user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/rtld/preinit/libfoo.c

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <assert.h>
+
+int fooDone = 0;
+
+// DSOs do not support pre-initialization functions.
+
+__attribute__((constructor))
+void fooInit() {
+	dprintf(1, "initialization function called in foo\n");
+
+	assert(fooDone == 0);
+	fooDone++;
+}
+
+int isFooDone() {
+	return fooDone;
+}

user/include/mlibc/tests/rtld/preinit/meson.build

@@ -0,0 +1,19 @@
+if host_machine.cpu_family() == 'riscv64'
+	# gp isn't initialized until after crt1.o runs, so to access
+	# globals in our pre-initializers we must disable it.
+	test_additional_link_args = ['-Wl,--no-relax']
+endif
+
+libfoo = shared_library('foo', ['libfoo.c'],
+	dependencies: libc_dep,
+	override_options: 'b_sanitize=none',
+	link_args: test_additional_link_args,
+)
+test_link_with = [libfoo]
+
+libfoo_native = shared_library('native-foo', 'libfoo.c',
+	link_args: ['-ldl'] + test_additional_link_args,
+	override_options: 'b_sanitize=none',
+	native: true
+)
+test_native_link_with = [libfoo_native]

user/include/mlibc/tests/rtld/preinit/test.c

@@ -0,0 +1,46 @@
+#include <stdio.h>
+#include <assert.h>
+
+int mainDone = 0;
+
+int isFooDone();
+
+void preInit1() {
+	// Use dprintf because stdout might not be initialized yet.
+	dprintf(1, "pre-initialization function 1 called in main executable\n");
+
+	assert(isFooDone() == 0);
+	assert(mainDone == 0);
+	mainDone++;
+}
+
+void preInit2() {
+	dprintf(1, "pre-initialization function 2 called in main executable\n");
+
+	assert(isFooDone() == 0);
+	assert(mainDone == 1);
+	mainDone++;
+}
+
+__attribute__((constructor))
+void mainInit() {
+	dprintf(1, "initialization function called in main executable\n");
+
+	assert(isFooDone() == 1);
+	assert(mainDone == 2);
+	mainDone++;
+}
+
+// Manually register the pre-initialization functions.
+__attribute__((used, section(".preinit_array")))
+static void (*preinitFunctions[])(void) = {
+	&preInit1,
+	&preInit2,
+};
+
+int main() {
+	assert(isFooDone() == 1);
+	assert(mainDone == 3);
+
+	return 0;
+}