From ReactOS

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, Windows 2003 and Vista, same for 64 Bit Versions:

typedef struct _BASEOBJECT
{
    HANDLE hHmgr;          // 0x00
    ULONG ulShareCount;    // 0x04
    USHORT cExclusiveLock; // 0x08
    USHORT BaseFlags;      // 0x0a
    PW32THREAD Tid;        // 0x0c
} BASEOBJECT, *POBJ; 
 
 typedef struct _BASEOBJECT DD_BASEOBJECT, *PDD_BASEOBJECT; //(correct?)

hHmgr

Gdi handle for the object, if the object is in the handle table. NULL otherwise.

ulShareCount

Number of references to the object.

cExclusiveLock

Number of exclusive locks held by a thread. These are only held for a short time and only for some of the object types.

BaseFlags

0x800 for objects that are allocated from a lookaside list (DC, region, bitmap, palette, font and brush)

See also

PW32THREAD