C - Common Failures

// List of Common Failures
// ------------------------------------------------------------------------
//     - mathematical overflows
//         - implement all available controller traps
//     - unexpected program states or values
//         - implement default branches
//         - make plausibility checks
//         - use greater-/smaller- instead equal-comparison
//     - invalid pointers
//     - memory failures
//     - overload
//         - make stress tests
//     - endless loops
//         - watchdog
//             - single reset inside main loop or idle task
//             - checks if all code segments were executed
//               can be made with flag-variables
//         - execution time observation
//     - stackoverflow
//         - detect by using stack-end-marker (magic-number)
//     - race condition
//         - control mutual used variables and resources
//     - dead lock
 
 
// Basics Controller Mistakes
// ------------------------------------------------------------------------
//    - not initialized ports
//        - read/write (TRIS), analog/digital (ANSEL)
//    - floating unused I/O pins
//        - configured as output and driven to a logic-low state
//    - ISR executes too often
//        - clear interrupt flag
//    - execution stops or ends in unexpected behavior
//        - missing endless main-loop
//    - copied code segements
//        - changed only one code position instead of both
//    - wrong function qualifiers
//        - static/extern
//    - wrong variable qualifiers
//        - static/extern, volatile, const
//    - other register values after watchdog-reset compared to power-on-reset
 
 
// Detailed Failure Descriptions
// ------------------------------------------------------------------------
 
//     Race Condition
//     --------------------------------------------------------------------
//         - very hard to find, because they depends on the execution oder
//           from parallel code
//         - and furthermore very hard to locate
//         - debugging changes the runtime behavior of the program
//           execution and so also the RC occurrence probability
//         - example of two parallel executions (both codes uses the same
//           counter variable)
//             code 1:                  code 2:
//                 ...                      ...
//                 cnt = cnt + 1;           cnt++;
//                 ...                      ...
//         - race condition (e.g. assembler directives)
//                 load cnt
//                 -->                      load cnt
//                                          increase cnt
//                                          store cnt
//                 increase cnt             <--
//                 store cnt
//
//           the counter will be incremented by 1, instead of 2
//
//         - solution depends on available methods
//             - locks, mutex, critical section, semaphore,
//               disable interrupt (for uC purpose), ...
//             - transactional memory
//
//             lock example:
// global:
    int  var;
    bool var_written;
 
// main loop:
    int var_buffer;
 
    if (!var_written)           // read value at single point
    {                           // and use the buffer variable,
        GID;                    // which is used only by this task
        var_buffer = var;
        GIE;
    }
 
    GID;                        // write buffer and set flag
    var_buffer  = x;
    var_written = true;
    GIE;
 
 
// isr:
    if (var_written)            // take over value
    {
        var         = var_buffer;
        var_written = false;
    }
 
//             transactional memory example:
//                 - start tansaction
//                 - execute transaction
//                 - test if transaction was interrupted
//                 - if yes redo
// global:
    int  var;
    bool var_valid;
 
// main loop:
    int var_buffer;
 
    do                          // if transaction was interrupted
    {                           // do it again
        var_valid = true;       // advantage: no interrupt disable necassary
        var_buffer = var;
    }
    while (!var_valid);
 
// isr:
    var_valid = false;