The cb and pGrid fields can be set by the application with CM_SETGRIDINFO before sending this message.

If pGrid is NULL, the fields cxGrid, cyGrid, sGridRows, sGridCols, and cGridSquares are updated by the container.

If pGrid is non-NULL, the container expects it to point to an array of GRIDSQUARE structures whose size is equal to cGridSquares*sizeof(GRIDSQUARE). The container will copy its internal grid square information into the GRIDINFO structure passed by the application.