What do these struct and typedef definitions mean?

Question

I have the following code, which works fine and has been in use for a quite a while now... but I have no idea what it means.

struct event_param
{
    int task:3;
    int param1;
    int param2;
};

#define SV_DRIVER_EVENTS_MASK_SIZE (SV_DRIVER_EVENT_LAST*sizeof(struct event_param))
typedef struct event_param driver_event_mask[SV_DRIVER_EVENTS_MASK_SIZE];
typedef driver_event_mask DriverEventMask;

• What does driver_event_mask represent?
• Why is there sizeof(struct event_param) inside the array?

Answer 1

The :3 thing represents a bit field . I'm not sure it matters much, though, since sizeof(struct event_param) will still almost certainly be 12 bytes, I think.

So what happens here is that an array of SV_DRIVER_EVENT_LAST of event_param structures is created. But I would also expect that the sizeof(struct event_param) is not necessary here -- the length of an array is defined in the number of units it contains, not in bytes.

1. driver_event_mask is just what it says. A type to represent an array of SV_DRIVER_EVENTS_MASK_SIZE struct event_param items.

2. sizeof(struct event_param) is not needed here, but doesn't hurt (ie won't cause bugs) since it just means you allocate about 12 times as much memory as you actually need.

Answer 2

how to typedef strcuts array in c?

How to define is already shown in your question, for why refer below:-

If a typedef name denotes a variable length array type, the length of the array is fixed at the time the typedef name is defined, not each time it is used:

what does DriverEventMask represents

DriverEventMask represent struct event_param

why there are sizeof(struct event_param)

So that SV_DRIVER_EVENTS_MASK_SIZE will represent the size of struct event_param. This struct contain bit fields and the way it is arranged is not sure until we know if there are any compiler directives to avoid padding.