Add a C++ backend for LibertyXDR

Also change the C backend so that it also de/serializes
unions without any other fields besides the tag.

tools/lxdrgen-cpp-posix.cpp

@@ -0,0 +1,67 @@
+// lxdrgen-cpp-posix.cpp: POSIX support code for lxdrgen-cpp.awk.
+//
+// Copyright (c) 2023, Přemysl Eric Janouch <p@janouch.name>
+// SPDX-License-Identifier: 0BSD
+#include <iconv.h>
+
+#include <cstdint>
+#include <string>
+
+// Various BSD derivatives may have a problem here.
+// Linux defines __STDC_ISO_10646__, but also supports "WCHAR_T".
+#ifdef APPLE
+#define ICONV_WCHAR "UTF-32"
+#else
+#define ICONV_WCHAR "WCHAR_T"
+#endif
+
+namespace LibertyXDR {
+
+bool utf8_to_wstring(const uint8_t *utf8, size_t length, std::wstring &wide) {
+	iconv_t conv = iconv_open(ICONV_WCHAR, "UTF-8");
+	if (conv == (iconv_t) -1)
+		return false;
+
+	wchar_t buffer[1024] = {};
+	char *start = (char *) buffer, *out = start, *in = (char *) utf8;
+	size_t available = sizeof buffer;
+	wide.clear();
+	while (iconv(conv, &in, &length, &out, &available) == (size_t) -1) {
+		if (errno != E2BIG) {
+			iconv_close(conv);
+			return false;
+		}
+
+		wide.append(buffer, (out - start) / sizeof *buffer);
+		out = start;
+		available = sizeof buffer;
+	}
+	wide.append(buffer, (out - start) / sizeof *buffer);
+	iconv_close(conv);
+	return true;
+}
+
+bool wstring_to_utf8(const std::wstring &wide, std::string &utf8) {
+	iconv_t conv = iconv_open("UTF-8", ICONV_WCHAR);
+	if (conv == (iconv_t) -1)
+		return false;
+
+	char buffer[1024] = {}, *out = buffer, *in = (char *) wide.data();
+	size_t available = sizeof buffer, length = wide.size() * sizeof wide[0];
+	utf8.clear();
+	while (iconv(conv, &in, &length, &out, &available) == (size_t) -1) {
+		if (errno != E2BIG) {
+			iconv_close(conv);
+			return false;
+		}
+
+		utf8.append(buffer, out - buffer);
+		out = buffer;
+		available = sizeof buffer;
+	}
+	utf8.append(buffer, out - buffer);
+	iconv_close(conv);
+	return true;
+}
+
+} // namespace LibertyXDR