What is a file descriptor and how does the kernel use it?

A strong answer

A strong candidate frames What is a file descriptor and how does the kernel use it? around the exact memory layout, compiler semantics, and hardware execution model covered in File Descriptors and Kernel File Tables. Explain file descriptors as indices into the per-process file descriptor table, referencing the open file description and inode.

In Scenario A, if you read 100 bytes from fd_a, fd_b remains at byte 0. Both descriptors target the same disk file, but their file positions never affect one another.

In Scenario B, if the parent reads 100 bytes from fd_c, the child reading from fd_c immediately begins at byte 101. Advancing the offset in one process advances it simultaneously in the other.

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: Trace integer file descriptors through the per-process descriptor table, system-wide open file table, and inode table.
• Mastery of core invariant: Differentiate shared file offsets across dup/fork from independent offsets across separate open calls.
• Mastery of core invariant: Identify the lowest-available-descriptor allocation rule used by the POSIX kernel.
• 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.