January 9, 2006

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Introduction to CPS 445: Systems Programming II

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• course objectives, outline, and policies

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Why study this stuff anyways?

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• communication and concurrency are everything in today's software
• ability to write reliable and secure code is indispensable (counter-terrorism)
• gateway to studies in networking and system administration

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Major Sources of Bugs

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• poor indentation
• failure to check return values, esp. NULL
• failure to check error codes

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Good Practices in C

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• always initialize pointer variables
  
  examples:

  Node* node_ptr = NULL;
  
  char* filename = "input.txt";
  
  FILE* myinstream = fopen (filename, "r");
  

• when allocating memory, always verify that the memory was allocated successfully
  
  example:

  if ((node_ptr = malloc (sizeof (Node))) == NULL) {
     fprintf (stderr, "out of memory!");
     exit (1);
  } else {
       ...
       exit (0);
     }
  

• once finished, always free memory that you explicitly allocated
  
  example:

  if ((node_ptr = malloc (sizeof (Node))) == NULL) {
     fprintf (stderr, "out of memory!");
     exit (1);
  } else {
       ...
       free (node_ptr);
       exit (0);
     }
  

• when opening a file, always verify that the file was opened successfully
  
  example:

  if ((fp = fopen (filename, "r")) == NULL) {
     fprintf (stderr, "cannot open %s\n", filename);
     exit (1);
  } else {
       ...
       exit (0);
     }
  

• always close files that you explicitly opened
  
  example:

  if ((fp = fopen (filename, "r")) == NULL) {
     fprintf (stderr, "cannot open %s\n", filename);
     exit (1);
  } else {
       ...
       fclose (fp);
       exit (0);
    }
  

• always print error and debugging messages to stderr (output written to stdout is buffered)
  
  example:

  if ((fp = fopen (filename, "r")) == NULL) {
     fprintf (stderr, "cannot open %s\n", filename);
     exit (1);
  } else {
       ...
    }
  

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Quick Review of Operating System Nomenclature

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• program vs. process
• process vs. thread
• multiprogramming (vs. batch processing)
• timesharing
• context switch
• context switch time
• quantum (time slice)
• system call
• process control block (or PCB)
• execution stack
• activation record
• multiprocessor system
• asynchronous operation
• concurrency
• communication
• (asynchronous or synchronous) interrupt (hardware)
• (asynchronous or synchronous) signal (software)
• device driver
• job scheduling
• process scheduling
• paging
  
  
• UNIX is a multiprogramming and timeshared OS

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Concurrency

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• achieved in UNIX with functions fork, wait, and exec
• processes with a common ancestor can communicate through pipes
• processes without common ancestor can communicate through signals, queues, semaphores, monitors, shared address space, messages

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Distributed Computation

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• client-server model, e.g., ftp and http
• object-based model
  • theme: objects passing messages to each other
  • encourages code reuse
• application (software) layers, e.g.,
  • MPI (Message Passing Interface)
  • CORBA (Common Object Request Broker Architecture)
  • RMI (Remote Method Invocation)
  • and other P2P (Peer to Peer) approaches

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Buffer Overflows

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• programs and data represented uniformly in main memory
  • programs are data
• layout of the stack in main memory (stack grows from high to low memory)
• how a root shell might be attained through telnet
• 1988, Robert Morris' worm exploited a buffer overflow in the finger daemon, nearly brought down the Internet
• CERT (Computer Emergency Response Team) formed
• some recent articles on buffer overflows
  • Detection and Prevention of Stack Buffer Overflow Attacks
    by Kuperman et al. from Communications of the ACM, Vol. 48, No. 11, pp. 50-56, November 2005.
  • USB Devices Can Crack Windows
    by Paul F. Roberts from eWeek.com, July 2005.
  • Buffer Overrun Madness
    by Rodney Bates from ACM Queue, Vol. 2, No. 3, pp. 12-14, May 2004.

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UNIX Standards

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• POSIX (Portable Operating System Interface)
  • IEEE standard for UNIX libraries to promote the development of reliable software
  • Solaris, Mac OS X, and many other flavors of UNIX are moving toward POSIX standards
    • POSIX threads