m68k: add makecontext() implementation

arch/m68k/makecontext.c

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2020 Ariadne Conill <ariadne@dereferenced.org>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * This software is provided 'as is' and without any warranty, express or
+ * implied.  In no event shall the authors be liable for any damages arising
+ * from the use of this software.
+ */
+
+#define _GNU_SOURCE
+#include <stddef.h>
+#include <stdarg.h>
+#include <ucontext.h>
+#include <string.h>
+#include <stdint.h>
+#include <stdio.h>
+#include "defs.h"
+
+
+extern void __start_context(void);
+
+
+void
+__makecontext(ucontext_t *ucp, void (*func)(void), int argc, ...)
+{
+	greg_t *sp;
+	va_list va;
+	int i;
+
+	/* set up and align the stack. */
+	sp = (greg_t *) ((uintptr_t) ucp->uc_stack.ss_sp + ucp->uc_stack.ss_size);
+	sp -= (argc + 1);
+	sp = (greg_t *) (((uintptr_t) sp & ~0x3));
+
+	/* set up the ucontext structure */
+	ucp->uc_mcontext.gregs[REG_SP] = (greg_t) sp;
+	ucp->uc_mcontext.gregs[REG_A6] = 0;
+
+	/* return address */
+	*sp++ = (greg_t) __start_context;
+
+	va_start(va, argc);
+
+	/* all arguments overflow into stack */
+	for (i = 0; i < argc; i++)
+		*sp++ = va_arg (va, greg_t);
+
+	va_end(va);
+}
+
+
+extern __typeof(__makecontext) makecontext __attribute__((weak, __alias__("__makecontext")));