Is it valid to use bit fields with union? - c

Question

I have used bit field with a structure like this,
struct
{
unsigned int is_static: 1;
unsigned int is_extern: 1;
unsigned int is_auto: 1;
} flags;
Now i wondered to see if this can be done with a union so i modified the code like,
union
{
unsigned int is_static: 1;
unsigned int is_extern: 1;
unsigned int is_auto: 1;
} flags;
I found the bit field with union works but all those fields in the union are given to a single bit as I understood from output. Now I am seeing it is not erroneous to use bit fields with union, but it seems to me that using it like this is not operationally correct. So what is the answer - is it valid to use bit field with union?

Answer 1

It is valid but as you found out, not useful the way you have done it there.
You might do something like this so you can reset all the bits at the same time using flags.
union {
struct {
unsigned int is_static: 1;
unsigned int is_extern: 1;
unsigned int is_auto: 1;
};
unsigned int flags;
};
Or you might do something like this:
union {
struct {
unsigned int is_static: 1;
unsigned int is_extern: 1;
unsigned int is_auto: 1;
};
struct {
unsigned int is_ready: 1;
unsigned int is_done: 1;
unsigned int is_waiting: 1;
};
};

Answer 2

You are given a gun and bullets. Is it okay to shoot your self in foot with it? Of course not, but nobody can stop you from doing this if you want to.
My point is, just like gun and bullets, union and bit fields are tools and they have their purpose, uses and "abuses". So using bitfields in union, as you have written above, is perfectly valid C but a useless piece of code. All the fields inside union share same memory so all the bitfields you mention are essentially same flag as they share same memory.

Answer 3

Here is an example on how to use bit fields with unions. I'm also showing how to arrange for MSB. In pictures it would look something like this:
MSB LSB
7 0
+------+-------+
| five | three |
| bits | bits |
+------+-------+
// A struct tag definition of
// two bit fields in an 8-bit register
struct fields_tag {
// LSB
unsigned int five:5;
unsigned int three:3;
// MSB
};
// here is a tag and typedef for less typing
// to modify the 8-bit value as a whole
// and read in parts.
typedef union some_reg_tag {
uint8_t raw;
struct fields_tag fields;
} some_reg_t;
Here is how to use the bit fields with Arduino
some_reg_t a_register;
a_register.raw = 0xC2; // assign using raw field.
Serial.print("some reg = "); // dump entire register
Serial.println(a_register.raw, HEX); // dump register by field
Serial.print("some reg.three = ");
Serial.println(a_register.fields.three, HEX);
Serial.print("some reg.five = ");
Serial.println(a_register.fields.five, HEX);
Here is the output showing the results
some reg = C2
some reg.three = 6
some reg.five = 2