Question

I have defined the following structure below.

typedef union
{
    struct
    {
        unsigned command:15;
        unsigned acknowledge:1;
    };
    struct
    {
        unsigned short uicommand;
    };
    struct
    {
        Byte command_bytes[2];
    };
}TCOMMAND;

After then declaring a variable of type TCOMMAND E.G

TCOMMAND mycommand
int x=sizeof(mycommand)

The value of x is 4 bytes instead of 2.
Enlightenment on the problem will be appreciated.
Thanks in advance.

Answer 1

In C/C++ unsigned means unsigned int and even if you don’t assign all of the bits of a bit field, the compiler will still size your structure to the data type specified.

So assuming a 32 bit int (which is the default in Visual C++) the first structure of your union is the same as:

struct
{
    unsigned int command:15;
    unsigned int acknowledge:1;
    unsigned int __unused:16;
};

Which is four bytes. Use an unsigned short in your first structure if you want it to be only two bytes:

struct
{
    unsigned short command:15;
    unsigned short acknowledge:1;
};