typedef struct {
       uint8_t* buf;
       size_t buflen;
   } API_t;

   void API_init_as_writer(API_t* p, uint8_t* buf, size_t buflen);

data API  -- empty decl (needs EmptyDataDecls extension)

   foreign import ccall unsafe "API.h API_init_as_writer"
       init :: Ptr API -> Ptr Word8 -> Int -> IO ()

writer <- initWriter 1024     -- inits with buflen of 1024 via mallocPlainForeignPtrBytes
    writeSomething writer
    writeSomethingElse writer
    ...
    bs <- getByteString writer    -- construct ByteString with PS
    writeFile "Some.file" bs

bs <- ByteString.readFile("Some.file")
    reader <- initReader bs
    info1 <- readSomething reader
    info2 <- readSomethingElse reader