What is a memory leak and how do you prevent it?

A strong answer

A strong candidate frames What is a memory leak and how do you prevent it? around the exact memory layout, compiler semantics, and hardware execution model covered in Dynamic Memory Lifecycles and Leak Prevention. Explain memory leaks, dangling pointers, and double frees, and how valgrind/ASan help detect them.

Unlike garbage-collected runtimes that scan object graphs automatically, C demands manual lifecycle management. Every byte allocated on the heap via malloc must be explicitly released using free.

If network connections disconnect frequently, this function leaks 4,096 bytes on every aborted handshake. Over days of operation, the process consumes gigabytes of memory until the operating system Out-Of-Memory (OOM) killer abruptly terminates the service.

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: Identify the four categories of dynamic memory lifecycle errors in C.
• Mastery of core invariant: Apply single-owner and borrowed-pointer patterns to manage heap lifetimes.
• Mastery of core invariant: Diagnose leaks and heap memory corruptions using AddressSanitizer and Valgrind.
• 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.