466: Operating Systems
=======================

Principles of operating system design, including process management, IPC, and networking.

Topics
------

- Process and thread management
- Scheduling algorithms
- Memory management and virtual memory
- File systems
- Inter-process communication (pipes, signals, sockets)
- Unix system call interface

Projects
--------

Custom Unix Shell (``466/shell/``)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The final iteration of a multi-stage shell project. Implements a POSIX-style command
interpreter with:

- Input/output redirection (``<``, ``>``, ``>>``)
- Pipes (``|``) chaining arbitrary commands
- Background execution (``&``)
- Built-in commands: ``cd``, ``exit``

.. code-block:: c

   int main(void) {
     char **tokenv;
     char *s;
     int tokenc;

     s = prompt();
     while (s != NULL) {
       tokenv = tokenize(s, &tokenc);
       if (tokenv[0] != NULL) {
         handle_tokens(tokenv, &tokenc);  /* dispatch: fork, pipe, redir, builtins */
       }
       free(tokenv);
       free(s);
       s = prompt();
     }
     return EXIT_SUCCESS;
   }

Socket Programming Utilities (``466/tok/socket.c``)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Low-level network programming examples using BSD sockets. Demonstrates TCP client/server
setup, ``connect``/``accept`` lifecycle, and streaming data over a socket.

.. code-block:: c

   int create_socket(u_short portnum) {
       struct sockaddr_in sa;
       gethostname(myname, MAXHOSTNAMELEN);
       hp = gethostbyname(myname);

       sa.sin_family = hp->h_addrtype;
       sa.sin_port   = htons(portnum);

       s = socket(AF_INET, SOCK_STREAM, 0);
       bind(s, (struct sockaddr *)&sa, sizeof sa);
       listen(s, 3);
       return s;
   }

   int wait_for_connection(int s) {
       struct sockaddr_in isa;
       int len = sizeof isa;
       return accept(s, (struct sockaddr *)&isa, &len);
   }

Token Ring Election (``466/tok/tok.c``)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Implements a distributed token ring leader-election algorithm using BSD sockets. Processes
communicate in a ring topology, passing tokens to elect a coordinator — demonstrating
socket setup, process coordination, and distributed consensus.

.. code-block:: c

   typedef union {
       char  as_char[2];
       short as_short;
   } tToken;                          /* 16-bit token value */

   #define NO_TOKEN  ((tToken)(short)-1)
   #define send_token(s, t) { tToken _t = t; write(s, &_t, sizeof(_t)); }

   /* Each process passes the token to the next in the ring;
    * a process keeps the token only if its ID is the highest seen. */

External Sort via Fork/Exec (``466/sort/main.c``)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Forks a child process and uses ``exec`` to pipe file input through the system ``sort``
utility, demonstrating the Unix process model, file descriptor manipulation, and
``fork``/``exec``/``wait`` patterns.

.. code-block:: c

   pipe(a);   /* parent → child (input to sort) */
   pipe(b);   /* child → parent (output from sort) */

   if (fork() == 0) {
       /* child: wire stdin/stdout to the pipes, then exec sort */
       dup2(a[0], STDIN_FILENO);
       dup2(b[1], STDOUT_FILENO);
       execvp("sort", (char *[]){ "sort", NULL });
   } else {
       /* parent: write lines, then read sorted output */
       while (fgets(line, sizeof line, infile))
           write(a[1], line, strlen(line));
       close(a[1]);   /* signal EOF to sort */
       FILE *fin = fdopen(b[0], "r");
       while (fgets(buf, sizeof buf, fin))
           printf("%s", buf);
   }

Directory Reader (``466/dir/main.c``)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reads and prints a file character-by-character using low-level POSIX I/O, an introductory
exercise in file descriptor usage and the ``read`` system call.

.. code-block:: c

   FILE *dir = fopen("dir", "r");
   char c;

   /* Read one byte at a time and print char + its ASCII value */
   while ((c = fgetc(dir)) != EOF)
       printf("%c %d\n", c, (unsigned char)c);