Question

My Questing is regarding structure padding? Can any one tell me what’s logic behind structure padding.

Example:

structure Node{
char c1;
short s1;
char c2;
int i1;
};

Can any one tell me how structure padding will apply on this structure?

Assumption: Integer takes 4 Byte.

Waiting for the answer.

Answer 1

How padding works depends entirely on the implementation.

For implementations where you have a two-byte short and four-byte int and types have to be aligned to a multiple of their size, you will have:

Offset  Var   Size
------  ----  ----
   0     c1      1
   1     ??      1
   2     s1      2
   4     c2      1
   5     ??      3
   8     i1      4
  12    next

An implementation is free to insert padding between fields of a structure and following the last field (but not before the first field) for any reason whatsoever. The ability to pad after a structure is important for aligning subsequent elements in an array. For example:

struct { int i1; char c1; };

may give you:

Offset  Var   Size
------  ----  ----
   0     i1      4
   4     c1      1
   5     ??      3
   8    next

Padding is usually done because either aligned data works faster, or misaligned data is illegal (some CPU architectures disallow misaligned access).