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Struct in union

Soren Hansen

Hi
I have some problems with struct inside a union
This is my code.

union{
	unsigned char id_byte[2];

	struct {
			unsigned char DLC:4;
			unsigned char RTR:1;
			unsigned int identifier:11;
			}del_element;
}can_descriptor;

when I write an_descriptor.del_element.identifier=0x0A;

nothing comes in id_byte array why?
The values I put in DLC and RTR are placed correct.

If i change the code to
union{
	unsigned char id_byte[2];

	struct {
			unsigned int identifier:11;
			unsigned char DLC:4;
			unsigned char RTR:1;
			}del_element;
}can_descriptor;
And writes 0x0A to identifier:
id_byte[0]=0
id_byte[1]=0x0A

but i shoud be
id_byte[0]=0x80
id_byte[1]=0x01

why?
This program is part of an CAN program for SJA1000, this funktion i made make it easy to write the CAN iddentifiers DLC and RTR bit in the controller.....if it worked.

Best Regrads
Soren

Parents
  • 0 in reply to Soren Hansen

    if I shifts, I need more code, becourse I need to shift different if i have a RTR bit set.

    I rather strongly suspect the opposite to be true. If the shift is, indeed, different depending on the status of some bits, that's impossible to model using bitfields alone, because the lengths, and thus by implication the positions, of bitfields inside a struct are fixed at compile time.
    It's much more transparent to have the argument of a shift operator be a variable or expression than having to do something like

    if(rtr)
   my_union.rtr_set.somebit = 1;
else
   my_union.rtr_clr.somebit = 1;

Reply
  • 0 in reply to Soren Hansen

    if I shifts, I need more code, becourse I need to shift different if i have a RTR bit set.

    I rather strongly suspect the opposite to be true. If the shift is, indeed, different depending on the status of some bits, that's impossible to model using bitfields alone, because the lengths, and thus by implication the positions, of bitfields inside a struct are fixed at compile time.
    It's much more transparent to have the argument of a shift operator be a variable or expression than having to do something like

    if(rtr)
   my_union.rtr_set.somebit = 1;
else
   my_union.rtr_clr.somebit = 1;

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