Can you explain this ?

[btgvesta]

Can you explain this code ?

#define UMWFromPointer(Pointer) ((umw)(Pointer))
#define PointerFromUMW(type, Value) (type *)(Value)

[bagaffey]

That type umw is an unsigned machine word. It's an alias for uintptr_t which is an unsigned int type that is guaranteed to be able to hold a pointer without losing any bits.

There are also 2 macros there that will get an umw from a Pointer or give a pointer to a certain type from a given UMW.

UMWFromPointer(Pointer), this converts a pointer into an integer that is large enough to hold it.
So, what you get is a numerical representation of the pointer that you pass to it.

PointerFromUMW(type, Value), this takes an integer like one created from UMWFromPointer(Pointer) and cast it back into a pointer of type type.
So, Value would be a UMW (an unsigned machine word)

Why do this at all? Sometimes an int field can be useful for storing a pointer especially when interfacing with OS APIs.
This is a method of serializing a pointer and getting a pointer back from that serialized data.