Test file to demonstrate structure packing on C

test_packing.c

/**
 * \file
 * Test to show how packed structures work in C.
 *
 * Eric S. Raymond  has written a great introduction to structure packing
 * (http://www.catb.org/esr/structure-packing/) 
 * 
 * @author: Lari Lehtomäki <[email protected]>
 * @date:   2015-05-29
 *
 */

#include <stdio.h>

/** 
 * Test structure
 * Try with and without the packed attribute.
 */
struct myStruct {
//struct __attribute__ ((__packed__)) myStruct {
	short unsigned int	a;
	      unsigned int	b;
	short unsigned int	c;
};

/** Buffer where the structs points. */
static char buffer[20];

/**
 * On little-endian machine (x86) the output with packed structure
 * should be something like:
 * \code
 *
 *	sym:   8
 *	sym.a: 2
 *	sym.k: 4
 *	sym.m: 2
 *		*si = (myStruct) {
 *			.a = 0x0123,
 *			.b = 0x456789ab,
 *			.c = 0xcdef
 *		};
 *	0x601041: 0x23 0x01 0xab 0x89 0x67 0x45 0xef 0xcd 
 *
 * \endcode	
 */
int main(int argc, const char *argv) {

	struct myStruct *si = (struct myStruct *)buffer;

	*si = (struct myStruct) {
		.a = 0x0123,
		.b = 0x456789ab,
		.c = 0xcdef
	};


	printf("sym:   %lu\n", sizeof(*si));
	printf("sym.a: %lu\n", sizeof(si->a));
	printf("sym.k: %lu\n", sizeof(si->b));
	printf("sym.m: %lu\n", sizeof(si->c));
	
	/* Printing of struct */
	printf("	*si = (struct myStruct) {\n");
	printf("		.a = %#06x,\n", si->a);
	printf("		.b = %#06x,\n", si->b);
	printf("		.c = %#06x \n", si->c);
	printf("	};\n");

	/* Printing of memory */
	void * p = &si;
	int i = 0;
	printf("%p: ", &(buffer[i])+1);
	for (i = 0; i < sizeof(*si); i++ ) {
		printf("%#04hhx ", (char)buffer[i]);
	}
	printf("\n");

	return 0;
}