Pointer basics: address-of and dereference

A strong answer

A strong candidate frames Pointer basics: address-of and dereference around the exact memory layout, compiler semantics, and hardware execution model covered in Your First Pointer: Addresses and Dereferencing. Explain what & and * do, and how a pointer stores a memory address.

Think back to Lesson 3, where we saw that C passes function arguments by value. Suppose you want to write a function swap(a, b) that exchanges the values of two integer variables x = 10 and y = 20 in main, or a function that updates a player's health and score at the same time. If you write:

Calling bad_swap(x, y) copies 10 into a and 20 into b, swaps the two local copies inside bad_swap, and then destroys a and b when the function returns—leaving x and y in main stuck at 10 and 20.

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: Explain that a pointer stores a memory address, not a value.
• Mastery of core invariant: Use the & operator to get a variable's address.
• Mastery of core invariant: Declare a pointer and dereference it with *.
• 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.