Finding Padding

The ANSI C offsetof(3C) macro may be used in a test program to determine the offset of each element in the structure. Knowing the offset and the size of each element, the location and size of any padding can be determined.

Example 3-2 Structure Padding

#include <sys/types.h>
#include <stdio.h>
#include <stddef.h>

struct device_reg {
	uint8_t			csr;
	uint32_t			data;
};

int main(void)
{
	printf("The offset of csr is %d, its size is %d.\n",
			offsetof(struct device_reg, csr), sizeof (uint8_t));
	printf("The offset of data is %d, its size is %d.\n",
			offsetof(struct device_reg, data), sizeof (uint32_t));
	return (0);
}

Here is a sample compilation with Sun WorkShop^TM Compiler C version 4.2 and a subsequent run of the program:

test% cc -Xa c.c

test% a.out

The offset of csr is 0, its size is 1.

The offset of data is 4, its size is 4.

Be aware that padding is dependent not only on the processor but also on the compiler.