ARM gcc structure alignment

test.h

#ifndef TEST_DEFINED
#define TEST_DEFINED

#define OFFSET(x, y)  &((x *)0)->y

typedef unsigned int __u32;
typedef int __s32;
typedef long long __s64;
typedef unsigned char __u8;

#endif

test1.c

#include "test.h"

#include <stdio.h>
#include <time.h>

struct v4l2_event_vsync {
        /* Can be V4L2_FIELD_ANY, _NONE, _TOP or _BOTTOM */
        __u8 field;
} __attribute__ ((packed));

struct v4l2_event {
        __u32                           type;
        union {
                struct v4l2_event_vsync vsync;
                __u8                    data[64];
        } u;
        __u32                           pending;
        __u32                           sequence;
        struct timespec                 timestamp;
        __u32                           reserved[9];
};

int main() {
        printf("sizeof v4l2_event = %d\n", sizeof(struct v4l2_event));

        printf("offsets of structure members\n");
        printf("offset of \'type\' = %d\n", (int)OFFSET(struct v4l2_event, type));
        printf("offset of \'u\' = %d\n", (int)OFFSET(struct v4l2_event, u));
        printf("offset of \'pending\' = %d\n", (int)OFFSET(struct v4l2_event, pending));
        printf("offset of \'sequence\' = %d\n", (int)OFFSET(struct v4l2_event, sequence));
        printf("offset of \'timestamp\' = %d\n", (int)OFFSET(struct v4l2_event, timestamp));
        printf("offset of \'reserved\' = %d\n", (int)OFFSET(struct v4l2_event, reserved));

        return 0;
}

test2.c

#include "test.h"

#include <stdio.h>
#include <time.h>

struct v4l2_event_vsync {
        /* Can be V4L2_FIELD_ANY, _NONE, _TOP or _BOTTOM */
        __u8 field;
} __attribute__ ((packed));

struct v4l2_event_ctrl {
        __u32 changes;
        __u32 type;
        union {
                __s32 value;
                __s64 value64;
        };
        __u32 flags;
        __s32 minimum;
        __s32 maximum;
        __s32 step;
        __s32 default_value;
};

struct v4l2_event_frame_sync {
        __u32 frame_sequence;
};

struct v4l2_event {
        __u32                           type;
        union {
                struct v4l2_event_vsync         vsync;
                struct v4l2_event_ctrl          ctrl;
                struct v4l2_event_frame_sync    frame_sync;
                __u8                            data[64];
        } u;
        __u32                           pending;
        __u32                           sequence;
        struct timespec                 timestamp;
        __u32                           id;
        __u32                           reserved[8];
};


int main() {
        printf("sizeof v4l2_event = %d\n", sizeof(struct v4l2_event));
        printf("sizeof v4l2_event_ctrl = %d\n", sizeof(struct v4l2_event_ctrl));

        printf("offsets of structure members\n");
        printf("offset of \'type\' = %d\n", (int)OFFSET(struct v4l2_event, type));
        printf("offset of \'u\' = %d\n", (int)OFFSET(struct v4l2_event, u));
        printf("offset of \'pending\' = %d\n", (int)OFFSET(struct v4l2_event, pending));
        printf("offset of \'sequence\' = %d\n", (int)OFFSET(struct v4l2_event, sequence));
        printf("offset of \'timestamp\' = %d\n", (int)OFFSET(struct v4l2_event, timestamp));
        printf("offset of \'id\' = %d\n", (int)OFFSET(struct v4l2_event, id));
        printf("offset of \'reserved\' = %d\n", (int)OFFSET(struct v4l2_event, reserved));
        return 0;
}