Thread: struct padding question..

Hi All,

Code:

 #include<stdio.h>
 
struct node
{
   char p;
   char c;
   char d;
    //int a;      //---------------------- Number 1
};

int main()
{ 
	struct node a;
         
    printf("size of node a:%d",sizeof(a));
	getch();
    return 0;
}

Here the Output is

3

Questions

1. If only char variables are present how will the processor access the memory address?
Assuming processors with fewer pins?

2. Shouldn't the variables be aligned at boundaries multiples of 2? What are criteria's for alignment of variables at even boundaries(basically when all)?

3. Introduction of Number 1 line will change the entire picture and structure is padded?

Thanks in advance

There are no standard answers for this. The compiler is responsible for padding and will do so at its own discretion.
Any x86 processor can access unaligned memory, though doing so may incur a speed hit. If the compiler chooses it, it may align them on a boundary of 2, 4 or 8 or an even higher number.
But the rules are specific to compilers and differ between them, so the questions are pointless.

Originally Posted by nonoob

Even if in your example the size of the struct is 3, I'm sure it's still aligned to some power-of-2 address. You may want to declare a second struct and see the address of where it's stored. I bet it's not 3 bytes from the first one.

I bet it is, actually. The compiler needs to ensure correct alignment, but it also doesn't want to waste memory pointlessly. If you declare an array of two of those, and take sizeof() on the array, I bet it's simply twice the size. Single chars don't need an alignment of more than 1, at least on any reasonable architecture.