Byte Alignment · Moule Lin

The starting address of a struct variable is divisible by the size of its widest basic-type member;
The offset of each member relative to the struct's starting address is an integer multiple of the member's size; if necessary, the compiler inserts padding bytes between members (internal padding);
The total size of the struct is an integer multiple of the size of its widest basic-type member; if necessary, the compiler adds padding bytes after the last member (trailing padding)

For example

struct student
{
    char name[5];
    int num;
    short score;
}

|char|char|char|char|
|char|—-|—-|—-|
|——–int——–|
|–short–|—-|—-|

The data needs only 11 bytes (5+4+2), but because int is 4-byte aligned by default, it is stored at a starting address divisible by 4. That is, if name[5] starts at 0, it occupies 5 bytes, and num starts at byte 8 (offset) (because int is 4 bytes, its starting position must be divisible by 4). So sizeof(student)=16, and a few bytes in between sit idle. This makes it easier for the computer to read and write data quickly; it trades space for time. Its data alignment is shown below:

struct student
{
    int num;
    char name[5];
    short score;
}