src/fiber.c, not used yet: a fiber runs a function on a stack of its
own, yielding back to the thread which resumed it, for a thread to
serve many clients with code written as if each had a thread.
The switch is our own on x86-64 (System V), aarch64, i386, arm (ARM or
Thumb code, with or without a floating point unit) and mips o32 (with
soft, 32 or 64 bit floating point registers, position independent or
not), Windows' fibers on Windows. It saves the registers a function
has to keep and swaps the stack. Elsewhere there are no fibers
(WITH_FIBERS is not defined), nor where x86-64 Linux checks returns
against a shadow stack, which fiberusable() tells: each client has a
thread there.
Stacks are mapped with a guard page below them. A fiber whose function
returned is started again with another, its stack kept. Switches are
annotated for AddressSanitizer.
tests/fibertest.c tests them: yielding and resuming, the integer and
floating point registers a function keeps across switches both ways, a
thousand fibers resumed at random each keeping its stack, recursion
deep in a stack, fibers sleeping on the event loop's timers. The units
case, which was the loop case, runs it with looptest.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
