Union inside a Structure.

[Jinzpaul4u]

Hello there,

Good wishes.

I have the basic idea about structure and union.but I found some codes, that using structure inside union , union inside structure, insdie a structure a union and again one more structure inside the same union.
So,where union inside structure is used? what is the advantage for it?

Best regards,

 

[bigdogguru]

Combining a structure within a union can streamline code generation.

You must be aware of the endianness of the compiler and/or device.

An example of the techniques use for ADC values:

union ADCvalue {
  unsigned int EntireWord;
  struct bytes {
     unsigned char MSB;
     unsigned char LSB;
  }
}

union ADCvalue ADCval;
int ADCResult ;

ADCval.bytes.MSB = ADRESH;    // retrieve and store ADC's MSB
ADCval.bytes.LSB = ADRESL;    // retrieve and store ADC's LSB

ADCResult = ADCval.EntireWord;          // retrieve 16bit result

Of course, depending on the endianness of the compiler and/or device, if applicable, you may need to reverse the order of MSB and LSB within the structure definition.

Another typical application is the requirement to access both the entire value, collection of bits or single bits of a register, such as various device specific header files utilized by compilers.

Example:

// Register: T1CON
extern volatile unsigned char           T1CON               @ 0x018;
#ifndef _LIB_BUILD
asm("T1CON equ 018h");
#endif
// bitfield definitions
typedef union {
    struct {
        unsigned TMR1ON                 :1;
        unsigned                        :1;
        unsigned nT1SYNC                :1;
        unsigned T1OSCEN                :1;
        unsigned T1CKPS0                :1;
        unsigned T1CKPS1                :1;
        unsigned TMR1CS0                :1;
        unsigned TMR1CS1                :1;
    };
    struct {
        unsigned                        :4;
        unsigned T1CKPS                 :2;
        unsigned TMR1CS                 :2;
    };
} T1CONbits_t;
extern volatile T1CONbits_t T1CONbits @ 0x018;
// bitfield macros
#define _T1CON_TMR1ON_POSN                                  0x0
#define _T1CON_TMR1ON_POSITION                              0x0
#define _T1CON_TMR1ON_SIZE                                  0x1
#define _T1CON_TMR1ON_LENGTH                                0x1
#define _T1CON_TMR1ON_MASK                                  0x1
#define _T1CON_nT1SYNC_POSN                                 0x2
#define _T1CON_nT1SYNC_POSITION                             0x2
#define _T1CON_nT1SYNC_SIZE                                 0x1
#define _T1CON_nT1SYNC_LENGTH                               0x1
#define _T1CON_nT1SYNC_MASK                                 0x4
#define _T1CON_T1OSCEN_POSN                                 0x3
#define _T1CON_T1OSCEN_POSITION                             0x3
#define _T1CON_T1OSCEN_SIZE                                 0x1
#define _T1CON_T1OSCEN_LENGTH                               0x1
#define _T1CON_T1OSCEN_MASK                                 0x8
#define _T1CON_T1CKPS0_POSN                                 0x4
#define _T1CON_T1CKPS0_POSITION                             0x4
#define _T1CON_T1CKPS0_SIZE                                 0x1
#define _T1CON_T1CKPS0_LENGTH                               0x1
#define _T1CON_T1CKPS0_MASK                                 0x10
#define _T1CON_T1CKPS1_POSN                                 0x5
#define _T1CON_T1CKPS1_POSITION                             0x5
#define _T1CON_T1CKPS1_SIZE                                 0x1
#define _T1CON_T1CKPS1_LENGTH                               0x1
#define _T1CON_T1CKPS1_MASK                                 0x20
#define _T1CON_TMR1CS0_POSN                                 0x6
#define _T1CON_TMR1CS0_POSITION                             0x6
#define _T1CON_TMR1CS0_SIZE                                 0x1
#define _T1CON_TMR1CS0_LENGTH                               0x1
#define _T1CON_TMR1CS0_MASK                                 0x40
#define _T1CON_TMR1CS1_POSN                                 0x7
#define _T1CON_TMR1CS1_POSITION                             0x7
#define _T1CON_TMR1CS1_SIZE                                 0x1
#define _T1CON_TMR1CS1_LENGTH                               0x1
#define _T1CON_TMR1CS1_MASK                                 0x80
#define _T1CON_T1CKPS_POSN                                  0x4
#define _T1CON_T1CKPS_POSITION                              0x4
#define _T1CON_T1CKPS_SIZE                                  0x2
#define _T1CON_T1CKPS_LENGTH                                0x2
#define _T1CON_T1CKPS_MASK                                  0x30
#define _T1CON_TMR1CS_POSN                                  0x6
#define _T1CON_TMR1CS_POSITION                              0x6
#define _T1CON_TMR1CS_SIZE                                  0x2
#define _T1CON_TMR1CS_LENGTH                                0x2
#define _T1CON_TMR1CS_MASK                                  0xC0

BigDog