Skip to main content
aj.sk
aj.sk
Associate III
February 26, 2019
Question

aci_gatt_update_char_value for 20 bytes structs

  • February 26, 2019
  • 4 replies
  • 859 views

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?

    This topic has been closed for replies.

    4 replies

    aj.sk
    aj.skAuthor
    Associate III
    March 5, 2019

    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
    rdscliff
    Associate III
    March 19, 2019

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

    kasi viswanadh
    kasi viswanadh
    Associate II
    May 11, 2019

    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
    Winfred LU
    ST Employee
    May 13, 2019

    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;

    Terms & ConditionsAccessibility statement