doutucui0133
doutucui0133
2014-04-05 23:03

如何对使用联合的C结构建模Go绑定?

  • unions
  • c
  • cgo
已采纳

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?

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2条回答

  • dongpao9165 dongpao9165 7年前

    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
    }
    
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  • doolo00026 doolo00026 7年前

    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. (If this were a port I'd use interfaces instead).

    Consider, similar to what he wrote:

    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

    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. So in this case n is 13 (three 4-byte int32 and one 1-byte uint8). Using unsafe:

    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

    void setStandardFile(*mifare_desfire_file_settings fs, uint32_t size) {
        fs.settings.standardfile.file_size = size
    }
    

    And write a passthrough function in Go:

    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.

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