Union Instruction in C-CodePudding

I'm trying to convert following code into union Instruction in C:


typedef struct  InstructionTypeI_t {
        unsigned long immediate: 16;
        unsigned char rt:5; // start a new byte
        unsigned char rs : 5;
        unsigned char opcode : 6;
} InstructionTypeI_t;



typedef struct  InstructionTypeJ_t {
        unsigned long address: 26;
        unsigned char opcodce: 6;
} InstructionTypeJ_t;

typedef struct InstructionTypeR{
    unsigned long funct: 6;
    unsigned char shamt:5; // start a new byte
    unsigned char rd : 5;
    unsigned char rt : 5;
    unsigned char rs: 5;
    unsigned char opcode: 6;
} InstructionTypeR_t;


/* TODO Task (d) add union Instruction here */
typedef union Instruction{
    InstructionTypeI_t i;
    InstructionTypeJ_t j;
    InstructionTypeR_t r;
}Instruction;

union Instruction instruction

   InstructionTypeI_t i;
    InstructionTypeJ_t j;
    InstructionTypeR_t r;

interestingly this works

instruction.i=i;
instruction.j=j;

but this converts values wrong meaning for example instruction.i.opcode and i.opcode have different vlaues

instruction.i=i;
instruction.j=j;
instruction.r=r;

Any ideas what the problem could be?

CodePudding user response：

Adjacent bitfields have to have the same underlying type, otherwise they're not merged into the same instance of the underlying type.

I.e. by having an unsigned long bitfield, followed by some unsigned char ones, you get a full unsigned long variable that only holds that single bitfield, and nothing else.

The solution is to use the same type for all those bitfields, probably uint32_t or at least unsigned int. I wouldn't use unsigned long due to its non-portable size.