[PATCH] MdePkg: correct and clarify documentation of VA_LIST in Base.h

[Zenith432 <zenith432@users.sourceforge.net>]

This is to resolve bug 457.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zenith432 <zenith432 at users.sourceforge.net>
---
 MdePkg/Include/Base.h | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/MdePkg/Include/Base.h b/MdePkg/Include/Base.h
index 02140a5a..19f36872 100644
--- a/MdePkg/Include/Base.h
+++ b/MdePkg/Include/Base.h
@@ -560,13 +560,14 @@ struct _LIST_ENTRY {
 //  VA_LIST  - typedef for argument list.
 //  VA_START (VA_LIST Marker, argument before the ...) - Init Marker for use.
 //  VA_END (VA_LIST Marker) - Clear Marker
-//  VA_ARG (VA_LIST Marker, var arg size) - Use Marker to get an argument from
-//    the ... list. You must know the size and pass it in this macro.
+//  VA_ARG (VA_LIST Marker, var arg type) - Use Marker to get an argument from
+//    the ... list. You must know the type and pass it in this macro.
 //  VA_COPY (VA_LIST Dest, VA_LIST Start) - Initialize Dest as a copy of Start.
 //
-//  example:
+//  Example:
 //
 //  UINTN
+//  EFIAPI
 //  ExampleVarArg (
 //    IN UINTN  NumberOfArgs,
 //    ...
@@ -582,7 +583,7 @@ struct _LIST_ENTRY {
 //    VA_START (Marker, NumberOfArgs);
 //    for (Index = 0, Result = 0; Index < NumberOfArgs; Index++) {
 //      //
-//      // The ... list is a series of UINTN values, so average them up.
+//      // The ... list is a series of UINTN values, so sum them up.
 //      //
 //      Result += VA_ARG (Marker, UINTN);
 //    }
@@ -591,6 +592,21 @@ struct _LIST_ENTRY {
 //    return Result
 //  }
 //
+//  Notes:
+//
+//  This set of macros is intended to support variadic functions that
+//  use the EFIAPI calling convention.  Variadic functions that use a
+//  native calling convention should use stdarg.h.
+//  In particular:
+//  -- VA_START may only be used in a variadic EFIAPI function.
+//  -- va_start may only be used in a variadic native function.
+//  -- VA_START, VA_END, VA_ARG and VA_COPY may only be used on a VA_LIST.
+//  -- va_start, va_end, va_arg and va_copy may only be used on a va_list.
+//  -- Both VA_LIST or va_list may be passed as arguments to functions
+//     of either EFIAPI or native calling conventions.
+//  -- VA_END, VA_ARG, VA_COPY, va_end, va_arg, and va_copy may be used
+//     in functions of either calling conventions.
+//
 
 /**
   Return the size of argument that has been aligned to sizeof (UINTN).
-- 
2.14.3