Libris Britannia 4/science library(b).zip/science library(b)/DJGPP/AETSK102.ZIP/contrib/tasks/src/task.cc

C/C++ Source or Header | 1991-12-01 | 15KB | 458 lines

/**********************************************************************
 *
 *  NAME:           task.cpp
 *
 *  DESCRIPTION:    routines for multitasking scheduler
 *
 *  copyright (c) 1991 J. Alan Eldridge
 *
 *  M O D I F I C A T I O N   H I S T O R Y
 *
 *  when        who                 what
 *  -------------------------------------------------------------------
 *  03/12/91    J. Alan Eldridge    created
 *
 *  Mar-May 91  JAE                 many, many revisions
 *
 *  10/23/91    JAE                 added code to force task stack size
 *                                  up to reasonable minimum value
 *
 *  10/26/91    JAE                 added support for DJ Delorie's port
 *                                  port of GNU C++ v. 1.39
 *
 *  11/09/91    JAE                 rewrote to use SysQP class for
 *                                  Sema, Task queues
 *
 *                                  tasks now can have priorities...
 *                                  (be careful of starvation!)
 *
 *  11/11/91    JAE                 added stack basher checking in class
 *                                  Task, called by scheduler
 *
 *  11/13/91    JAE                 moved semas to their own file
 *
 *  11/30/91    jae                 added fDelete flag to Task, so you can
 *                                  dynamically create a new Task on the heap,
 *                                  and then proceed, knowing that the storage
 *                                  will eventually be freed when the task
 *                                  is killed or commits sucicide ...
 *
 *                                  added new queue pZombies, and code in
 *                                  Task::suicide() and scheduer() to support
 *                                  this feature
 *
 *                                  added CHECK_... defines to control
 *                                  run-time checking for fatal errors
 *                                  (calling TaskFatal() if necessary)
 *
 *********************************************************************/

#include    "aedef.h"
#include    "task.h"

//------------------------------------------------------------
//  local defines to control error checking/reporting:
//  each of these enables to code to check for an error condition
//  and call TaskFatal() with a message if error occurred
//------------------------------------------------------------

#define CHECK_SUSPEND       1   //  set to 1 to check for attempt to
                                //  suspend non-current task

#define CHECK_STACK_OVER    1   //  set to 1 to check for stack overflow

#define CHECK_STACK_ALLOC   1   //  set to 1 to check for failed attempt
                                //  to allocate a Task's stack

#define CHECK_QUEUE_ALLOC   1   //  set to 1 to check for failure to create
                                //  the three system task queues

//------------------------------------------------------------
//  copyright notice: do not remove this definition!
//------------------------------------------------------------

static const char copyright[] = "Task++ v. " TASKPP_VERSION
    " Copyright (c) 1991 J. Alan Eldridge";

//------------------------------------------------------------
//  Stack fill value
//------------------------------------------------------------

const uchar Task::stkval = 0xAE;

//------------------------------------------------------------
//  ********** SCHEDULER **********
//------------------------------------------------------------

//------------------------------------------------------------
//  the scheduler's data storage is implemented entirely
//  by these static variables
//------------------------------------------------------------

//  static SLList   tasks;      //  list of Tasks

//  This variable "tasks" has been changed to a ptr to a
//  list of tasks. This was done at the suggestion of Peter W.
//  at Borland Tech Support. Here's the scenario, since
//  it is of interest to anyone creating global objects:
//
//  The order of calling global object constructors inside one
//  module is well defined: top to bottom, left to right.
//  The order of calling the constructors in multiple modules
//  is "implementation-defined" and subject to change without
//  notice. That is, I cannot ensure that the constructor for
//  "tasks" is called before the constructors for any global Task
//  instances. The only way I can control when the constructor
//  is called is to allocate the list using operator new.  Thus,
//  the ptr is initialized to 0 (explicitly, since we need to
//  make sure it is done before the startup code calls constructors
//  and zeros uninitialized globals). Then, the code to install
//  a task in the list (sch_AddTask()) checks the value, and
//  creates the list if it doesn't already exist. There is the
//  slight inefficiency caused by the extra level of indirection,
//  but this is the only reliable way to get the results I need.

static SysPriQP *pRunning = 0;  //  ptr to list of running Tasks
static SysQP    *pBlocked = 0;  //  ptr to list of blocked Tasks
static SysQP    *pZombies = 0;  //  ptr to list of zombie Tasks
                                //  (scheduler will kill them...)

static jmp_buf  schEnv;         //  environment in scheduler

//------------------------------------------------------------
//  the global CurrTask lets an application function know
//  who's currently executing
//------------------------------------------------------------

Task    *CurrTask = 0;

//------------------------------------------------------------
//  these 4 calls are the entire interface to the scheduler:
//  SchAddTask(), SchResume(), SchWakeup(), scheduler()
//------------------------------------------------------------

//------------------------------------------------------------
//  SchAddTask --  add a new task to the scheduler task list
//  WARNING: this should only be called by class Task constructor
//------------------------------------------------------------

inline void
SchAddTask(Task *t)
{
    if (!pRunning) {
        pRunning = new SysPriQP;
        pBlocked = new SysQP;
        pZombies = new SysQP;
#if CHECK_QUEUE_ALLOC
        if (!(pRunning && pBlocked && pZombies))
            TaskFatal("can't create task queues!");
#endif
    }

    pRunning->Add(t);
}

//------------------------------------------------------------
//  SchResume  --  return to setjmp() in scheduler()
//------------------------------------------------------------

inline void
SchResume(int code = 1)
{
    longjmp(schEnv, code);
}

//------------------------------------------------------------
//  SchWakeup() -- wakeup any sleeping or blocked tasks
//------------------------------------------------------------

inline void
SchWakeup()
{
    int cnt = pBlocked->Cnt();

    while (cnt-- > 0) {
        Task    *pt = GetNxtTask(*pBlocked);

        if (pt->maybeWake())
            pRunning->Add(pt);
        else
            pBlocked->Add(pt);
    }
}

//------------------------------------------------------------
//  scheduler() --  this is the main scheduler loop
//
//  if the arg is 0, it means somebody wants to shut
//  down all tasks... we do so, then resume the scheduler
//  at the setjmp(), where it will return on its own stack
//
//  otherwise...  every time a task surrenders control it
//  returns to the setjmp() near the beginning.  we then:
//
//      check for stack overflow
//      if it's not a zombie...
//          place it on the right queue (ready or blocked)
//
//  then we go into a simple loop:
//
//  while (there are tasks left) loop
//      wakeup anybody who can be awakened
//      get next one off ready queue
//      if nobody ready, continue
//      if it hasn't been initialized,
//          initialize it
//      else
//          make it return to where it suspended itself
//  end loop
//
//  returns 1 if forced down by scheduler(0), zero
//  if tasks terminated normally, and optionally calls TaskFatal()
//  if stack basher has been detected
//
//------------------------------------------------------------

int
scheduler(int tasking)
{
    if (!tasking) {
        CurrTask = 0;
        SchResume(-1);
    }

    int code = setjmp(schEnv);

    //  check state of CurrTask and put on a task queue

    if (code >= 0 && CurrTask) {
#if CHECK_STACK_OVER
        if (!CurrTask->stackok()) {
            //  die if we blew out the stack
            TaskFatal("stack overflow!");
        } else
#endif
        {
            //  add to appropriate queue
            if (CurrTask->isReady())
                pRunning->Add(CurrTask);
            else if (!CurrTask->isZombie())
                pBlocked->Add(CurrTask);
            else
                pZombies->Add(CurrTask);
        }
    }

    //  clean up any zombies left around

    int zcnt = pZombies->Cnt();

    while (zcnt-- > 0) {
        Task    *pt = GetNxtTask(*pZombies);

        if (pt->shouldDelete())
            delete pt;
    }

    //  find next eligible task to run and set it off

    while (code >= 0 && (pRunning->Cnt() > 0 || pBlocked->Cnt() > 0)) {
        SchWakeup();
        if (!(CurrTask = GetNxtTask(*pRunning)))
            continue;
        if (!CurrTask->isInited())
            CurrTask->init();
        else
            CurrTask->resume(CurrTask->timeOut() ? -1 : 1);
    }

    CurrTask = 0;
    return code < 0;
}

//------------------------------------------------------------
//  ********** CLASS TASK MEMBER FUNCTIONS **********
//------------------------------------------------------------

//------------------------------------------------------------
//  constructor for a Task: set flags, allocate stack space
//------------------------------------------------------------

Task::Task(char *tname, int stk): stklen(stk), tskname(tname)
{
    fInited = 0;
    fReady = 1;
    fTimed = 0;
    fZombie = 0;
    fDelete = 0;

    tskpri = 1;
    if (stk < StackMin)
        stk = StackMin;
    stack = new uchar [ stk ];
#if CHECK_STACK_ALLOC
    if (!stack)
        TaskFatal("can't allocate stack for task %s!", tname);
#endif
    memset(stack, stkval, stk);
    SchAddTask(this);
}

//------------------------------------------------------------
//  Task::timeOut()  --  return & reset timed out flag
//------------------------------------------------------------

int
Task::timeOut()
{
    int timedOut = fTimed;

    fTimed = 0;
    return timedOut;
}

//------------------------------------------------------------
//  Task::init()    --  initialize a task: switch stacks and
//  call the virtual TaskMain() function, never to return
//------------------------------------------------------------

//  this function is used as a wrapper so that the "this"
//  pointer is saved on the stack; that way, if a task returns,
//  it immediately commits suicide

static void
callTaskMain(Task *t)
{
    t->TaskMain();  //  call the main function ...
    t->suicide();   //  if it returns, the task wants to die
}

void
Task::init()
{
    fInited = 1;    //  we are initialized

    //  switch to private stack now

    static jmp_buf  stkswitch;

    if (!setjmp(stkswitch)) {
#if     defined(__TURBOC__)
        uchar far   *farstk = (uchar far *)stack;

        stkswitch[ 0 ].j_ss = FP_SEG(farstk);
        stkswitch[ 0 ].j_sp = FP_OFF(farstk) + stklen;
#elif   defined(__GNUG__)
        stkswitch[ 0 ].esp = (unsigned int)stack + stklen;
#endif
        longjmp(stkswitch, 1);
    }

    //  start task running ...
    callTaskMain(CurrTask); //  ... and never return
}

//------------------------------------------------------------
//  Task::stackok() --  have we blown out bottom of stack?
//------------------------------------------------------------

int
Task::stackok()
{
    const int   StackChk = 64;

    for (int i = 0; i < StackChk; i++)
        if (stack[ i ] != stkval)
            return 0;

    return 1;
}

//------------------------------------------------------------
//  Task::suicide() --  become a zombie (scheduler will finish off)
//------------------------------------------------------------

void
Task::suicide()
{
    fReady = 0;
    fZombie = 1;
    if (this == CurrTask)       //  if we are the current task, then we
        suspend();              //  aren't on any queues -- just go back
    else {
        pRunning->Del(this);    //  we're on one of these queues, so we
        pBlocked->Del(this);    //  just delete from both (one will succeed)
        pZombies->Add(this);    //  put on Zombie queue (scheduler will kill)
    }
}

//------------------------------------------------------------
//  Task::suspend() --  voluntarily give up control to scheduler
//------------------------------------------------------------

int
Task::suspend()
{
#if CHECK_SUSPEND
    if (this != CurrTask)
        TaskFatal("suspend task <%s>: not running!", name());
#endif

    int n;

    if (!(n = setjmp(tskEnv)))
        SchResume();
    return n;
}

//------------------------------------------------------------
//  Task::block()   --  block for an event, possibly with
//  a timeout period specified in milliseconds...
//  since the PC clock ticks 18.2 times a second, the granularity
//  of these calls is not too good, but it'll have to do
//------------------------------------------------------------

inline clock_t
msecToTicks(clock_t msec)
{
    const clock_t   msecPerTick = 10000 / 182;

    return (msec + msecPerTick / 2) / msecPerTick;
}

int
Task::block(clock_t msec)
{
    if (msec > -1) {
        wakeUp = clock() + msecToTicks(msec);
        fTimed = 1;
    }
    fReady = 0;
    return suspend() > 0;
}

//------------------------------------------------------------
//  Task::unblock() --  return to running queue
//------------------------------------------------------------

void
Task::unblock()
{
    fTimed = 0;
    fReady = 1;
    if (pBlocked->Del(this))
        pRunning->Add(this);
}

//------------------------------------------------------------
//  Task::maybeWake()   --  check the clock, and if the sleeping
//  task has slept long enough, return TRUE to unblock
//------------------------------------------------------------

int
Task::maybeWake()
{
    return fReady = (fTimed && clock() >= wakeUp);
}