Design a full user-space coroutine runtime in C.

A strong answer

A strong candidate frames Design a full user-space coroutine runtime in C. around the exact memory layout, compiler semantics, and hardware execution model covered in Project: High-Performance Cooperative Coroutine Runtime and Scheduler. Discuss stack management, context switching, M:N scheduling, cooperative I/O integration, and synchronization primitives.

The solution deployed by production runtimes like Go, Erlang, and Tokio is the User-Space Coroutine Runtime:

• Provide an imperative, sequential programming API: fibers execute coro_read(), coro_sleep_ms(), and chan_send() as if they were standard blocking calls.
• Retain sub-microsecond memory overhead: each fiber gets a compact, guarded virtual memory stack.
• Perform sub-5-nanosecond context switches: state transfers execute entirely in user-space assembly with zero kernel system calls.

In this Capstone, you will unify every low-level mechanism mastered across the Expert Tier into a complete, standalone C coroutine runtime:

1. Memory: Virtual stack allocation via mmap with mprotect(PROT_NONE) guard pages and stack pooling.
2. Context Switching: A hand-crafted 14-instruction x86-64 assembly routine (coro_switch.S) preserving the System V callee-saved register set.
3. Execution Control: Automatic exit handling via entry trampolines and formal state machine tracking.
4. Asynchronous I/O: Non-blocking socket polling and timers driven by Linux epoll and timerfd.
5. Synchronization: Cooperative fiber mutexes and bounded CSP channels enforcing backpressure.

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: Integrate mmap virtual stack allocation, mprotect guard pages, and recycled stack pooling.
• Mastery of core invariant: Wire the 14-instruction x86-64 assembly context switch with exit trampolines and lifecycle tracking.
• Mastery of core invariant: Unify asynchronous socket polling and timerfd events inside a central non-blocking scheduler loop.
• 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.