memory allocation for structures elements

Question

Hi I am having difficulties in understanding about how the memory is allocated to the structure elements.

For example if i have the below structure and the size of c

Answer 1

It seems that on your system, double has to have the alignment of 8.

struct temp {
    int a;     // size is 4
    // padding 4 bytes
    double b;  // size is 8
    char c;    // size is 1
    // padding 7 bytes
    double d;  // size is 8
    int e;     // size is 4
    // padding 4 bytes
}; 
// Total 4+4+8+1+7+8+4+4 = 40 bytes

Compiler adds an extra 4 bytes to the end of struct to make sure that array[1].b will be properly aligned.

Without end padding (assuming array is at address 0):

&array[0]   == 0
&array[1]   == 36
&array[1].b == 36 + 8 == 44  
44 % 8 == 4  ->  ERROR, not aligned!

With end padding (assuming array is at address 0):

&array[0]   == 0
&array[1]   == 40
&array[1].b == 40 + 8 == 48
48 % 8 == 0  ->  OK!

Note that sizes, alignments, and paddings depend on target system and compiler in use.