Generic Pointer Question

[Nimbecile]

I'm testing the LMDB package and I'm stuck. When I retrieve key/value pairs 
they are saved to MDB_val structures:
    
    
    type
      Val* = object
        mvSize*: csize      #*< size of the data item
        mvData*: pointer    #*< address of the data item
    

I tried casting the generic pointer (mvData) to a cstring, but the data is not 
zero terminated so I get garbage attached. Knowing the starting address and 
number of bytes, what is the best way to capture the data (which is a string in 
my instance)? Thanks!