C Struct Padding Example

struct_padding.c

#include <stdio.h>

/*Author: https://github.com/911992*/

typedef struct s{
	int b;
	char f;
	char _data[9];
};

int main(void) {
	printf("sizeof(struct s) : %d\n",sizeof(struct s)); // 16
	struct s _s;
	memset(&_s,0,sizeof(struct s));
	_s.b= 0xaaaaaaaa;
	_s.f = 0xbb;
	memset(_s._data,0xcc,sizeof(_s._data));
	printf("_data len : %d\n",sizeof(_s._data)); // returns 9, but actually 11
	printf("a:%x , f:%x\n",_s.b,_s.f&0xff); // 0xaaaaaaaa , 0xbb
	printf("arr: {");
	for(int a=0;a<sizeof(_s._data);a++){
		printf(" %x ",_s._data[a]&0xff); 
	} // { cc  cc  cc  cc  cc  cc  cc  cc  cc }
	printf("}\nall_struct arr: {");
	char* _ptr = (char*)&_s;
	for(int a=0;a<sizeof(struct s);a++){
		printf(" %x ",_ptr[a]&0xff); 
	} // { aa  aa  aa  aa  bb  cc  cc  cc  cc  cc  cc  cc  cc  cc  0  0 }
  //                   Mind the these two zeros, as padded bytes ^^^^
	printf("}\n");
	//surprise here
	printf("Altering padded data\n");
	_s._data[9] = 0xee; // expecting error? but no, becasue of padding(at the end)
	_s._data[10] = 0xdd; // expecting error? but no, becasue of padding(at the end)
	
	printf("}\nall_struct arr(after alter): {");
	for(int a=0;a<sizeof(struct s);a++){
		printf(" %x ",_ptr[a]&0xff);
	} //{ aa  aa  aa  aa  bb  cc  cc  cc  cc  cc  cc  cc  cc  cc  ee  dd }
	printf("}\n");
	return 0;
}