Why can a struct be larger than the sum of its members?

A strong answer

A strong candidate frames Why can a struct be larger than the sum of its members? around the exact memory layout, compiler semantics, and hardware execution model covered in Struct Layout, Padding and Alignment. Explain alignment, internal and tail padding, and ordering that reduces waste.

Lesson 1 mentioned that the size of a struct is not the sum of its members. Take a struct with a char, an int and another char. The data is 1 + 4 + 1, which is 6 bytes, yet sizeof reports 12. Half of the object seems to be missing from the accounting.

It matters because the waste multiplies. A million of these structs cost 12 MB instead of 8 MB for exactly the same information, and the extra bytes have to travel through the cache too. To see where the bytes go, you need the idea of alignment.

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

• Mastery of core invariant: Compute member offsets and the total size of a struct from the alignment rules.
• Mastery of core invariant: Explain why padding exists, both between members and at the end.
• Mastery of core invariant: Reorder members to reduce padding.
• Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
• Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

• Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
• Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
• Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.