user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/options/posix/generic/resolv_conf.cpp

@@ -0,0 +1,42 @@
+#include <mlibc/resolv_conf.hpp>
+#include <mlibc/allocator.hpp>
+#include <stdio.h>
+#include <ctype.h>
+
+namespace mlibc {
+
+frg::optional<struct nameserver_data> get_nameserver() {
+	auto file = fopen("/etc/resolv.conf", "r");
+	if (!file)
+		return frg::null_opt;
+
+	char line[128];
+	struct nameserver_data ret;
+	while (fgets(line, 128, file)) {
+		char *pos;
+		if (!strchr(line, '\n') && !feof(file)) {
+			// skip truncated lines
+			for (int c = getc(file); c != '\n' && c != EOF; c = getc(file));
+			continue;
+		}
+
+		// TODO(geert): resolv.conf can actually have multiple nameservers
+		// but we just pick the first one for now
+		if (!strncmp(line, "nameserver", 10) && isspace(line[10])) {
+			char *end;
+			for (pos = line + 11; isspace(*pos); pos++);
+			for (end = pos; *end && !isspace(*end); end++);
+			*end = '\0';
+			ret.name = frg::string<MemoryAllocator>(
+					pos, end - pos, getAllocator());
+			break;
+		}
+	}
+
+	fclose(file);
+	if(ret.name.empty())
+		return frg::null_opt;
+	return ret;
+}
+
+} // namespace mlibc