Add ucontext coroutine implementation for generic fallback.

coroutine/ucontext/Context.c

@@ -0,0 +1,16 @@
+/*
+ *  This file is part of the "Coroutine" project and released under the MIT License.
+ *
+ *  Created by Samuel Williams on 24/6/2019.
+ *  Copyright, 2019, by Samuel Williams. All rights reserved.
+*/
+
+#include "Context.h"
+
+void coroutine_trampoline(void * _start, void * _context)
+{
+    coroutine_start start = _start;
+    struct coroutine_context * context = _context;
+
+    start(context->from, context);
+}

coroutine/ucontext/Context.h

@@ -0,0 +1,66 @@
+/*
+ *  This file is part of the "Coroutine" project and released under the MIT License.
+ *
+ *  Created by Samuel Williams on 24/6/2019.
+ *  Copyright, 2019, by Samuel Williams. All rights reserved.
+*/
+
+#pragma once
+
+#include <assert.h>
+#include <stddef.h>
+#include <ucontext.h>
+
+#define COROUTINE __attribute__((noreturn)) void
+
+struct coroutine_context
+{
+    ucontext_t state;
+    struct coroutine_context * from;
+};
+
+typedef COROUTINE(* coroutine_start)(struct coroutine_context *from, struct coroutine_context *self);
+
+COROUTINE coroutine_trampoline(void * _start, void * _context);
+
+static inline void coroutine_initialize_main(struct coroutine_context * context) {
+    context->from = NULL;
+    getcontext(&context->state);
+}
+
+static inline void coroutine_initialize(
+    struct coroutine_context *context,
+    coroutine_start start,
+    void *stack,
+    size_t size
+) {
+    assert(start && stack && size >= 1024);
+
+    coroutine_initialize_main(context);
+
+    context->state.uc_stack.ss_size = size;
+    // Despite what it's called, this is not actually a stack pointer. It points to the address of the stack allocation (the lowest address).
+    context->state.uc_stack.ss_sp = (char*)stack;
+    context->state.uc_stack.ss_flags = 0;
+    context->state.uc_link = NULL;
+
+    makecontext(&context->state, (void(*)(void))coroutine_trampoline, 2, (void*)start, (void*)context);
+}
+
+static inline struct coroutine_context * coroutine_transfer(struct coroutine_context * current, struct coroutine_context * target)
+{
+    struct coroutine_context * previous = target->from;
+
+    target->from = current;
+    swapcontext(&current->state, &target->state);
+    target->from = previous;
+
+    return target;
+}
+
+static inline void coroutine_destroy(struct coroutine_context * context)
+{
+    context->state.uc_stack.ss_sp = NULL;
+    context->state.uc_stack.ss_size = 0;
+    context->from = NULL;
+}