aci_gatt_update_char_value for 20 bytes structs

[aj.sk]

I have an issue with an aci_gatt_update_char_value call to update a 20 byte characteristics.

So I defined a data struct that holds all my values.

typedef struct data {
		uint16_t var1;
		uint16_t var2;
		uint16_t var3;
		uint32_t var4;
		uint16_t var5;
		uint16_t var6;
		uint16_t var7;
		uint16_t var8;
		uint16_t var9;
	} dataset;

and then call the update function (*data is a pointer to the initialized struct)

ret = aci_gatt_update_char_value(service_handle_runtime, char_handle_du, 0, 20, (uint8_t*)&data);

If I define the struct only containing uint8_t or uint16_t everything arrives as expected. But with the uint32_t value I get somehow an offset or whatever.

Are there any restrictions when I have a characteristic with 20 bytes?

[aj.sk]

In case someone ever runs into this, just copy the struct into a uint8_t array buffer and hand the buffer to the aci_gatt_update_char_value() function.

[rdscliff]

Yeah, this looks like a compiler variable alignment problem. Nothing to do with the function itself.

[kasi viswanadh]

Basically u need to define 32 bit variables first and then 16 bits and 8bits in the struct to save space. Some compilers optimise it.

[Winfred LU]

Please declare the structure with the "packed" attribute to prevent compiler adding padding in the structure.

With GCC, declare your structure with "__attribute__((packed))":

typdef struct __attribute__((packed)) data {
	uint16_t var1;
	uint16_t var2;
	uint16_t var3;
	uint32_t var4;
	uint16_t var5;
	uint16_t var6;
	uint16_t var7;
	uint16_t var8;
	uint16_t var9;
} dataset;

In case of ArmCC, it is equivalent to:

typedef __packed struct data {
	uint16_t var1;
	uint16_t var2;
	uint16_t var3;
	uint32_t var4;
	uint16_t var5;
	uint16_t var6;
	uint16_t var7;
	uint16_t var8;
	uint16_t var9;
} dataset;