Thread: bit fields in a structure

Firstly, you want to do this to ensure your bit fields do not take up the same bits:

Code:

struct test {
                  union test1{
                                        struct{
                                                short int         Version:4,
                                                                      Ack_Req:1,
                                                                      Ack_Rsp:1,
                                                                      Reserved:10;
                                        };
                                         short int       var;
                   }test11;
}test22;

Second, the outermost struct is pointless, unless you have other elements in it besides the union.

Third, the order of the bit fields is not guaranteed and is compiler dependent. Accessing the struct (containing all the bit fields) and the var member interchangeably yields undefined, compiler dependent, behavior. It is up to the programmer to ensure that this does not happen. If you assign "val" a value, it would then be undefined behavior to get the bits fields (except to assign a value to them, thereby invalidating the previous assignment). Similarly if you assign values to any of the bit fields, then it is undefined behavior to access val for anything but assignment.

All that said, if you do determine the order of the bit fields that the compiler uses, it is pretty safe to use the union as a converting mechanism. That assumes that you are planning on using just the one compiler.

All that said, if you do determine the order of the bit fields that the compiler uses, it is pretty safe to use the union as a converting mechanism. That assumes that you are planning on using just the one compiler.

Not to mention padding or structure alignment, which is likely to be another problem. (I think it applies to bit fields.) And edianness would be an issue too.

Since you already have the data you want to put into the union in a variable, why not just store the data in the variable and use bit shifts whenever you want to access a given variable? You could create a macro or function to do this. Alternatively, assign each field of the structure individually. It's really the easiest way.