Understanding data structure in usbjoy library from v-usb project

Question

good day to you all.

I am trying to make my own joystick, so I have been looking for references and then I found usbjoy project from v-usb wiki page (http://vusb.wikidot.com/project:usbjoy).

Then, I found data structure for buttons, taken from common.h in zip file from the website.

typedef struct
{
  uchar x; //Byte-0, ﾕ (0...255)
  uchar y; //Byte-1, Y (0...255)
        uchar z; //Byte-2, Handle-1 (0...255)
  uchar p; //Byte-3, Handle-2 (0...255)

  union {
    uchar buttons; //Byte-4, buttons 8
    struct
    {
      uchar btn1:    1; //0, 1
      uchar btn2:    1; //0, 1
      uchar btn3:    1; //0, 1
      uchar btn4:    1; //0, 1
      uchar btn5:    1; //0, 1
      uchar btn6:    1; //0, 1
      uchar btn7:    1; //0, 1
      uchar btn8:    1; //0, 1
    } b;
  } u;
        union {
    uchar but; //Byte-5, buttons 4
    struct
    {
      uchar btn9:    1; //0, 1
      uchar btn10:   1; //0, 1
      uchar btn11:   1; //0, 1
      uchar btn12:   1; //0, 1
      uchar padding: 4; //Not use
                } b;
  } w;
} t_PsxController;

I understand that x and y are for left analog pad, z and p are for right analog pad, and u and w are for buttons. My questions are:

1. Why are u and w declared as unions?
2. Will the struct inside the unions ever be used?
3. What is the size of t_PsxController?
4. And finally, what do colons in uchar btn1: 1; and codes below it mean?

Answer 1

Why are u and w declared as unions?

You will be using one button from eight button at a time right ? Only one member of data structure need to be accessed so union is used .Understand the difference between structure and union. Joystick can be utilized left pad ,right pad and

Will the struct inside the unions ever be used?

Yes it represent different button so it will be used.

What is the size of t_PsxController?

t_PsxController is structure and maximum size of structure is summation of all member of structure.

And finally, what do colons in uchar btn1: 1; and codes below it mean?

Inside structure of union uchar btn1: 1 represents unsigned char bit field that occupies 1 bits

Inside structure of union uchar padding: 4 represents unsigned char bit field that occupies 4 bits

Answer 2

t_PsxController is a struct 6 bytes in size. Each byte is numbered in the comments of the code you posted. The colon : in some of the lines directs the compiler to devote a certain number of bits (1 or 4 in this example) to the item, rather than an entire byte. This makes each of the unions only 1 byte long.

t_PsxController controller;

will declare a struct called controller that you can use later. It is 6 bytes long.

To access members of the struct, use the . dot operator. The identifier you use will determine which member of the union you are accessing. E.g.,

controller.x = 23; // assigns a value to the byte 0
controller.u.b.btn1 = 1; // assigns a 1 to the first bit of the byte 4
uchar x = controller.u.buttons; // assigns 128 to x

You may want to use a pointer to controller at some point, especially when passing into a function. You then need to use the -> operator as well as the . dot.

t_PsxController *ctlr = controller;

ctlr->u.b.btn2 = 1; // Now ctlr->u.buttons is 192