Sets the double pointed to by @value corresponding to the value of the
given field. Caller is responsible for making sure the field exists
and has the correct type.
Return: %TRUE if the value could be set correctly. If there was no field
with @fieldname or the existing field did not contain a double, this
function returns %FALSE.
Sets the double pointed to by @value corresponding to the value of the given field. Caller is responsible for making sure the field exists and has the correct type.