Design

Context switch

Each platform has its own hand-written switch:

• x86_64: Saves/restores RBP, RBX, R12-R15, RSP, RIP (8 registers, 64 bytes)
• ARM64: Saves/restores X19-X30, SP, LR, D8-D15 (callee-saved per AAPCS64)
• Windows x64: Additionally saves XMM6-XMM15 and TEB fiber storage fields

The assembly lives in the header, as __asm__ blocks on Unix and byte arrays on Windows, so there is no separate assembler step.

Threads

• Each coroutine must only be accessed from one thread at a time
• coro::running() is thread-local - safe to call from any thread
• task_runner is not thread-safe - use one per thread
• Creating and destroying coroutines from different threads is safe

Why not C++20 coroutines

C++20 coroutines are stackless: they suspend only at a co_await or co_yield in the coroutine body, and every caller on the way has to be a coroutine too. A ucoro coroutine has its own stack, so any function it calls can yield. That makes it a fit for deep recursion, wrapping callback APIs, fibers and game logic. See Advanced examples.