diff --git a/include/stdarg.h b/include/stdarg.h
index 6a8fee6a..4f065d0a 100644
--- a/include/stdarg.h
+++ b/include/stdarg.h
@@ -1,6 +1,7 @@
 /* -*-comment-start: "//";comment-end:""-*-
  * GNU Mes --- Maxwell Equations of Software
  * Copyright © 2017,2018,2019 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ * Copyright © 2021 W. J. van der Laan <laanwj@protonmail.com>
  *
  * This file is part of GNU Mes.
  *
@@ -26,7 +27,19 @@
 
 #define va_arg8(ap, type) va_arg(ap, type)
 
-#else // ! SYSTEM_LIBC
+#elif __GNUC__ && __riscv
+
+// GCC on RISC-V always passes arguments in registers.  Implementing
+// these macros without the use of built-ins would be very involved.
+typedef __builtin_va_list va_list;
+
+#define va_start(v, l)     __builtin_va_start (v, l)
+#define va_end(v)          __builtin_va_end (v)
+#define va_arg(v, l)       __builtin_va_arg (v, l)
+#define va_arg8(ap, type)  va_arg (ap, type)
+#define va_copy(d, s)      __builtin_va_copy (d, s)
+
+#else // ! SYSTEM_LIBC && ! __riscv
 
 #include <sys/types.h>