error: pasting "." and "x" does not give a valid preprocessing token

Make sure you are not doing the mistake of putting "##" before macro argument i.e VAR,
it should be put only between macro argument and the string to be concatenated.

Change something like this
"arg->VAR. ## VAR ## _val,"
to "arg->VAR.VAR ## _val,"

Also, no need of "##" for the line below for calling the function.

"G(win-> ## FUNC,(c, win,"
to "G(win->FUNC,(c, win,"