Question

Why the concept of padding is added only when there are multiple members of a structure and why is it not included when there is a single basic data type member ?

if we consider on a 32bit machine

struct 
{
    char a;
} Y;

There is no padding and sizeof Y comes to 1 byte .

If we consider this structure

struct 
{
    char a;
    int b;
} X;

Sizeof X will be 8bytes .

My question is
Why was padding adding in the second case ? If it is for efficient access by the machine which normally reads data in blocks of multiples of 4bytes then why was there no padding in the first case ?

Answer 1

Padding is added in the second case because, on your machine, an int is aligned to 4 bytes. So it has to reside at an address that is divisible to 4.

0x04   0x05   0x06   0x07   0x08   0x09   0x0A   0x0B

  a      b      b      b      b     

If no padding is added, the int member starts at address 0x05, which is wrong. With 3 added padding bytes:

0x04   0x05   0x06   0x07   0x08   0x09   0x0A   0x0B

  a   |      padding      |   b      b      b      b

Now the int is at 0x08, which is OK.