Multiple modifiers (2) in Keyboard Input Report for custom HID keyboard

Question

I'm building a macro keyboard and one of the functions I'm trying to implement is Ctrl+Shift+R, but in the definitions, only one modifier exists in the fixed 8 byte string. How do I implement additional modifiers?

Answer 1

USB keyboards almost always have a HID Report Descriptor that defines each inbound keyboard report as follows:

Bit:        7   6   5   4   3   2   1   0
          +---+---+---+---+---+---+---+---+
Byte 0    | RG| RA| RS| RC| LG| LA| LS| LC|  Modifier bits (LC=Left Control, LS= Left Shift, etc)
          +---+---+---+---+---+---+---+---+
Byte 1    |        Reserved byte          |
          +---+---+---+---+---+---+---+---+
Byte 2    |        Key 1                  |
          +---+---+---+---+---+---+---+---+
Byte 3    |        Key 2                  |
          +---+---+---+---+---+---+---+---+
Byte 4    |        Key 3                  |
          +---+---+---+---+---+---+---+---+
Byte 5    |        Key 4                  |
          +---+---+---+---+---+---+---+---+
Byte 6    |        Key 5                  |
          +---+---+---+---+---+---+---+---+
Byte 7    |        Key 6                  |
          +---+---+---+---+---+---+---+---+

Each modifier key is represented as a single bit in byte 0. To indicate that multiple modifier keys are pressed you would "or" the values together. You could code something like:

#define MOD_LEFT_CONTROL 0b00000001
#define MOD_LEFT_SHIFT   0b00000010
#define MOD_LEFT_ALT     0b00000100
.
.
#define KEY_R            0x15
.
.
modifiers = MOD_LEFT_CONTROL | MOD_LEFT_SHIFT;
reserved = 0;
key[0] = KEY_R;

It is possible to define a HID Report Descriptor that allows modifier key usages to be included in the 6-byte key array but there is usually no need to do that - and the above scheme uses less space anyway.