Effects of padding on variable declaration:

testingPadding.c

/* Roosembert Palacios © 2015, Released under GPLv3 License: http://www.gnu.org/licenses/gpl-3.0.fr.html
 * This piece of code is intended to show the effects of memory padding and the importance of the variable
 * Allocation order. Please note that compiler warning "-Wpadded" is intentional and can be disabled by
 * commenting line (10: #pragma GCC diagnostic warning "-Wpadded").
 */
#include <stdio.h>
#pragma GCC diagnostic ignored "-Wpointer-to-int-cast"
#pragma GCC diagnostic ignored "-Wint-to-pointer-cast"
#pragma GCC diagnostic ignored "-Wformat"
#pragma GCC diagnostic warning "-Wpadded"

struct S1{
    double a;
    double b;
    int c;
    short d;
    char e;
};

struct S2{
    char a;
    double b;
    int c;
    double d;
    short e;
};

struct S3{
    char a;
    short b;
    int c;
    double d;
    double e;
};

const char structDefinitions[]="\
struct S1{	struct S2{	struct S3{		\n\
    double a;	    char a;	    char a;		\n\
    double b;       double b;       short b;	\n\
    int c;          int c;          int c;		\n\
    short d;        double d;       double d;	\n\
    char e;         short e;        double e;	\n\
};              };		    };				\n\
";


struct S1 s1;
struct S2 s2;
struct S3 s3;

void main(void){
	printf("This program is intended to show the effects of compiler padding and variable order on struct size\n\
based in conjectures on Alignment in C seminar by Sven-Hendrik Haase. Online at http://goo.gl/jXDdMX\n\
©2015 - Roosembert Palacios, Released under GPLv3 License: http://www.gnu.org/licenses/gpl-3.0.fr.html\n\
\n");

	printf("Struct definitions:\n%s",structDefinitions);
	printf("============================================================\n");
	printf("s1.member:address(size)\ts2.member:address(size)\ts3.member:address(size)\n");
	printf("------------------------------------------------------------\n");
	printf("s1->a:0x%x:(%d)\ts2->a:0x%x:(%d)\ts3->a:0x%x:(%d)\n", &s1.a, (int)(((int)&s1.b)-((int)&s1.a)), &s2.a, (int)(((int)&s2.b)-((int)&s2.a)), &s3.a, (int)(((int)&s3.b)-((int)&s3.a)));
	printf("s1->c:0x%x:(%d)\ts2->c:0x%x:(%d)\ts3->c:0x%x:(%d)\n", &s1.b, (int)(((int)&s1.c)-((int)&s1.b)), &s2.b, (int)(((int)&s2.c)-((int)&s2.b)), &s3.b, (int)(((int)&s3.c)-((int)&s3.b)));
	printf("s1->c:0x%x:(%d)\ts2->c:0x%x:(%d)\ts3->c:0x%x:(%d)\n", &s1.c, (int)(((int)&s1.d)-((int)&s1.c)), &s2.c, (int)(((int)&s2.d)-((int)&s2.c)), &s3.c, (int)(((int)&s3.d)-((int)&s3.c)));
	printf("s1->d:0x%x:(%d)\ts2->d:0x%x:(%d)\ts3->d:0x%x:(%d)\n", &s1.d, (int)(((int)&s1.e)-((int)&s1.d)), &s2.d, (int)(((int)&s2.e)-((int)&s2.d)), &s3.d, (int)(((int)&s3.e)-((int)&s3.d)));
	printf("s1->e:0x%x:(%d)\ts2->e:0x%x:(%d)\ts3->e:0x%x:(%d)\n", &s1.e, (int)(((int)&s1.a+sizeof(s1))-((int)&s1.e)), &s2.e, (int)(((int)&s2.a+sizeof(s2))-((int)&s2.e)), &s3.e, (int)(((int)&s3.a+sizeof(s3))-((int)&s3.e)));
}