Index: flang/runtime/environment.h
===================================================================
--- flang/runtime/environment.h
+++ flang/runtime/environment.h
@@ -41,12 +41,37 @@
 };
 extern ExecutionEnvironment executionEnvironment;
 
+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, 8> 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.
+// The \p value descriptor is only read/written into if it is already allocated.
+// Returns a STATUS as described in the standard.
+CppTypeFor<TypeCategory::Integer, 8> RTNAME(ArgumentValue)(
+    CppTypeFor<TypeCategory::Integer, 8> n, Descriptor &value);
+
+// Try to get the value of the n'th argument. Write up an error message
+// otherwise.
+// The \p value descriptor is only read/written into if it is already allocated.
+// Returns a STATUS as described in the standard.
+CppTypeFor<TypeCategory::Integer, 8> RTNAME(ArgumentValueVerbose)(
+    CppTypeFor<TypeCategory::Integer, 8> n, Descriptor &value,
+    Descriptor &errmsg);
+
+// Try to get the significant length of the n'th argument.
+// Returns 0 if it doesn't manage.
+CppTypeFor<TypeCategory::Integer, 8> RTNAME(ArgumentLength)(
+    CppTypeFor<TypeCategory::Integer, 8> n);
 }
 } // namespace Fortran::runtime