create a packed struct

Question

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 · Accepted Answer

@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
",sizeof(short_single_t));
	printf("sizeof(short_double_t)==%zu
",sizeof(short_double_t));
	printf("sizeof(struct PackedStructure)==%zu
",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.

create a packed struct

Top Replies