Why must you handle partial reads and writes?

A strong answer

A strong candidate frames Why must you handle partial reads and writes? around the exact memory layout, compiler semantics, and hardware execution model covered in Unbuffered I/O: read, write, and Partial Transfers. Explain that read() and write() may transfer fewer bytes than requested and must be called in a loop until all data is transferred.

When writing to an ext4 or APFS file on a local NVMe drive, this code often succeeds: the kernel page cache accepts the entire payload and returns len.

However, as soon as this identical function writes to a TCP network socket, an anonymous pipe, or a virtual terminal, it begins intermittently corrupting data or truncating payloads.

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: Differentiate ssize_t return values across success, EOF, and error conditions.
• Mastery of core invariant: Construct production-grade read_all and write_all loops that handle partial transfers.
• Mastery of core invariant: Handle transient system call interruptions by checking errno for EINTR.
• 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.