Structure Padding in visual basics tool.

[Jeevan Rudramurthy]

We are trying to see the size of the structure its getting 8 bytes,and get padded with remaining bytes we want to use only 4 bytes only 32bit frame. How to make 32 bit frame. so that we want to use only 4 bytes,Can any one help me out in finding the solutions we are using Visual basics tool.

typedef struct STTCADispCtrl
{
	unsigned char label;
	unsigned char sdi:2;
	unsigned char ia:1;
	unsigned char As:2;
	unsigned char Att:1;
	unsigned char Dtif:1;
	unsigned int Range:9;
	unsigned char Ac:2;
	unsigned char Mil:3;
	unsigned char ssm:2;
	unsigned char parity:1;
}ST_TCAS_MODE_DISP_CTRL;

int main()
{

	ST_TCAS_MODE_DISP_CTRL display;

	printf("sizeof ST_TCAS_MODE_DISP_CTRL: %d sizeof unsigned int: %d\n", sizeof(display), sizeof(unsigned int));

	return 0;
}

[2kaud]

The minimum number of bytes this will pack to is 5 by setting the Struct Member Alignment to 1 byte.

label 1 byte

sdi, ia, As, Att, Dtif 1 byte (only 7 bits used)

make Range short int so 2 bytes but starts on a byte boundary.

Ac, Mil, sam, ssm, parity 1 byte

so total for the struct is 5 bytes.

If you want this packed into 4 bytes, then Range needs to be split into 2 parts - 1 bit char and and 8 bit char giving

	unsigned char label;
	unsigned char sdi : 2;
	unsigned char ia : 1;
	unsigned char As : 2;
	unsigned char Att : 1;
	unsigned char Dtif : 1;
	unsigned char RangeTop : 1;
	unsigned char RangeBot : 8;
	unsigned char Ac : 2;
	unsigned char Mil : 3;
	unsigned char ssm : 2;
	unsigned char parity : 1;

and using 4 bytes.