diff --git a/flang/runtime/command.h b/flang/runtime/command.h
--- a/flang/runtime/command.h
+++ b/flang/runtime/command.h
@@ -13,12 +13,29 @@
 #include "entry-names.h"
 
 namespace Fortran::runtime {
+class Descriptor;
+
 extern "C" {
 // 16.9.51 COMMAND_ARGUMENT_COUNT
 //
 // Lowering may need to cast the result to match the precision of the default
 // integer kind.
 CppTypeFor<TypeCategory::Integer, 4> RTNAME(ArgumentCount)();
+
+// 16.9.83 GET_COMMAND_ARGUMENT
+// We're breaking up the interface into several different functions, since most
+// of the parameters are optional.
+
+// Try to get the value of the n'th argument.
+// Returns a STATUS as described in the standard.
+CppTypeFor<TypeCategory::Integer, 4> RTNAME(ArgumentValue)(
+    CppTypeFor<TypeCategory::Integer, 4> n, const Descriptor *value,
+    const Descriptor *errmsg);
+
+// Try to get the significant length of the n'th argument.
+// Returns 0 if it doesn't manage.
+CppTypeFor<TypeCategory::Integer, 4> RTNAME(ArgumentLength)(
+    CppTypeFor<TypeCategory::Integer, 4> n);
 }
 } // namespace Fortran::runtime