How the sizeof() function works for Structures in C?

Question

The Structure is defined as follows

typedef struct Sample
{
  int test;
  char strtest;
} Sample;

In Main Function, I called Sizeof the structure.

sizeof(struct Sample)

I've heard the return value of sizeof on structures could be incorrect. If this is the case, what should I do to get the right value?

Answer 1

It does return a reliable value - just not always the value you expect.

In Sample structure, you are assuming a 1 byte char and 4 byte int, but you do not get a result of "5" .

Because the structure is padded so that elements start on their natural boundaries. you are more likely to get a result of "8".

Wiki explains this pretty well: http://en.wikipedia.org/wiki/Sizeof - at "Structure Padding", near the bottom.