[英]How to model Go bindings to C structs that use unions?
I'm currently writing a Go wrapper for the libfreefare .我目前正在为libfreefare编写Go 包装器。 The API of the libfreefare contains the following function:
libfreefare 的 API 包含以下函数:
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?包装这样一个功能的理想解决方案是什么? If the
struct mifare_desfire_file_settings
wouldn't contain any unions, my wrapper would probably look like this:如果
struct mifare_desfire_file_settings
不包含任何联合,我的包装器可能看起来像这样:
type DESFireFileSettings struct {
// all fields exported, no methods
}
func (t DESFireTag) FileSettings(fileNo byte) (DESFireFileSettings, error)
How should I proceed?我应该如何进行?
You need to consider how you would update fields in the union. 您需要考虑如何更新联合中的字段。 Obviously, you can't let users do that without validation.
显然,如果没有验证,你不能让用户这样做。 They could do inconsistent updates.
他们可以做不一致的更新。 Consider doing something like this:
考虑做这样的事情:
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
}
I partially agree with peterSO, but I'd take a different approach to the DESFireFileSettings
struct since this is a wrapper and not a port. 我部分同意peterSO,但我对
DESFireFileSettings
结构采取了不同的方法,因为这是一个包装器而不是端口。 (If this were a port I'd use interfaces instead). (如果这是一个端口,我会使用接口)。
Consider, similar to what he wrote: 考虑一下,类似于他所写的:
type StandardFile struct {
FileSize uint32
_ [9]byte // for padding to size of ValueFile
}
type ValueFile struct {
LowerLimit int32
UpperLimit int32
LimitedCreditValue int32
LimitedCreditEnabled uint8
}
type LinearRecordFile struct {
Record_size uint32
MaxNumberOfRecords uint32
CurrentNumberOfRecords uint32
_ [1]byte // for padding to size of ValueFile
}
However, for DESFireFileSettings 但是,对于DESFireFileSettings
type DESFireFileSettings struct {
fs C.mifare_desfire_get_file_settings
}
Note that in Go unions are recorded as [n]byte where n is the biggest size of the union. 请注意,在Go中,联合会被记录为[n]个字节,其中n是联合的最大大小。 So in this case n is 13 (three 4-byte int32 and one 1-byte uint8).
所以在这种情况下,n是13(三个4字节int32和一个1字节uint8)。 Using unsafe:
使用不安全:
func (dfs *DESFireFileSettings) SetStandardFile(standardFile StandardFile) error {
// If not valid, return an error
dfs.fs.settings = *((*[13]byte)(unsafe.Pointer(&standardFile)))
return nil
}
If you want to avoid unsafe, you can make it more explicit with a C function 如果您想避免不安全,可以使用C函数使其更明确
void setStandardFile(*mifare_desfire_file_settings fs, uint32_t size) {
fs.settings.standardfile.file_size = size
}
And write a passthrough function in Go: 在Go中写一个passthrough函数:
func (dfs *DESFireFileSettings) SetStandardFile(standardFile StandardFile) error {
// If not valid, return an error
C.setStandardFile(&dfs.fs, C.uint32_t(standardFile.FileSize))
}
I, personally, prefer directly setting it with unsafe
in this context, but both ways are valid. 我个人而言,更喜欢在这种情况下直接设置它是
unsafe
,但两种方式都是有效的。
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