When should you use a read-write lock instead of a mutex?

A strong answer

A strong candidate frames When should you use a read-write lock instead of a mutex? around the exact memory layout, compiler semantics, and hardware execution model covered in Read-Write Locks and Barrier Synchronization. Explain read-mostly workloads where multiple concurrent readers are safe, but writers need exclusive access.

Standard mutual exclusion locks (pthread_mutex_t) are strictly pessimistic: they assume every critical section mutates data.

In this system:

• 99.9% of all operations are read-only queries.
• 0.1% of operations are cache updates inserting a newly resolved domain.

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: Permit concurrent shared reads while enforcing exclusive writes using pthread_rwlock_t.
• Mastery of core invariant: Diagnose and prevent writer starvation using writer-preference lock attributes.
• Mastery of core invariant: Synchronize multi-threaded computation phases across threads using pthread_barrier_t.
• 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.