user: implement mlibc as the libc, finally.

It's finally done..

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/mlibc/options/lsb/generic/dso_exit.cpp

@@ -0,0 +1,88 @@
+
+// for memcpy()
+#include <string.h>
+
+#include <bits/ensure.h>
+#include <mlibc/allocator.hpp>
+
+#include <frg/eternal.hpp>
+#include <frg/vector.hpp>
+
+struct ExitHandler {
+	void (*function)(void *);
+	void *argument;
+	void *dsoHandle;
+};
+
+using ExitQueue = frg::vector<ExitHandler, MemoryAllocator>;
+
+ExitQueue &getExitQueue() {
+	// use frg::eternal to prevent the compiler from scheduling the destructor
+	// by generating a call to __cxa_atexit().
+	static frg::eternal<ExitQueue> singleton(getAllocator());
+	return singleton.get();
+}
+
+extern "C" int __cxa_atexit(void (*function)(void *), void *argument, void *handle) {
+	ExitHandler handler;
+	handler.function = function;
+	handler.argument = argument;
+	handler.dsoHandle = handle;
+	getExitQueue().push(handler);
+	return 0;
+}
+
+extern "C" void __dlapi_exit();
+
+extern "C" void __cxa_finalize(void *dso) {
+	ExitQueue &eq = getExitQueue();
+	for (size_t i = eq.size(); i > 0; i--) {
+		auto &handler = eq[i - 1];
+		if (!handler.function)
+			continue;
+
+		if (!dso || handler.dsoHandle == dso) {
+			handler.function(handler.argument);
+			handler.function = nullptr;
+		}
+	}
+}
+
+// In static builds, these should be provided by the crtbegin.o/crtend.o that
+// is linked into the executable.
+#ifndef MLIBC_STATIC_BUILD
+// This is referenced by the compiler when generating constructors for global
+// C++ objects so that it can call __cxa_finalize with a unique argument.
+extern "C" { [[gnu::visibility("hidden")]] void *__dso_handle; }
+#else
+extern "C" void *__dso_handle;
+#endif
+
+[[gnu::destructor]] void __mlibc_do_destructors() {
+	// In normal programs this call to __cxa_finalize is provided by libgcc.
+	__cxa_finalize(&__dso_handle);
+}
+
+void __mlibc_do_finalize() {
+	// Invoke any handlers registered with atexit (NOT associated with a DSO).
+	// Note that we deliberately do not invoke other handlers here, since
+	// that would destroy mlibc's global objects including stdout and flushing
+	// open FILEs, but we'd like those to be available to [[gnu::destructor]]
+	// functions which we invoke below.
+	ExitQueue &eq = getExitQueue();
+	for (size_t i = eq.size(); i > 0; i--) {
+		auto &handler = eq[i - 1];
+		if (!handler.function)
+			continue;
+
+		if (!handler.dsoHandle) {
+			handler.function(handler.argument);
+			handler.function = nullptr;
+		}
+	}
+
+	// Call fini/fini_array functions of each loaded object. This is necessary
+	// to implement [[gnu::destructor]]. Note that C++ applications will call
+	// __cxa_finalize from here.
+	__dlapi_exit();
+}