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Why does a struct's size exceed the sum of its fields?

Explain compiler struct padding, alignment requirements, and how to minimize padding.

TRT practice prompt — not a verified question from a named employer.

The Runtime Theory Team2 min read
Learn it firstStructures, Alignment, and PaddingPro — members onlyInspect hardware memory alignment, calculate compiler struct padding and trailing bytes, and optimize composite data layouts.

A strong answer

A strong candidate frames Why does a struct's size exceed the sum of its fields? around the exact memory layout, compiler semantics, and hardware execution model covered in Structures, Alignment, and Padding. Explain compiler struct padding, alignment requirements, and how to minimize padding.

When defining a custom record type in C, it is natural to treat a struct as the simple mathematical sum of its constituent members.

The struct consumes 12 bytes in RAM—exactly double the memory occupied by the actual data. Half the allocated space consists of unused, invisible bytes.

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: Calculate internal and trailing struct padding bytes introduced by the compiler.
  • Mastery of core invariant: Inspect field byte offsets using the offsetof macro from stddef.h.
  • Mastery of core invariant: Reorder struct members by descending alignment to minimize memory waste.
  • 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.

This answer walks

Practice follow-ups

  1. 01How does the compiler, OS kernel, or hardware memory model enforce the invariants of Structures, Alignment, and Padding under optimization (`-O2`/`-O3`)?
  2. 02What production bug or undefined behavior occurs when Structures, Alignment, and Padding is misused across module or thread boundaries, and which sanitizer (`ASan`, `UBSan`, `TSan`) catches it?

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