Function sto_subfield 

pub unsafe extern "C" fn sto_subfield(
    read_ptr: u32,
    read_len: u32,
    field_id: u32,
) -> i64

Index into a xrpld serialized object and return the location and length of a subfield. Except for Array subtypes the offset and length refer to the payload of the subfield not the entire subfield. Use SUB_OFFSET and SUB_LENGTH macros to extract return value. @param read_ptr The memory location of the stobject @param read_len The length of the stobject @param field_id The Field Code of the subfield @return high-word (most significant 4 bytes excluding the most significant bit (MSB)) is the field offset relative to read_ptr and the low-word (least significant 4 bytes) is its length. MSB is sign bit, if set (negative) return value indicates error (typically error means could not find.)