Overview of potential upgrade issues (Visual C++)

Over the years, the Microsoft C++ compiler has undergone many changes, along with changes in the C++ language itself, the C++ Standard Library, the C runtime (CRT), and other libraries such as MFC and ATL. As a result, when upgrading an application from an earlier version of Visual Studio you might encounter compiler and linker errors and warnings in code that previously compiled cleanly. The older the original code base, the greater the potential for such errors. This overview summarizes the most common classes of issues you are likely to encounter, and provides links to more detailed information.


In the past, we have recommended that upgrades that span several versions of Visual Studio should be performed incrementally one version at a time. We no longer recommend this approach. We have found that it's almost always simpler to upgrade to the most current version of Visual Studio no matter how old the code base.

Questions or comments about the upgrade process can be sent to vcupgrade@microsoft.com.

Library and toolset dependencies


This section applies to applications and libraries built with Visual Studio 2013 and earlier. The toolsets used in Visual Studio 2015, Visual Studio 2017 and Visual Studio 2019 are binary compatible. For more information, see C++ Binary Compatibility between Visual Studio 2015 and Visual Studio 2019.

When upgrading an application to a new version of Visual Studio, it's both advisable and in many cases necessary to also upgrade all libraries and DLLs that the application links to. It requires either that you have access to the source code, or that the library vendor can provide new binary files compiled with the same major version of the compiler. If one of these conditions is true, then you can skip this section, which deals with the details of binary compatibility. If neither is the case, then you might not be able to use the libraries in your upgraded application. The information in this section will help you understand whether you can proceed with the upgrade.


The .obj and .lib file formats are well-defined and rarely change. Sometimes additions are made to these file formats, but these additions generally don't affect the ability of newer toolsets to consume object files and libraries produced by older toolsets. The major exception here is if you compile using /GL (Whole Program Optimization). If you compile using /GL, the resulting object file can only be linked using the same toolset that was used to produce it. So, if you produce an object file with /GL and using the Visual Studio 2017 (v141) compiler, you must link it using the Visual Studio 2017 (v141) linker. It's because the internal data structures within the object files are not stable across major versions of the toolset, and newer toolsets don't understand the older data formats.

C++ doesn't have a stable application binary interface (ABI). But Visual Studio maintains a stable C++ ABI for all minor versions of a release. Visual Studio 2015 (v140), Visual Studio 2017 (v141), and Visual Studio 2019 (v142) vary only in their minor version. They all have the same major version number, which is 14. For more information, see C++ Binary Compatibility between Visual Studio 2015 and Visual Studio 2019.

If you have an object file that has external symbols with C++ linkage, that object file may not link correctly with object files produced by a different major version of the toolset. There are many possible outcomes: the link may fail entirely (for example, if name decoration changed). The link may succeed, and things may not work at runtime (for example, if type layout changed). Or in many cases, things may happen to work and nothing will go wrong. Also note, while the C++ ABI is not stable, the C ABI and the subset of the C++ ABI required for COM are stable.

If you link to an import library, any later version of the Visual Studio redistributable libraries that preserve ABI compatibility may be used at runtime. For example, if your app is compiled and linked using the Visual Studio 2015 Update 3 toolset, you can use any Visual Studio 2017 or Visual Studio 2019 redistributable, because the 2015, 2017, and 2019 libraries have preserved backward binary compatibility. The reverse isn't true: You can't use a redistributable for an earlier version of the toolset than you used to build your code, even if they have a compatible ABI.


If you compile a source file using a particular version of the Visual Studio C++ libraries header files (by #including the headers), the resulting object file must be linked with the same version of the libraries. So, for example, if your source file is compiled with the Visual Studio 2015 Update 3 <immintrin.h>, you must link with the Visual Studio 2015 Update 3 vcruntime library. Similarly, if your source file is compiled with the Visual Studio 2017 version 15.5 <iostream>, you must link with the Visual Studio 2017 version 15.5 Standard C++ library, msvcprt. Mixing-and-matching is not supported.

For the C++ Standard Library, mixing-and-matching has been explicitly disallowed via use of #pragma detect_mismatch in the standard headers since Visual Studio 2010. If you try to link incompatible object files, or if you try to link with the wrong standard library, the link will fail.

For the CRT, mixing-and-matching was never supported, but it often just worked, at least until Visual Studio 2015 and the Universal CRT, because the API surface did not change much over time. The Universal CRT broke backwards compatibility so that in the future we can maintain backwards compatibility. In other words, we have no plans to introduce new, versioned Universal CRT binaries in the future. Instead, the existing Universal CRT is now updated in-place.

To provide partial link compatibility with object files (and libraries) compiled with older versions of the Microsoft C Runtime headers, we provide a library, legacy_stdio_definitions.lib, with Visual Studio 2015 and later. This library provides compatibility symbols for most of the functions and data exports that were removed from the Universal CRT. The set of compatibility symbols provided by legacy_stdio_definitions.lib is sufficient to satisfy most dependencies, including all of the dependencies in libraries included in the Windows SDK. However, there are some symbols that were removed from the Universal CRT for which it's not possible to provide compatibility symbols. These symbols include some functions (for example, __iob_func) and the data exports (for example, __imp___iob, __imp___pctype, __imp___mb_cur_max).

If you have a static library that was built with an older version of the C Runtime headers, we recommend the following actions, in this order:

  1. Rebuild the static library using the new version of Visual Studio and the Universal CRT headers to support linking with the Universal CRT. This approach is the fully supported (and thus best) option.

  2. If you can't (or don't want to) rebuild the static library, you may try linking with legacy_stdio_definitions.lib. If it satisfies the link-time dependencies of your static library, you will want to thoroughly test the static library as it's used in the binary, to make sure that it isn't adversely affected by any of the behavioral changes that were made to the Universal CRT.

  3. If your static library’s dependencies are not satisfied by legacy_stdio_definitions.lib or if the library doesn't work with the Universal CRT due to the aforementioned behavioral changes, we would recommend encapsulating your static library into a DLL that you link with the correct version of the Microsoft C Runtime. For example, if the static library was built using Visual Studio 2013, you would want to build this DLL using Visual Studio 2013 and the Visual Studio 2013 C++ libraries as well. By building the library into a DLL, you encapsulate the implementation detail that is its dependency on a particular version of the Microsoft C Runtime. You'll want to be careful that the DLL interface doesn't leak details of which C Runtime it uses, for example, by returning a FILE* across the DLL boundary or by returning a malloc-allocated pointer and expecting the caller to free it.

Use of multiple CRTs in a single process isn't in and of itself problematic (indeed, most processes will end up loading multiple CRT DLLs; for example, Windows operating system components will depend on msvcrt.dll and the CLR will depend on its own private CRT). Problems arise when you jumble state from different CRTs. For example, you should not allocate memory using msvcr110.dll!malloc and attempt to deallocate that memory using msvcr120.dll!free, and you should not attempt to open a FILE using msvcr110!fopen and attempt to read from that FILE using msvcr120!fread. As long as you don’t jumble state from different CRTs, you can safely have multiple CRTs loaded in a single process.

For more information, see Upgrade your code to the Universal CRT.

Errors due to project settings

To begin the upgrade process, open an older project/solution/workspace in the latest version of Visual Studio. Visual Studio will create a new project based on the old project settings. If the older project has library or include paths that are hard-coded to non-standard locations, it's possible that the files in those paths won’t be visible to the compiler when the project uses the default settings. For more information, see Linker OutputFile setting.

In general, now is a great time to organize your project code properly in order to simplify project maintenance and help get your upgraded code compiling as quickly as possible. If your source code is already well-organized, and your older project is compiled with Visual Studio 2010 or later, you can manually edit the new project file to support compilation on both the old and new compiler. The following example shows how to compile for both Visual Studio 2015 and Visual Studio 2017:

<PlatformToolset Condition="'$(VisualStudioVersion)'=='14.0'">v140</PlatformToolset>
<PlatformToolset Condition="'$(VisualStudioVersion)'=='15.0'">v141</PlatformToolset>

LNK2019: Unresolved external

For unresolved symbols, you might need to fix up your project settings.

  • If the source file is in a non-default location, did you add the path to the project’s include directories?

  • If the external is defined in a .lib file, have you specified the lib path in the project properties, and is the correct version of the .lib file located there?

  • Are you attempting to link to a .lib file that was compiled with a different version of Visual Studio? If so, see the previous section on library and toolset dependencies.

  • Do the types of the arguments at the call site actually match an existing overload of the function? Verify the underlying types for any typedefs in the function’s signature and in the code that calls the function are what you expect them to be.

To troubleshoot unresolved symbol errors, you can try using dumpbin.exe to examine the symbols defined in a binary. Try the following command line to view symbols defined in a library:

dumpbin.exe /LINKERMEMBER somelibrary.lib

/Zc:wchar_t (wchar_t Is Native Type)

(In Microsoft Visual C++ 6.0 and earlier, wchar_t was not implemented as a built-in type, but was declared in wchar.h as a typedef for unsigned short.) The C++ standard requires that wchar_t is a built-in type. Using the typedef version can cause portability problems. If you upgrade from earlier versions of Visual Studio and encounter compiler error C2664 because the code is trying to implicitly convert a wchar_t to unsigned short, we recommend that you change the code to fix the error, instead of setting /Zc:wchar_t-. For more information, see /Zc:wchar_t (wchar_t Is Native Type).

Upgrading with the linker options /NODEFAULTLIB, /ENTRY, and /NOENTRY

The /NODEFAULTLIB linker option (or the Ignore All Default Libraries linker property) tells the linker not to automatically link in the default libraries such as the CRT. It means that each library has to be listed as input individually. This list of libraries is given in the Additional Dependencies property in the Linker section of the Project Properties dialog.

Projects that use this option present a problem when upgrading, because the contents of some of the default libraries were refactored. Because each library has to be listed in the Additional Dependencies property or on the linker command line, you need to update the list of libraries to use all the current names.

The following table shows the libraries whose contents changed starting with Visual Studio 2015. To upgrade, you need to add the new library names in the second column to the libraries in the first column. Some of these libraries are import libraries, but that shouldn’t matter.

If you were using: You need to use these libraries:
libcmt.lib libcmt.lib, libucrt.lib, libvcruntime.lib
libcmtd.lib libcmtd.lib, libucrtd.lib, libvcruntimed.lib
msvcrt.lib msvcrt.lib, ucrt.lib, vcruntime.lib
msvcrtd.lib msvcrtd.lib, ucrtd.lib, vcruntimed.lib

The same issue applies also if you use the /ENTRY option or the /NOENTRY option, which also have the effect of bypassing the default libraries.

Errors due to improved language conformance

The Microsoft C++ compiler has continuously improved its conformance to the C++ standard over the years. Code that compiled in earlier versions might fail to compile in later versions of Visual Studio because the compiler correctly flags an error that it previously ignored or explicitly allowed.

For example, the /Zc:forScope switch was introduced early in the history of MSVC. It permits non-conforming behavior for loop variables. That switch is now deprecated and might be removed in future versions. It is highly recommended to not use that switch when upgrading your code. For more information, see /Zc:forScope- is deprecated.

One example of a common compiler error you might see when upgrading is when a non-const argument is passed to a const parameter. Older versions of the compiler did not always flag it as an error. For more information, see The compiler's more strict conversions.

For more information on specific conformance improvements, see Visual C++ change history 2003 - 2015 and C++ conformance improvements in Visual Studio.

Errors involving <stdint.h> integral types

The <stdint.h> header defines typedefs and macros that, unlike built-in integral types, are guaranteed to have a specified length on all platforms. Some examples are uint32_t and int64_t. The <stdint.h> header was added in Visual Studio 2010. Code that was written before 2010 might have provided private definitions for those types and those definitions might not always be consistent with the <stdint.h> definitions.

If the error is C2371, and a stdint type is involved, it probably means that the type is defined in a header either in your code or a third-party lib file. When upgrading, you should eliminate any custom definitions of <stdint.h> types, but first compare the custom definitions to the current standard definitions to ensure you are not introducing new problems.

You can press F12 (Go to Definition) to see where the type in question is defined.

The /showIncludes compiler option can be useful here. In the Property Pages dialog box for your project, open the C/C++ > Advanced page and set Show Includes to Yes. Then rebuild your project and see the list of #includes in the output window. Each header is indented under the header that includes it.

Errors involving CRT functions

Many changes have been made to the C runtime over the years. Many secure versions of functions have been added, and some have been removed. Also, as described earlier in this article, Microsoft’s implementation of the CRT was refactored in Visual Studio 2015 into new binaries and associated .lib files.

If an error involves a CRT function, search Visual C++ change history 2003 - 2015 or C++ conformance improvements in Visual Studio to see if those articles contain any additional information. If the error is LNK2019, unresolved external, make sure the function has not been removed. Otherwise, if you are sure that the function still exists, and the calling code is correct, check to see whether your project uses /NODEFAULTLIB. If so, you need to update the list of libraries so that the project uses the new universal (UCRT) libraries. For more information, see the section above on Library and dependencies.

If the error involves printf or scanf, make sure that you are not privately defining either function without including stdio.h. If so, either remove the private definitions or link to legacy_stdio_definitions.lib. You can set this library in the Property Pages dialog under Configuration Properties > Linker > Input, in the Additional Dependencies property. If you are linking with Windows SDK 8.1 or earlier, then add legacy_stdio_definitions.lib.

If the error involves format string arguments, it's probably because the compiler is stricter about enforcing the standard. For more information, see the change history. Pay close attention to any errors here, because they can potentially represent a security risk.

Errors due to changes in the C++ standard

The C++ standard itself has evolved in ways that are not always backward compatible. The introduction in C++11 of move semantics, new keywords, and other language and standard library features can potentially cause compiler errors and even different runtime behavior.

For example, an old C++ program might include the iostream.h header. This header was deprecated early in the history of C++ and was eventually removed completely from Visual Studio. In this case, you will need to use <iostream> and rewrite your code. For more information, see Updating old iostreams code.

C4838: narrowing conversion warning

The C++ standard now specifies that conversions from unsigned to signed integral values are considered as narrowing conversions. The compiler did not raise this warning prior to Visual Studio 2015. Inspect each occurrence to make sure the narrowing doesn't impact the correctness of your code.

Warnings to use secure CRT functions

Over the years, secure versions of C runtime functions have been introduced. Although the old, non-secure versions are still available, it's recommended to change your code to use the secure versions. The compiler will issue a warning for usage of the non-secure versions. You can choose to disable or ignore these warnings. To disable the warning for all projects in your solution, open View > Property Manager, select all projects for which you want to disable the warning, then right-click on the selected items and choose Properties. In the Property Pages dialog under Configuration Properties > C/C++ > Advanced, select Disable Specific Warnings. Click the drop-down arrow and then click on Edit. Enter 4996 into the text box. (Don't include the 'C' prefix.) For more information, see Porting to use the Secure CRT.

Errors due to changes in Windows APIs or obsolete SDKs

Over the years, Windows APIs and data types have been added, and sometimes changed or removed. Also, other SDKs that did not belong to the core operating system have come and gone. Older programs may therefore contain calls to APIs that no longer exist. They may also contain calls to APIs in other Microsoft SDKs that are no longer supported. If you see an error involving a Windows API or an API from an older Microsoft SDK, it's possible that an API has been removed and/or superseded by a newer, more secure function.

For more information about the current API set and the minimum supported operating systems for a specific Windows API, see Microsoft API and reference catalog and navigate to the API in question.

Windows version

When upgrading a program that uses the Windows API either directly or indirectly, you will need to decide the minimum Windows version to support. In most cases, Windows 7 is a good choice. For more information, see Header file problems. The WINVER macro defines the oldest version of Windows that your program is designed to run on. If your MFC program sets WINVER to 0x0501 (Windows XP) you will get a warning because MFC no longer supports XP, even though the compiler itself has an XP mode.

For more information, see Updating the Target Windows Version and More outdated header files.


ATL and MFC are relatively stable APIs but changes are made occasionally. For more information, see Visual C++ change history 2003 - 2015, What's New for Visual C++ in Visual Studio, and C++ conformance improvements in Visual Studio.

LNK 2005 _DllMain@12 already defined in MSVCRTD.lib

This error can occur in MFC applications. It indicates an ordering issue between the CRT library and the MFC library. MFC must be linked first so that it provides new and delete operators. To fix the error, use the /NODEFAULTLIB switch to Ignore these default libraries: MSVCRTD.lib and mfcs140d.lib. Then add these same libs as additional dependencies.

32 vs 64 bit

If your original code is compiled for 32-bit systems, you have the option of creating a 64-bit version instead of or in addition to a new 32-bit app. In general, you should get your program compiling in 32-bit mode first, and then attempt 64-bit. Compiling for 64-bit is straightforward, but in some cases it can reveal bugs that were hidden by 32-bit builds.

Also, you should be aware of possible compile-time and runtime issues relating to pointer size, time and size values, and format specifiers in printf and scanf functions. For more information, see Configure Visual C++ for 64-bit, x64 targets and Common Visual C++ 64-bit Migration Issues. For additional migration tips, see Programming Guide for 64-bit Windows.

Unicode vs MBCS/ASCII

Before Unicode was standardized, many programs used the Multibyte Character Set (MBCS) to represent characters that were not included in the ASCII character set. In older MFC projects, MBCS was the default setting, and when you upgrade such a program, you will see warnings that advise to use Unicode instead. You may choose to disable or ignore the warning if you decide that converting to Unicode isn't worth the development cost. To disable it for all projects in your solution, open View > Property Manager, select all projects for which you want to disable the warning, then right-click on the selected items and choose Properties. In the Property Pages dialog, select Configuration Properties > C/C++ > Advanced. In the Disable Specific Warnings property, open the drop-down arrow, and then choose Edit. Enter 4996 into the text box. (Don't include the 'C' prefix.) Choose OK to save the property, then choose OK to save your changes.

For more information, see Porting from MBCS to Unicode. For general information about MBCS vs. Unicode, see Text and Strings in Visual C++ and Internationalization .

See also

Upgrading projects from earlier versions of Visual C++
C++ conformance improvements in Visual Studio