typedef struct PID
{
     double     SetPoint;         // Desired Value

     double     Proportion;         // Proportional Const
     double     Integral;         // Integral Const
     double     Derivative;         // Derivative Const

     double     LastError;         // Error[-1]
     double     PrevError;         // Error[-2]
     double     SumError;         // Sums of Errors
}   PID;

/*=========================================================================*
     Perform One PID Iteration
*=========================================================================*/

double PIDCalc ( PID     *pp,    double NextPoint )

{   double     dError, Error;

     pp->SumError += (Error = pp->SetPoint - NextPoint);
     dError = pp->LastError - pp->PrevError;
     pp->PrevError = pp->LastError;
     pp->LastError = Error;
     return     (   pp->Proportion * Error
                     +   pp->Integral * pp->SumError
                     +   pp->Derivative * dError
                 );
}

/*=========================================================================*
     Initialize PID Structure
*=========================================================================*/

void PIDInit   ( PID *pp )

{       memset ( pp,0,sizeof(PID) );
}

/***************************************************************************
     Main Program
***************************************************************************/

double sensor ( void )                         // Dummy Sensor Function
{       return 100.0;
}

void actuator ( double rDelta )             // Dummy Actuator Function
{}

void main ( void )
{     PID             sPID;                              // PID Control Structure
     double         rOut;                              // PID Response (Output)
     double         rIn;                              // PID Feedback (Input)

     PIDInit ( &sPID );                         // Initialize Structure
     sPID.Proportion     = 0.5;                 // Set PID Coefficients
     sPID.Integral       = 0.5;
     sPID.Derivative     = 0.0;
     sPID.SetPoint       = 100.0;                 // Set PID Setpoint

     for (;;)                                         // Mock Up of PID Processing
     {     rIn = sensor ();                         // Read Input
         rOut = PIDCalc ( &sPID,rIn );         // Perform PID Interation
         actuator ( rOut );                     // Effect Needed Changes
     }
}