windows-server-2003/inetcore/outlookexpress/inetcomm/exrwlck/exrwlck.cpp

//
// This file contains test implmentations of reader and writer locks.
// These are intended to be used with the template class in rw.h so that
// different implementations can be plugged in and tested.
// 
// The semantics of the read/write classes should be as follows : 
//	Functions CAN NOT be recursively called,
//	Multiple Readers should be able to enter the lock 
//	Only a single writer may execute at a time.
//


#ifndef WIN16


#include	<windows.h>
#include	<limits.h>

#define Assert(x)	// Just define a dummy Assert, so we don't get
                    // compilation errors from exrwlck.h

#include "badstrfunctions.h"
#include	"exrwlck.h"

#ifdef	DEBUG
#ifndef	_VALIDATE
#define	_VALIDATE( f )	if( (f) ) ; else DebugBreak() 
#endif
#else
#ifndef	_VALIDATE
#define	_VALIDATE( f ) 
#endif
#endif

long	const	BlockValue = (-LONG_MAX) / 2; 
                            // Large in magnitude, negative value.  Used to 
                            // indicate a waiting writer in cReadLock


CExShareLock::CExShareLock( ) : cReadLock( 0  ), cOutRdrs( 0 )	{
    InitializeCriticalSection( &critWriters ) ;
    hWaitingWriters = CreateSemaphore( NULL, 0, 1, NULL ) ;
    hWaitingReaders = CreateSemaphore( NULL, 0, LONG_MAX, NULL ) ;
}

CExShareLock::~CExShareLock( ) {
    CloseHandle( hWaitingWriters ) ;
    CloseHandle( hWaitingReaders ) ;
    DeleteCriticalSection( &critWriters ) ;
}


void
CExShareLock::ShareLock( ) {
    long	sign = InterlockedIncrement( &cReadLock ) ;
    if( sign > 0 ) {
        return ;
    }	else 	{
        // There must be a writer in the lock.  Wait for him to leave.
        // The InterlockedIncrement recorded our presence so that the writer
        // can later release the correct number of threads.
        WaitForSingleObject( hWaitingReaders, INFINITE ) ;
    }
}

void
CExShareLock::ShareUnlock( ) {
    //
    // Leave the lock.  The return value will be negative if there is a writer
    // waiting.
    BOOL fWriterWaiting = InterlockedDecrement( &cReadLock ) < 0 ;

    if( fWriterWaiting ) {
        //
        // The following increment occurs when there is writer waiting, but
        // readers own the lock.  So although cReadLock is temporarily inaccurate
        // about the number of readers waiting for the lock, it is not inaccurate 
        // when it matters in WriteUnlock (which assumes a writer owns the lock.)
        //
        long junk = InterlockedIncrement( &cReadLock ) ;	// restore the value in cReadLock, so that we
                                                // end up with an accurate count of readers waiting
                                                // for entry.  

        long sign = InterlockedDecrement( &cOutRdrs ) ;	// Make sure we don't lose track of the 
                                                // number for readers who have left the lock.
        //
        // Are we the last reader out of the lock ?
        //
        if( sign == 0 ) {
            //
            // Definately the last reader out !
            //
            ReleaseSemaphore( hWaitingWriters, 1, &junk ) ;
        }
    }
}

void
CExShareLock::ExclusiveLock( ) {
    // Only one writer allowed to try for the lock at a time.
    //
    EnterCriticalSection( &critWriters ) ;

    //
    // Need to track the number of readers who leave the lock while we 
    // are trying to grab it.
    //
    cOutRdrs = 0 ;
    // Grab the lock 
 	long	oldsign = InterlockedExchange( &cReadLock, BlockValue ) ;
    // How many readers left while we grabbed the lock ??
    long	oldval = InterlockedExchange( &cOutRdrs, oldsign ) ;

    //
    // Accurately track all the readers who left the lock.
    //
    long	cursign = 1 ;	// Initialize to 1 so that if while loop not executed
                            // following if statement works correctly.
    while( oldval++ ) 
        cursign = InterlockedDecrement( &cOutRdrs ) ; 

    //
    // Do we own the lock ?  Only if there were no readers, or they have all left already.
    //
    if( oldsign == 0 || cursign == 0 ) {
        // We have the lock
    }	else	{
        // Wait for a reader to signal us.
        WaitForSingleObject( hWaitingWriters, INFINITE ) ;
    }
}



void
CExShareLock::ExclusiveUnlock( ) 	{

    // Estimate how many readers are waiting for the lock
    long	cWaiting = cReadLock - BlockValue ;

    // This Exchange allows any readers who have just arrived to grab the lock.
    // Also, it accounts for cWaiting of the blocked readers.
    long	cNewWaiting = InterlockedExchange( &cReadLock, cWaiting ) - BlockValue ;
    
    // cNewWaiting is the EXACT number of blocked readers - we will increment cReadLock
    // until we have accounted for the difference between our estimate and the correct
    // number !
    long	cTotal = cNewWaiting ;	// Save cNewWaiting for later use
    while( cNewWaiting-- > cWaiting ) 
        InterlockedIncrement( &cReadLock ) ;

    if( cTotal > 0 ) {
        long	junk = 0 ;
        ReleaseSemaphore( hWaitingReaders, cTotal, &junk ) ;	// let all those readers go!
    }
    // Let the next writer take his shot at the lock!
    LeaveCriticalSection( &critWriters ) ;
}


BOOL
CExShareLock::SharedToExclusive( )	{

    // tbd - implement this!
    return( FALSE ) ;
}


#endif