memory alignment __attribute__( ( aligned ( 8 ) ) )

Question

I got a program in a book

#include 
int main( )
{
    struct data
    {
        int a    __attribute__( ( aligned ( 8 ) ) ) ;
        char ch          


        

Answer 1

A sizeof is always a multiple of largest alignment, as it is actually reports 'step' in chars in array of given type which include padding as between members so padding between array elements required to align them.

Memory layout of struct data will look like this:

 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 ... <--offset
|                                               |
|          struct data   (element [0])          |      (element [1])
|                                               |
|     a     |ch|::::::::|     s     |:::::::::::|     a     |ch|::::...
|                                               |
|<----------- sizeof(struct data) ------------->|

|::::| is padding

• a is at offset 0 which is obviously a multiple of 8;
• ch is at offset 4 which is (also obviously) a multiple of 1;
• s is at offset 8 which is multiple of 4;
• sizeof(struct data) is equal to offset of [1] (16) which required to be multiple of max(8,1,4) = 8