user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/rtld/destroy2/meson.build

@@ -0,0 +1 @@
+# only the test executable is required

user/include/mlibc/tests/rtld/destroy2/test.c

@@ -0,0 +1,43 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <unistd.h>
+#include <string.h>
+#include <sys/wait.h>
+
+// Make sure files are flushed on process exit.
+
+int main() {
+	int pipefd[2];
+	if (pipe(pipefd) < 0) {
+		perror("pipe");
+		return -1;
+	}
+
+	pid_t child = fork();
+	if (child < 0) {
+		perror("fork");
+		return -1;
+	}
+
+	// This should sit in libc's buffer and be flushed on exit.
+	const char *message = "hello world";
+
+	if (child == 0) {
+		// Child
+		dup2(pipefd[1], STDOUT_FILENO);
+		printf("%s", message);
+		return 0;
+	}
+
+	// Parent
+	wait(NULL);
+
+	char buffer[64];
+	ssize_t len = read(pipefd[0], buffer, sizeof(buffer));
+	assert(len > 0);
+	buffer[len] = 0;
+
+	return strcmp(buffer, message) != 0;
+}
+