Access to Domain Pressure Field or mesh
 

I want to define 'pressure field' (P) in nuSgsWallFunctionFvPatchScalarField.C. (OpenFOAM/OpenFOAM-1.6/src/turbulenceModels/incompressible/LES/derivedFvPatchFields/wallFunctions/nuSgsWallFunctions/nuSgsWallFunction). I need pressure field in order to calculate grad(p) to use in this file. I can access to p only on the patches which don't let us calculate grad(p). Are there any ways to get 'p' field in such file?
Furthermore mesh is not declared in this code! Are there any ways to declare mesh in this code??

thanks in advance