Missing example with string in key/value

[recoilme]

Hi
I try implement string key/value storage and found this to hard for me
If i right understand - sophia not store strings as cstring, but as array of bytes
If i right code in errorExit - will not work, (cast to cstring in Nim will fail with segfault)

I found the strange workarround how set string and get it with casting to array[0,char]
It's work, but i dont understand why) May you explain how do it right?

My code is:

import sophia,strutils

proc free(obj: pointer) {.importc: "free", header: "<stdio.h>"}

proc errorExit() =
  var size: cint
  var error = env.getstring("sophia.error", addr size);
  #cast to csting not working?
  var msg = $(cast[ptr cstring](error)[])
  echo("Error: " & msg)
  free(error)
  discard env.destroy()

# Create a Sophia environment
var env = env()

# Set directory and add a db named test
discard env.setstring("sophia.path", "sophia3".cstring, 0)
discard env.setstring("db", "db".cstring, 0)

# Get the db
var db = env.getobject("db.db")

# Open the environment
var rc = env.open()
if (rc == -1):  errorExit()
echo "Opened Sophia env"

# Store strings
var o:pointer
var key:string = "hello"
var value:string = "hello world"

o = document(db)
discard o.setstring("key".cstring, addr key[0], (key.len).cint)
discard o.setstring("value".cstring, addr value[0], (value.len).cint)
rc =  db.set(o);
if (rc == -1): errorExit()
o = document(db)
discard o.setstring("key".cstring, addr key[0], (key.len).cint)
o = db.get(o)
if (o!=nil):
  var valPointer = o.getstring("value".cstring, nil)
  #very strange but work
  var returnVal = $(cast[ptr array[0,char]](valPointer)[])
  echo returnVal
  echo returnVal.len
else: 
  echo "not found"  
echo "Yup, it's gone, all good"

# Clean up
discard destroy(env)