Layout, Padding & offsetof

The compiler may insert holes

Each member must sit at an offset that is a multiple of its alignment. The compiler inserts invisible padding to make that true, and pads the struct's total size so arrays of it stay aligned:

struct Packed {
    char  c;      /* offset 0 */
                  /* 3 bytes padding */
    int   i;      /* offset 4 */
    char  d;      /* offset 8 */
                  /* 3 bytes padding (so sizeof works for arrays) */
};                /* sizeof == 12, not 6 */

Reordering members largest-first usually shrinks the struct. But do not memorize layouts — measure them:

#include <stddef.h>
offsetof(struct Packed, i)   /* == 4: the offset as actually compiled */
sizeof(struct Packed)        /* == 12 */

offsetof answers "where is this member really" under this compiler, this standard, these flags. Code that files into binary formats uses it — or better, uses it in a _Static_assert to fail the build if the layout ever drifts:

_Static_assert(offsetof(struct Header, magic) == 0, "header layout");

Alignment is a property of types

int aligns to 4, double to 8, pointers to 8 (on 64-bit). The struct's own alignment is the max of its members'. That is why member order matters and why the trailing padding exists.