Techwiki:Win32k/BASEOBJECT

The GDI Base Object, which is a header at the top of every GDI Object. The structure is called _BASEOBJECT and is defined as follows for Windows XP and Windows 2003 :

struct _BASEOBJECT
{
   HANDLE hHmgr; // Handle for this object
   ULONG ulShareCount;  
   LONG cExclusiveLock; // lock with InterlockedIncrement or not
   ULONG BaseFlags;     
   PW32THREAD Tid;   // contain which thread it belong to, PsGetCurrentThread()
} *POBJ;

The pointer to a _BASEOBJECT is known as a POBJ.