The VS2017 compiler reports 'warning C6001: Using
uninitialized memory 'Marker'.' for VA_LIST
variables.

To fix this issue declare a VA_LIST global variable
and use this to initialise VA_LIST variables before
use.

Note: The VA_LIST cannot be assigned a NULL value
because some compilers define VA_LIST to be a
structure.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
---
 MdePkg/Library/BaseLib/SwitchStack.c           | 9 +++++++++
 MdePkg/Library/BasePrintLib/PrintLib.c         | 5 +++++
 MdePkg/Library/BasePrintLib/PrintLibInternal.c | 9 +++++++++
 3 files changed, 23 insertions(+)

diff --git a/MdePkg/Library/BaseLib/SwitchStack.c b/MdePkg/Library/BaseLib/SwitchStack.c
index cb9f69f1eaceba690b48e9ca6b8a9af2e348bddd..e1bb524819b3de3521c5461ce681aa3a6c186f2c 100644
--- a/MdePkg/Library/BaseLib/SwitchStack.c
+++ b/MdePkg/Library/BaseLib/SwitchStack.c
@@ -2,12 +2,20 @@
   Switch Stack functions.
 
   Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2019, ARM Ltd. All rights reserved.<BR>
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
 
 #include "BaseLibInternals.h"
 
+//
+// Declare a VA_LIST global variable that is used to initialise VA_LIST
+// variables before use. The VA_LIST cannot be assigned a NULL value
+// because some compilers define VA_LIST to be a structure.
+//
+STATIC VA_LIST gNullVaList;
+
 /**
   Transfers control to a function starting with a new stack.
 
@@ -57,6 +65,7 @@ SwitchStack (
   //
   ASSERT (((UINTN)NewStack & (CPU_STACK_ALIGNMENT - 1)) == 0);
 
+  Marker = gNullVaList;
   VA_START (Marker, NewStack);
 
   InternalSwitchStack (EntryPoint, Context1, Context2, NewStack, Marker);
diff --git a/MdePkg/Library/BasePrintLib/PrintLib.c b/MdePkg/Library/BasePrintLib/PrintLib.c
index af771652e4b0aebd616973ba1089ae5bc2b6f0c0..67c5f3dd547cea5447075ef88d697879883ba5ab 100644
--- a/MdePkg/Library/BasePrintLib/PrintLib.c
+++ b/MdePkg/Library/BasePrintLib/PrintLib.c
@@ -3,6 +3,7 @@
 
   Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
   Portions copyright (c) 2008 - 2009, Apple Inc. All rights reserved.<BR>
+  Copyright (c) 2019, ARM Ltd. All rights reserved.<BR>
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -177,6 +178,7 @@ UnicodeSPrint (
   VA_LIST Marker;
   UINTN   NumberOfPrinted;
 
+  Marker = gNullVaList;
   VA_START (Marker, FormatString);
   NumberOfPrinted = UnicodeVSPrint (StartOfBuffer, BufferSize, FormatString, Marker);
   VA_END (Marker);
@@ -337,6 +339,7 @@ UnicodeSPrintAsciiFormat (
   VA_LIST Marker;
   UINTN   NumberOfPrinted;
 
+  Marker = gNullVaList;
   VA_START (Marker, FormatString);
   NumberOfPrinted = UnicodeVSPrintAsciiFormat (StartOfBuffer, BufferSize, FormatString, Marker);
   VA_END (Marker);
@@ -614,6 +617,7 @@ AsciiSPrint (
   VA_LIST Marker;
   UINTN   NumberOfPrinted;
 
+  Marker = gNullVaList;
   VA_START (Marker, FormatString);
   NumberOfPrinted = AsciiVSPrint (StartOfBuffer, BufferSize, FormatString, Marker);
   VA_END (Marker);
@@ -774,6 +778,7 @@ AsciiSPrintUnicodeFormat (
   VA_LIST Marker;
   UINTN   NumberOfPrinted;
 
+  Marker = gNullVaList;
   VA_START (Marker, FormatString);
   NumberOfPrinted = AsciiVSPrintUnicodeFormat (StartOfBuffer, BufferSize, FormatString, Marker);
   VA_END (Marker);
diff --git a/MdePkg/Library/BasePrintLib/PrintLibInternal.c b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
index b6ec5ac4fbb98982f8ccaf3908c2a91ce583e31e..11392f2a5d12eb059611c3ff77b27b602f9b9a40 100644
--- a/MdePkg/Library/BasePrintLib/PrintLibInternal.c
+++ b/MdePkg/Library/BasePrintLib/PrintLibInternal.c
@@ -2,12 +2,20 @@
   Print Library internal worker functions.
 
   Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2019, ARM Ltd. All rights reserved.<BR>
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
 
 #include "PrintLibInternal.h"
 
+//
+// Declare a VA_LIST global variable that is used to initialise VA_LIST
+// variables before use. The VA_LIST cannot be assigned a NULL value
+// because some compilers define VA_LIST to be a structure.
+//
+extern VA_LIST gNullVaList;
+
 #define WARNING_STATUS_NUMBER         5
 #define ERROR_STATUS_NUMBER           33
 
@@ -1256,6 +1264,7 @@ BasePrintLibSPrint (
   VA_LIST  Marker;
   UINTN    NumberOfPrinted;
 
+  Marker = gNullVaList;
   VA_START (Marker, FormatString);
   NumberOfPrinted = BasePrintLibSPrintMarker (StartOfBuffer, BufferSize, Flags, FormatString, Marker, NULL);
   VA_END (Marker);
--
'Guid(CE165669-3EF3-493F-B85D-6190EE5B9759)'