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What is the difference between a variable and a pointer in C?

Explain that a variable holds a value directly while a pointer holds the memory address of a value, and how dereferencing works.

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

The Runtime Theory Team2 min read
Learn it firstVariables, Assignment, and Basic ExpressionsLearn what a variable is, how to declare and assign int, float, double, and char values, and how to build arithmetic expressions and print results.

A strong answer

A strong candidate frames What is the difference between a variable and a pointer in C? around the exact memory layout, compiler semantics, and hardware execution model covered in Variables, Assignment, and Basic Expressions. Explain that a variable holds a value directly while a pointer holds the memory address of a value, and how dereferencing works.

Suppose you are writing a program to calculate the total price of two identical items, each costing $9.99. You might be tempted to write:

This works for one calculation, but it is not a program — it is just a hard-coded sentence. If the price changes to $12.50, you must edit and recompile the text itself.

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 what a variable is and how it stores a value in memory.
  • Mastery of core invariant: Declare and initialize variables of type int, float, double, and char.
  • Mastery of core invariant: Write assignment statements and arithmetic expressions with +, -, *, /, and %.
  • 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. 01What does the * operator do when declaring a pointer versus when reading through it?
  2. 02What is a null pointer and why is dereferencing it dangerous?

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