create a packed struct

[Jonathan]

Long story short, I need to create a packed struct.

I knew the pragma way, to change aligment, didn't work. I looked up and found the __packed (I'm using Keil µVision) here: www.keil.com/.../armcc_chr1359124229695.htm

So in the end, here's my theoritically bulletproof code:

#pragma pack(push)
#pragma pack(1)

__packed struct PackedStructure
{
  __packed uint16_t packedValue;
};

#pragma pack(pop)

And now what does sizeof(struct PackedStructure) return ? 4...

Note I've tried pretty much any combination of pragma pack and __packed on structure/member, with always the same result.

And this:

typedef __packed __attribute__((packed)) __attribute__((aligned(1))) struct
{
  __packed uint16_t packedValue;
  __packed uint16_t packedValue2;
} Event;

Gives me a size of 8.

Anyone can tell me how to get a packed structure? Thanks!

[Keton]

@Jonathan I've checked this on x64 using gcc, nRF52 using both Keil and gcc and Raspberry pi using gcc. Each time following code:

typedef struct
{
	uint16_t test;
} short_single_t;

typedef struct
{
	uint16_t test;
	uint16_t test2;
} short_double_t;

struct PackedStructure
{
  uint16_t packedValue;
};

#ifdef NRF52
#define printf NRF_LOG_INFO
#endif

static void struct_size_test(void)
{
	printf("sizeof(short_single_t)==%zu\n",sizeof(short_single_t));
	printf("sizeof(short_double_t)==%zu\n",sizeof(short_double_t));
	printf("sizeof(struct PackedStructure)==%zu\n",sizeof(struct PackedStructure));
}

gives me this result:

sizeof(short_single_t)==2
sizeof(short_double_t)==4
sizeof(struct PackedStructure)==2

According to this guide behavior I see is mandated by C standard. Shorts (uint16_t is short) are self-aligned to 2 byte boundary so struct containing only shorts will have 2 byte alignment and no padding.

[Jonathan]

I had tried the various optimization myself without any success. And of course, that's right, if it was thumb, you wouldn't get 2 bytes alignment on x86-64. Thanks for your time!