open, close, read, write, lseek, stat, sync -
- directories: mkdir, rmdir, link, unlink, mount, umount -
- users + security: chown, chmod, getuid, setuid -
- interprocess communication: signals, pipe -
- networking: socket, accept, snd, recv, connect -
- time: gettimeofday -
- terminal: -
- -
Sample Unix System calls (mostly POSIX) -
- int read(int fd, void*, int) -
- int write(int fd, void*, int) -
- off_t lseek(int fd, off_t, int [012]) -
- int close(int fd) -
- int fsync(int fd) -
- int open(const char*, int flags [, int mode]) -
  - O_RDONLY, O_WRONLY, O_RDWR, O_CREAT -
  -
- mode_t umask(mode_t cmask) -
- int mkdir(char *path, mode_t mode); -
- DIR *opendir(char *dirname) -
- struct dirent *readdir(DIR *dirp) -
- int closedir(DIR *dirp) -
- int chdir(char *path) -
- int link(char *existing, char *new) -
- int unlink(char *path) -
- int rename(const char*, const char*) -
- int rmdir(char *path) -
- int stat(char *path, struct stat *buf) -
- int mknod(char *path, mode_t mode, dev_t dev) -
- int fork() -
  - returns childPID in parent, 0 in child; only - difference -
  -
- int getpid() -
- int waitpid(int pid, int* stat, int opt) -
  - pid==-1: any; opt==0||WNOHANG -
  - returns pid or error -
  -
- void _exit(int status) -
- int kill(int pid, int signal) -
- int sigaction(int sig, struct sigaction *, struct sigaction *) -
- int sleep (int sec) -
- int execve(char* prog, char** argv, char** envp) -
- void *sbrk(int incr) -
- int dup2(int oldfd, int newfd) -
- int fcntl(int fd, F_SETFD, int val) -
- int pipe(int fds[2]) -
  - writes on fds[1] will be read on fds[0] -
  - when last fds[1] closed, read fds[0] retursn EOF -
  - when last fds[0] closed, write fds[1] kills SIGPIPE/fails - EPIPE -
  -
- int fchown(int fd, uind_t owner, gid_t group) -
- int fchmod(int fd, mode_t mode) -
- int socket(int domain, int type, int protocol) -
- int accept(int socket_fd, struct sockaddr*, int* namelen) -
  - returns new fd -
  -
- int listen(int fd, int backlog) -
- int connect(int fd, const struct sockaddr*, int namelen) -
- void* mmap(void* addr, size_t len, int prot, int flags, int fd, - off_t offset) -
- int munmap(void* addr, size_t len) -
- int gettimeofday(struct timeval*) -
-

- -

See the reference page for links to -the early Unix papers. - -

Class structure

- -

Lab: minimal OS for x86 in an exokernel style (50%) -
- kernel interface: hardware + protection -
- libOS implements fork, exec, pipe, ... -
- applications: file system, shell, .. -
- development environment: gcc, bochs -
- lab 1 is out -
- -
Lecture structure (20%) -
- homework -
- 45min lecture -
- 45min case study -
- -
Two quizzes (30%) -
- mid-term -
- final's exam week -
- -

- -

Case study: the shell (simplified)

- -

interactive command execution and a programming language -
Nice example that uses various OS abstractions. See Unix -paper if you are unfamiliar with the shell. -
Final lab is a simple shell. -

Basic structure: -

-      
-       while (1) {
-	    printf ("$");
-	    readcommand (command, args);   // parse user input
-	    if ((pid = fork ()) == 0) {  // child?
-	       exec (command, args, 0);
-	    } else if (pid > 0) {   // parent?
-	       wait (0);   // wait for child to terminate
-	    } else {
-	       perror ("Failed to fork\n");
-            }
-        }
-

The split of creating a process with a new program in fork and exec -is mostly a historical accident. See the assigned paper for today. -

Example: -
```
-        $ ls
-
```
-
why call "wait"? to wait for the child to terminate and collect -its exit status. (if child finishes, child becomes a zombie until -parent calls wait.) -
I/O: file descriptors. Child inherits open file descriptors -from parent. By convention: -
- file descriptor 0 for input (e.g., keyboard). read_command: -
```
-     read (1, buf, bufsize)
-
```
  -
- file descriptor 1 for output (e.g., terminal) -
```
-     write (1, "hello\n", strlen("hello\n")+1)
-
```
  -
- file descriptor 2 for error (e.g., terminal) -
-
How does the shell implement: -
```
-     $ls > tmp1
-
```
-just before exec insert: -
```
-    	   close (1);
-	   fd = open ("tmp1", O_CREAT|O_WRONLY);   // fd will be 1!
-
```
-
The kernel will return the first free file descriptor, 1 in this case. -

How does the shell implement sharing an output file: -

-     $ls 2> tmp1 > tmp1
-

-replace last code with: -

-
-	   close (1);
-	   close (2);
-	   fd1 = open ("tmp1", O_CREAT|O_WRONLY);   // fd will be 1!
-	   fd2 = dup (fd1);
-

-both file descriptors share offset -

how do programs communicate? -

-        $ sort file.txt | uniq | wc
-

-or -

-	$ sort file.txt > tmp1
-	$ uniq tmp1 > tmp2
-	$ wc tmp2
-	$ rm tmp1 tmp2
-

-or -

-        $ kill -9
-

A pipe is an one-way communication channel. Here is an example -where the parent is the writer and the child is the reader: -

-
-	int fdarray[2];
-	
-	if (pipe(fdarray) < 0) panic ("error");
-	if ((pid = fork()) < 0) panic ("error");
-	else if (pid > 0) {
-	  close(fdarray[0]);
-	  write(fdarray[1], "hello world\n", 12);
-        } else {
-	  close(fdarray[1]);
-	  n = read (fdarray[0], buf, MAXBUF);
-	  write (1, buf, n);
-        }
-

How does the shell implement pipelines (i.e., cmd 1 | cmd 2 |..)? -We want to arrange that the output of cmd 1 is the input of cmd 2. -The way to achieve this goal is to manipulate stdout and stdin. -

The shell creates processes for each command in -the pipeline, hooks up their stdin and stdout correctly. To do it -correct, and waits for the last process of the -pipeline to exit. A sketch of the core modifications to our shell for -setting up a pipe is: -

	    
-	    int fdarray[2];
-
-  	    if (pipe(fdarray) < 0) panic ("error");
-	    if ((pid = fork ()) == 0) {  child (left end of pipe)
-	       close (1);
-	       tmp = dup (fdarray[1]);   // fdarray[1] is the write end, tmp will be 1
-	       close (fdarray[0]);       // close read end
-	       close (fdarray[1]);       // close fdarray[1]
-	       exec (command1, args1, 0);
-	    } else if (pid > 0) {        // parent (right end of pipe)
-	       close (0);
-	       tmp = dup (fdarray[0]);   // fdarray[0] is the read end, tmp will be 0
-	       close (fdarray[0]);
-	       close (fdarray[1]);       // close write end
-	       exec (command2, args2, 0);
-	    } else {
-	       printf ("Unable to fork\n");
-            }
-

Why close read-end and write-end? multiple reasons: maintain that -every process starts with 3 file descriptors and reading from an empty -pipe blocks reader, while reading from a closed pipe returns end of -file. -
How do you background jobs? -
```
-        $ compute &
-
```
-
How does the shell implement "&", backgrounding? (Don't call wait -immediately). -
More details in the shell lecture later in the term. - - - - -- cgit v1.2.3