How to model Go bindings to C structs that use unions?

I'm currently writing a Go wrapper for the libfreefare. The API of the libfreefare contains the following function:

struct mifare_desfire_file_settings {
    uint8_t file_type;
    uint8_t communication_settings;
    uint16_t access_rights;
    union {
	struct {
	    uint32_t file_size;
	} standard_file;
	struct {
	    int32_t lower_limit;
	    int32_t upper_limit;
	    int32_t limited_credit_value;
	    uint8_t limited_credit_enabled;
	} value_file;
	struct {
	    uint32_t record_size;
	    uint32_t max_number_of_records;
	    uint32_t current_number_of_records;
	} linear_record_file;
    } settings;
};

int		 mifare_desfire_get_file_settings (MifareTag tag, uint8_t file_no, struct mifare_desfire_file_settings *settings);

What is the ideomatic solution for wrapping such a function? If the struct mifare_desfire_file_settings wouldn't contain any unions, my wrapper would probably look like this:

type DESFireFileSettings struct {
    // all fields exported, no methods
}

func (t DESFireTag) FileSettings(fileNo byte) (DESFireFileSettings, error)

How should I proceed?

You need to consider how you would update fields in the union. Obviously, you can't let users do that without validation. They could do inconsistent updates. Consider doing something like this:

package mifare
const (
MDFTStandarDataFile            = 0x00
MDFTBackupDataFile             = 0x01
MDFTValueFileWithBackup        = 0x02
MDFTLinearRecordFileWithBackup = 0x03
MDFTCyclicRecordFileWithBackup = 0x04
)
type StandardFile struct {
FileSize uint32
}
type ValueFile struct {
LowerLimit           int32
UpperLimit           int32
LimitedCreditValue   int32
LimitedCreditEnabled uint8
}
type LinearRecordFile struct {
Record_size            uint32
MaxNumberOfRecords     uint32
CurrentNumberOfRecords uint32
}
type DESFireFileSettings struct {
FileType              uint8
CommunicationSettings uint8
AccessRights          uint16
settings              struct {
StandardFile
ValueFile
LinearRecordFile
}
}
func (fs *DESFireFileSettings) StandardFile() (StandardFile, error) {
// if not valid for FileType, return error
return fs.settings.StandardFile, nil
}
func (fs *DESFireFileSettings) SetStandardFile(standardFile StandardFile) error {
// if not valid for FileType, return error
fs.settings.StandardFile = standardFile
return nil
}
func (fs *DESFireFileSettings) ValueFile() (ValueFile, error) {
// if not valid for FileType, return error
return fs.settings.ValueFile, nil
}
func (fs *DESFireFileSettings) SetValueFile(valueFile ValueFile) error {
// if not valid for FileType, return error
fs.settings.ValueFile = valueFile
return nil
}
func (fs *DESFireFileSettings) LinearRecordFile() (LinearRecordFile, error) {
// if not valid for FileType, return error
return fs.settings.LinearRecordFile, nil
}
func (fs *DESFireFileSettings) SetLinearRecordFile(linearRecordFile LinearRecordFile) error {
// if not valid for FileType, return error
fs.settings.LinearRecordFile = linearRecordFile
return nil
}