Using the Microsoft C Runtime with User-Mode Drivers and Desktop Apps

If you are building applications or drivers for Windows 10, you only need to read this section. If you are using a version of Visual Studio earlier than Visual Studio 2015, skip this section and start with Redistributing the C Runtime (applies to before Visual Studio 2015).

Starting in Visual Studio 2015, the Universal C Runtime (UCRT) encompasses the C runtime. The other pieces required for a complete program (C/C++ Language Features, C++ Library) are provided by Visual Studio in the VC++ Redistributable. To avoid a runtime redistribution requirement, those pieces are statically linked.

Warning

When building a user-mode driver project in Visual Studio, if you set PlatformToolset to WindowsUserModeDriver10.0, the toolset ignores any runtime library specified in the project and instead links statically against the VC++ Runtime and dynamically against the UCRT. When using this toolset, this hybrid linking behavior cannot be reconfigured.

If you're not using the WindowsUserModeDriver10.0 toolset, use the following procedure to make modifications (for example include another DLL):

1. Set to link statically in general: Properties > C/C++ > Code Generation > Runtime Library = Multi-threaded (/MT)
2. Remove the statically linked UCRT: Properties > Linker > Input > Ignore Specific Default Libraries += libucrt.lib
3. Add the dynamically linked UCRT: Properties > Linker > Input > Additional Dependencies += ucrt.lib, Properties > Linker > Input > Ignore Specific Default Libraries += libucrt.lib

Redistributing the C Runtime (applies to before Visual Studio 2015)

Note

All information below this point applies only to pre-2015. Prior to 2015, there were two separate versions of the C Runtime: the Visual C++ Runtime (VCRT, for example msvcr120.dll) and the legacy Windows CRT (msvcrt.dll).

Visual Studio installs the latest version of the VCRT into the System32 directory. If the file is not in this location, you can copy it directly into the build directory of your Visual C++ project.

If your user-mode driver or desktop application uses the VCRT, you must distribute the appropriate dynamic-link libraries. Use the Visual C++ Redistributable Package (VCRedist_x86.exe, VCRedist_x64.exe, VCRedist_arm.exe). Chain the redistributable package in with other binaries, and the redistributable package will receive automatic updates.

If you want to achieve isolation or avoid the dependency on the VC++ Redistributable, you can link statically to the CRT instead. While non-driver projects are usually able to copy the specific Visual C/C++ DLLs to the application local folder (where the application is installed) to avoid a dependency on the VC++ Redistributable, app-local deployment is not appropriate for a driver.

Do not copy individual CRT components to System32 instead of using a redistributable package. This may cause the CRT not to be serviced automatically, and potentially to be overwritten.

The following special considerations apply for printer drivers:

• These drivers should include the required CRT files in the INF, so the CRT files are copied to the driver store as part of the driver payload.
• V4 print drivers cannot use a co-installer for setup, so the INF must copy relevant binaries of the C/C++ runtime library to the driver store. To do this, reference the appropriate files in the [COPY_FILES] section of the driver package.
• V3 print drivers should not use co-installers for setup, as they are not run during Point and Print connections. These drivers should reference the appropriate files in the [COPY_FILES] section of the driver package.

The following is an example of how to include the CRT binaries in the [COPY_FILES] section of an INF:

[COPY_FILES]
;CRT
Msvcr120.dll
; other files

* [SourceDisksFiles]
Msvcr120.dll = 2 
; other files

* [SourceDisksNames.amd64]
1 = %Location%,,,
2 = %Location%,,,"amd64"

For UMDF drivers:

• Statically link your driver against the CRT to include the runtime in the binary. In this case, you do not need to redistribute the CRT.

Linking your code with the C Runtime libraries (applies to before Visual Studio 2015)

To determine which DLLs you must redistribute with your application, collect a list of the DLLs that your application depends on. One way to collect the list is to run Dependency Walker (depends.exe).

For more information, see Determining Which DLLs to Redistribute and Choosing a Deployment Method.

You cannot redistribute all of the files that are included in Visual Studio; you are only permitted to redistribute the files that are specified in Redistributable Code for Visual Studio 2013 Preview and Visual Studio 2013 SDK Preview. Debug versions of applications and the various Visual C++ dynamic-link libraries are not redistributable.

The following libraries contain the C run-time library functions:

Term	Description
Msvcr120.dll	C runtime
Msvcp120.dll	C++ runtime
Msvcr120d.dll	Debug version of C runtime - no redistribution allowed
Msvcp120d.dll	Debug version of C++ runtime - no redistribution allowed