A strong answer
A strong candidate frames How does a struct lay out members and copy them? around the exact memory layout, compiler semantics, and hardware execution model covered in Structs: Grouping Related Data. Explain fixed offsets, brace initialization, and memberwise value copying for records.
Information about one thing usually comes in several pieces of different types. A contact has a name, which is text, an age, which is an integer, and a balance, which is a number with a fraction. An array holds only elements of one type. You could keep three arrays, names, ages and balances, and use the same index in all of them, but that is fragile. If one array is sorted or one element is removed and the others are not, the records silently fall out of step. A function that works on a contact would also need three separate parameters.
Declaring. A struct defines a new type from members. Note the semicolon after the closing brace, which is easy to forget:
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: Declare a struct, create objects of it and access their members.
- Mastery of core invariant: Explain where each member lives and why member access costs nothing extra.
- Mastery of core invariant: Predict the effect of copying a struct and then modifying the copy.
- 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
-O0debug build. - Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.