#endif
#if OS_WIN
  DWORD got;
  SimulateIOError(SQLITE_IOERR);
  if( !ReadFile(id->h, pBuf, amt, &got, 0) ){
    got = 0;
  }
  if( got==amt ){
    return SQLITE_OK;
  }else{
    return SQLITE_IOERR;
  }
#endif
}

................................................................................
** A lock is obtained on byte 0 before acquiring either a read lock or
** a write lock.  This prevents two processes from attempting to get a
** lock at a same time.  The semantics of sqliteOsReadLock() require that
** if there is already a write lock, that lock is converted into a read
** lock atomically.  The lock on byte 0 allows us to drop the old write
** lock and get the read lock without another process jumping into the
** middle and messing us up.  The same argument applies to sqliteOsWriteLock().
**




** There are a finite number of read locks under windows.  That number
** is determined by the following variable:
*/

#define MX_LOCKBYTE 10240


















































/*
** Change the status of the lock on the file "id" to be a readlock.
** If the file was write locked, then this reduces the lock to a read.
** If the file was read locked, then this acquires a new read lock.
**
** Return SQLITE_OK on success and SQLITE_BUSY on failure.
*/

int sqliteOsReadLock(OsFile *id){
#if OS_UNIX
  int rc;
  sqliteOsEnterMutex();
  if( id->pLock->cnt>0 ){
    if( !id->locked ){
      id->pLock->cnt++;
................................................................................
#if OS_WIN
  int rc;
  if( id->locked>0 ){
    rc = SQLITE_OK;
  }else{
    int lk = (sqliteRandomInteger() & 0x7ffffff)%MX_LOCKBYTE + 1;
    int res;
    if( (res = LockFile(id->h, 0, 0, 1, 0))!=0 ){

      UnlockFile(id->h, 1, 0, MX_LOCKBYTE, 0);
      res = LockFile(id->h, lk, 0, 1, 0);
      UnlockFile(id->h, 0, 0, 1, 0);

    }
    if( res ){
      id->locked = lk;
      rc = SQLITE_OK;
    }else{
      rc = SQLITE_BUSY;
    }
  }
................................................................................
#endif
#if OS_WIN
  int rc;
  if( id->locked<0 ){
    rc = SQLITE_OK;
  }else{
    int res;
    if( (res = LockFile(id->h, 0, 0, 1, 0))!=0 ){
      if( id->locked==0 || UnlockFile(id->h, id->locked, 0, 1, 0) ){
        res = LockFile(id->h, 1, 0, MX_LOCKBYTE, 0);
      }else{
        res = 0;
      }
      UnlockFile(id->h, 0, 0, 1, 0);




    }
    if( res ){

      id->locked = -1;
      rc = SQLITE_OK;
    }else{
      rc = SQLITE_BUSY;
    }
  }
  return rc;
................................................................................
  }
  sqliteOsLeaveMutex();
  id->locked = 0;
  return rc;
#endif
#if OS_WIN
  int rc;
  if( id->locked==0 ){
    rc = SQLITE_OK;
  }else if( id->locked<0 ){
    UnlockFile(id->h, 1, 0, MX_LOCKBYTE, 0);
    rc = SQLITE_OK;

    id->locked = 0;
  }else{
    UnlockFile(id->h, id->locked, 0, 1, 0);
    rc = SQLITE_OK;
    id->locked = 0;
  }

  return rc;
#endif
}

/*
** Get information to seed the random number generator.  The seed
** is written into the buffer zBuf[256].  The calling function must
** supply a sufficiently large buffer.

#endif
#if OS_WIN
  DWORD got;
  SimulateIOError(SQLITE_IOERR);
  if( !ReadFile(id->h, pBuf, amt, &got, 0) ){
    got = 0;
  }
  if( (int)got==amt ){
    return SQLITE_OK;
  }else{
    return SQLITE_IOERR;
  }
#endif
}

................................................................................
** A lock is obtained on byte 0 before acquiring either a read lock or
** a write lock.  This prevents two processes from attempting to get a
** lock at a same time.  The semantics of sqliteOsReadLock() require that
** if there is already a write lock, that lock is converted into a read
** lock atomically.  The lock on byte 0 allows us to drop the old write
** lock and get the read lock without another process jumping into the
** middle and messing us up.  The same argument applies to sqliteOsWriteLock().
**
** Locks must be obtained in an area that does not overlap the "real data area"
** otherwise read/write operations will conflict with lock operations. Locking beyond EOF
** is allowed in windows.
**
** There are a finite number of read locks under windows.  That number
** is determined by the following variable:
*/

#define MX_LOCKBYTE 0xFFF0

#if OS_WIN

// get the platform id to decide how to calculate the lock offset

int mkPlatformId(void){
 
 static long init=0;
 static int pid=VER_PLATFORM_WIN32_WINDOWS;
 OSVERSIONINFOA info;	
 
 if (!init) {
  if (InterlockedIncrement(&init)==1)
   {
    info.dwOSVersionInfoSize=sizeof(info);
   if (GetVersionEx(&info)) pid=info.dwPlatformId;
   }
  } 
 return pid;  
}

// locks and unlocks beyond eof. uses platformid to move the lock as far as possible.
int mklock(HANDLE h, WORD base, WORD size)
{
 if (mkPlatformId()==VER_PLATFORM_WIN32_WINDOWS)
  return LockFile(h,0xFFFF0000+base,0,size,0);
 else
  return LockFile(h,base,0xFFFFFFFF,size,0);
}

int mkunlock(HANDLE h, WORD base, WORD size)
{
 if (mkPlatformId()==VER_PLATFORM_WIN32_WINDOWS)
  return UnlockFile(h,0xFFFF0000+base,0,size,0);
 else
  return UnlockFile(h,base,0xFFFFFFFF,size,0);
}

//obtain the sync lock on a handle

void synclock(HANDLE h){ 
 while (!mklock(h,0,1)) Sleep(1);
}

void syncunlock(HANDLE h){ 
 mkunlock(h,0,1);
}

#endif

/*
** Change the status of the lock on the file "id" to be a readlock.
** If the file was write locked, then this reduces the lock to a read.
** If the file was read locked, then this acquires a new read lock.
**
** Return SQLITE_OK on success and SQLITE_BUSY on failure.
*/

int sqliteOsReadLock(OsFile *id){
#if OS_UNIX
  int rc;
  sqliteOsEnterMutex();
  if( id->pLock->cnt>0 ){
    if( !id->locked ){
      id->pLock->cnt++;
................................................................................
#if OS_WIN
  int rc;
  if( id->locked>0 ){
    rc = SQLITE_OK;
  }else{
    int lk = (sqliteRandomInteger() & 0x7ffffff)%MX_LOCKBYTE + 1;
    int res;

    
    synclock(id->h);
    if (id->locked<0) mkunlock(id->h,1,MX_LOCKBYTE); // release write lock if we have it
    res=mklock(id->h,lk,1);
    syncunlock(id->h);
    
    if( res ){
      id->locked = lk;
      rc = SQLITE_OK;
    }else{
      rc = SQLITE_BUSY;
    }
  }
................................................................................
#endif
#if OS_WIN
  int rc;
  if( id->locked<0 ){
    rc = SQLITE_OK;
  }else{
    int res;





    

    synclock(id->h);
    if (id->locked>0) mkunlock(id->h,id->locked,1); // release read lock
    res=mklock(id->h,1,MX_LOCKBYTE);
    syncunlock(id->h);
   

    if(res){
      id->locked = -1;
      rc = SQLITE_OK;
    }else{
      rc = SQLITE_BUSY;
    }
  }
  return rc;
................................................................................
  }
  sqliteOsLeaveMutex();
  id->locked = 0;
  return rc;
#endif
#if OS_WIN
  int rc;


  if(id->locked<0 ) {


    mkunlock(id->h,1,MX_LOCKBYTE);
  }else if (id->locked>0) {



    mkunlock(id->h,id->locked,1);
  }
  id->locked = 0;
  return SQLITE_OK;
#endif
}

/*
** Get information to seed the random number generator.  The seed
** is written into the buffer zBuf[256].  The calling function must
** supply a sufficiently large buffer.