Design a high-throughput HTTP server in C.

A strong answer

A strong candidate frames Design a high-throughput HTTP server in C. around the exact memory layout, compiler semantics, and hardware execution model covered in Project: High-Throughput HTTP/1.1 Web Server. Discuss epoll event loop, thread pool for parsing, keep-alive, zero-copy file serving, and backpressure.

Building an HTTP/1.1 static file server touches every subsystem of C systems engineering:

1. Network multiplexing: Handling thousands of active TCP streams on a single thread without head-of-line blocking.
2. Buffer management: Reading streaming HTTP requests with variable line lengths without heap allocation churn.
3. Data transfer bottlenecks: Bypassing user-space memory copies when streaming multi-megabyte files.
4. Protocol semantics: Handling persistent HTTP/1.1 Connection: keep-alive requests across pipelined commands.

A naive implementation that spawns a thread per connection and copies file blocks through user-space char buf[4096] arrays consumes gigabytes of memory and spends over 60% of its execution time executing memcpy and context switches.

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: Construct an event-driven HTTP/1.1 server using Linux Edge-Triggered epoll.
• Mastery of core invariant: Eliminate user-space memory copies via zero-copy sendfile kernel pipelines.
• Mastery of core invariant: Maintain persistent HTTP/1.1 keep-alive connection state machines without per-request allocations.
• 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.