AddressSanitizer (ASan) for Windows with MSVC


We are pleased to announce AddressSanitizer (ASan) support for the MSVC toolset. ASan is a fast memory error detector that can find runtime memory issues such as use-after-free and perform out of bounds checks. Support for sanitizers has been one of our more popular suggestions on Developer Community, and we can now say that we have an experience for ASan on Windows, in addition to our existing support for Linux projects.

At Microsoft, we believe that developers should be able to use the tools of their choice, and we are glad to collaborate with the LLVM team to introduce more of their tooling into Visual Studio. In the past, we also incorporated tools like clang-format, and most recently, clang-tidy. MSVC support for ASan is available in our second Preview release of Visual Studio 2019 version 16.4.

To bring ASan to Windows and MSVC, we made the following changes:

  • The ASan runtime has been updated to better target Windows binaries
  • The MSVC compiler can now instrument binaries with ASan
  • CMake and MSBuild integrations have been updated to support enabling ASan
  • The Visual Studio debugger now can detect ASan errors in Windows binaries
  • ASan can be installed from the Visual Studio installer for the C++ Desktop workload

When you’re debugging your ASan-instrumented binary in Visual Studio, the IDE Exception Helper will be displayed when an issue is encountered, and program execution will stop. You can also view detailed ASan logging information in the Output window.

ASan Exception in Visual Studio

Installing ASan support for Windows

ASan is included with the C++ Desktop workload by default for new installations. However, if you are upgrading from an older version of Visual Studio 2019, you will need to enable ASan support in the Installer after the upgrade:

Visual Studio Installer with ASan checkbox

You can click Modify on your existing Visual Studio installation from the Visual Studio Installer to get to the screen above.

Note: if you run Visual Studio on the new update but have not installed ASan, you will get the following error when you run your code:

LNK 1356 – cannot find library ‘clang_rt.asan_dynamic-i386.lib’


Turning on ASan for Windows MSBuild projects

You can turn on ASan for an MSBuild project by right-clicking on the project in Solution Explorer, choosing Properties, navigating under C/C++ > General, and changing the Enable Address Sanitizer (Experimental) option. The same approach can be used to enable ASan for MSBuild Linux projects.

Enabling ASan in the MSBuild project properties

Note: Right now, this will only work for x86 Release targets, though we will be expanding to more architectures in the future.


Turning on ASan for Windows CMake projects

To enable ASan for CMake projects targeting Windows, do the following:

  1. Open the Configurations dropdown at the top of the IDE and click on Manage Configurations. This will open the CMake Project Settings UI, which is saved in a CMakeSettings.json file.
  2. Click the Edit JSON link in the UI. This will switch the view to raw .json.
  3. Under the x86-Release configuration, add the following property: "addressSanitizerEnabled": true

You may get a green squiggle under the line above with the following warning: Property name is not allowed by the schema. This is a bug that will be fixed shortly – the property will in fact work.

Here is an image of the relevant section of the CMakeSettings.json file after the change:

Enabling ASan in the CMakeSettings.json file

We will further simplify the process for enabling ASan in CMake projects in a future update.


Contributions to ASan Runtime

To enable a great Windows experience, we decided to contribute to the LLVM compiler-rt project and reuse their runtime in our implementation of ASan. Our contributions to the ASan project include bug fixes and improved interception for HeapAlloc , RtlAllocateHeap , GlobalAlloc , and LocalAlloc , along with their corresponding Free , ReAllocate , and Size functions. Anyone can enable these features by adding the following to the ASAN_OPTIONS environment variable for either Clang or MSVC on Windows:

Additional options can be added with a colon at the end of the line above.


Changes to MSVC to enable ASan

To enable ASan, c1.dll and c2.dll have been modified to add instrumentation to programs at compile time. For a 32-bit address space, about 200 MB of memory is allocated to represent (or ‘shadow’) the entire address space. When an allocation is made, the shadow memory is modified to represent that the allocation is now valid to access. When the allocation is freed or goes out of scope, the shadow memory is modified to show that this allocation is no longer valid. Memory accesses which are potentially dangerous are checked against their entry in the shadow memory to verify that the memory is safe to access at that time. Violations are reported to the user as output from either stderr or an exception window in Visual Studio. The allocation data in the shadow memory is checked before the access happens. The AddressSanitizer algorithm enables error reports to show exactly where the problem occurred and what went wrong.

This means that programs compiled with MSVC + ASan also have the appropriate clang_rt.asan library linked for their target. Each library has a specific use case and linking can be complicated if your program is complex.


Compiling with ASan from the console

For console compilation, you will have to link the ASan libraries manually. Here are the libraries required for an x86 target:

  • clang_rt.asan-i386.lib – static runtime compatible with /MT CRT.
  • clang_rt.asan_cxx-i386.lib -static runtime component which adds support for new and delete, also compatible with /MT CRT.
  • clang_rt.asan_dynamic-i386.lib – dynamic import library, compatible with /MD CRT.
  • clang_rt.asan_dynamic-i386.dll – dynamic runtime DLL, compatible with /MD.
  • clang_rt.asan_dynamic_runtime_thunk-i386.lib – dynamic library to import and intercept some /MD CRT functions manually.
  • clang_rt.asan_dll_thunk-i386.lib – import library which allows an ASAN instrumented DLL to use the static ASan library which is linked into the main executable. Compatible with /MT CRT.

Once you have selected the correct ASan runtime to use, you should add /wholearchive:<library to link> to your link line and add the appropriate library to your executables. The clang_rt.asan_dynamic_i386.dll is not installed into System32, so when running you should make sure it is available in your environment’s search path.

Some additional instructions:

  • When compiling a single static EXE: link the static runtime (asan-i386.lib) and the cxx library if it is needed.
  • When compiling an EXE with the /MT runtime which will use ASan-instrumented DLLs: the EXE needs to have asan-i386.lib linked and the DLLs need the clang_rt.asan_dll_thunk-i386.lib. This allows the DLLs to use the runtime linked into the main executable and avoid a shadow memory collision problem.
  • When compiling with the /MD dynamic runtime: all EXE and DLLs with instrumentation should be linked with copies of asan_dynamic-i386.lib and clang_rt.asan_dynamic_runtime_thunk-i386.lib. At runtime, these libraries will refer to the clang_rt.asan_dynamic-i386.dll shared ASan runtime.

The ASan runtime libraries patch memory management functions at run-time and redirect executions to an ASan wrapper function which manages the shadow memory. This can be unstable if the runtime environment differs from what the libraries have been written to expect. Please submit any compile or run time errors which you encounter while using ASan via the feedback channels below!


Send us feedback!

Your feedback is key for us to deliver the best experience running ASan in Visual Studio. We’d love for you to try out the latest Preview version of Visual Studio 2019 version 16.4 and let us know how it’s working for you, either in the comments below or via email. If you encounter problems with the experience or have suggestions for improvement, please Report A Problem or reach out via Developer Community. You can also find us on Twitter @VisualC.

Augustin Popa



  • Markus43

    This is very good news, many thanks!
    I already know ASan with clang-cl, which currently AFAIK does not support exceptions and only supports release builds. Do these restrictions also apply here?

    • Avatar
      Matthew McGovern

      I think clang-cl does support exceptions, but yes this implementation is also using the compiler-rt from LLVM so all the same restrictions apply here.

      A note: The real restriction is that the debug runtime is not supported. This doesn’t mean that you need to build with full optimization or build without debug symbols. Certainly don’t build with ASAN on and then release that binary as a product, this is strictly for debugging or fuzzing. The /MTd and /MDd runtimes are not supported, but setting /MT or /MD along with /Zi for debug info and /Od to disable optimization gives you a ‘release build’ but with full debugging info.

      • Markus43

        I agree that ASan is very usable also without support for debug runtimes, thanks for clarifying.
        Regarding exceptions, clang-cl of course supports exceptions. clang-cl with ASan enabled however does not work with exceptions, because of . AFAIK this comes from the fact that the companies who developed the Windows port (Google for Chrome, Mozilla for Firefox) ban exceptions in their code bases, so they didn’t have much of an issue with that … Are you planning to fix this specific issue for the MSVC port? I believe this would make ASan usable for many more developers!
        How does the MSVC port of ASan currently handle exceptions, what happens if an exception is thrown / catched?

    • Avatar
      Kyle Reed

      In the first screenshot, the green squiggle shown is actually a warning reported by /analyze detected by PREFast.
      Background code analysis is on by default so everyone using 2019 should be running /analyze by default.

    • Avatar
      Matthew McGovern

      It’s possible! Open issues/suggestions with Visual Studio and ask for it to be added. User feedback counts for a lot when deciding priorities before a sprint starts.

    • Avatar
      Matthew McGovern

      ApplicationVerifier and ASan shouldn’t be used at the same time. I don’t know if they are strictly incompatible but it wouldn’t get you anything extra to use both and would probably be kind of slow.

  • Myungbae Son
    Myungbae Son

    I’ve just seen following critiques in reddit discussion that I tend to agree, so I decided to link them here:

    In short, could you explain the rationale on putting those settings in CMakeSettings.json for CMake projects? Is this the final design or to be integrated more smoothly with CMake?

    • Avatar
      Matthew McGovern

      I did not handle the cmake integration so I can’t speak to their rationale. Any usability issues are almost surely bugs which should be filed via the channels at the end of the blog post. We love fixing issues that frustrate people, and occasionally we miss things while alpha/beta testing so please do reach out.

    • Avatar
      Matthew McGovern

      It’s on the list of things to do but is lower priority than x64 support and a few other items. You can still turn on debug info with cl and link and set optimizations as needed for an excellent debugging experience, just don’t link the MDd or MTd runtimes since they won’t work as expected with the clang runtime libs.

  • Avatar
    Edward Lambert (SI)

    Does this not work on Windows 7, bombs out with “Exception thrown at 0x77BA191E (ntdll.dll) in fm.exe: 0xC0000139: Entry Point Not Found.” straight away

    • Avatar
      Matthew McGovern

      It should work on Win7, can you please provide feedback via the email, visual studio feedback hub, or via the developer community? It helps if you can include a small C file and instructions to reproduce the issue you’re seeing (VS version, windows patch verison, compiler-flags etc).

  • Avatar
    Mårten Nordheim

    Will linking from the command-line be more streamlined in the future? For example for clang you just specify -fsanitize=address for the linker as well, instead of this matrix of choices.


  • Avatar
    John Schroedl

    Exciting news. I won’t be able to use it until x64 support arrives.. but when it does, will it work with a mixed-mode application?
    ex. Native .exe entry point which loads .NET via C++/CLI? I don’t expect it to flag anything from the managed area, just want to know if it can monitor the native portions of the application.

  • Avatar
    Hristo Hristov

    CMake generation started for configuration: ‘Ninja-MSVC-x64-Debug’.
    1> CMakeSettings ‘addressSanitizerEnabled’ value is inconsistent with CMAKE_C_FLAGS or CMAKE_CXX_FLAGS.
    1> Environment settings:
    1> CXXFLAGS=-fsanitize=address
    1> CFLAGS=-fsanitize=address
    1> LDFLAGS=/wholearchive:clang_rt.asan_dynamic-i386.lib /wholearchive:clang_rt.asan_dynamic_runtime_thunk-i386.lib /wholearchive:vcasan.lib
    [CMake] CMake version: 3.15.19080502-MSVC_2
    1> [CMake] — The C compiler identification is MSVC 19.24.28207.0
    1> [CMake] — The CXX compiler identification is MSVC 19.24.28207.0
    1> [CMake] — Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Preview/VC/Tools/MSVC/14.24.28207/bin/HostX64/x64/cl.exe
    1> [CMake] — Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Preview/VC/Tools/MSVC/14.24.28207/bin/HostX64/x64/cl.exe — broken
    1> [CMake] CMake Error at C:/Program Files (x86)/Microsoft Visual Studio/2019/Preview/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.15/Modules/CMakeTestCCompiler.cmake:60 (message):
    1> [CMake] The C compiler
    1> [CMake]
    1> [CMake] “C:/Program Files (x86)/Microsoft Visual Studio/2019/Preview/VC/Tools/MSVC/14.24.28207/bin/HostX64/x64/cl.exe”
    1> [CMake]
    1> [CMake] is not able to compile a simple test program.
    1> [CMake]
    1> [CMake] It fails with the following output:
    1> [CMake]
    1> [CMake] Change Dir: X:/GitHub/GunBox/__build-output/Ninja-MSVC-x64-Debug/CMakeFiles/CMakeTmp
    1> [CMake]
    1> [CMake] Run Build Command(s):C:/Program Files (x86)/Microsoft Visual Studio/2019/Preview/Common7/IDE/CommonExtensions/Microsoft/CMake/Ninja/ninja.exe cmTC_20743 && [1/2] Building C object CMakeFiles\cmTC_20743.dir\testCCompiler.c.obj
    1> [CMake] cl : Command line warning D9002 : ignoring unknown option ‘-fsanitize=address’
    1> [CMake] [2/2] Linking C executable cmTC_20743.exe
    1> [CMake] FAILED: cmTC_20743.exe
    1> [CMake] cmd.exe /C “cd . && “C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe” -E vs_link_exe –intdir=CMakeFiles\cmTC_20743.dir –rc=C:\PROGRA~2\WI3CF2~1\10\bin\10.0.18362.0\x64\rc.exe –mt=C:\PROGRA~2\WI3CF2~1\10\bin\10.0.18362.0\x64\mt.exe –manifests — “C:\PROGRA~2\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.24.28207\bin\Hostx64\x64\link.exe” /nologo CMakeFiles\cmTC_20743.dir\testCCompiler.c.obj /out:cmTC_20743.exe /implib:cmTC_20743.lib /pdb:cmTC_20743.pdb /version:0.0 /machine:x64 /wholearchive:clang_rt.asan_dynamic-i386.lib /wholearchive:clang_rt.asan_dynamic_runtime_thunk-i386.lib /wholearchive:vcasan.lib /debug /INCREMENTAL /subsystem:console kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib && cmd.exe /C “cd /D X:\GitHub\GunBox\__build-output\Ninja-MSVC-x64-Debug\CMakeFiles\CMakeTmp && powershell -noprofile -executionpolicy Bypass -file C:/SDKs/vcpkg/scripts/buildsystems/msbuild/applocal.ps1 -targetBinary X:/GitHub/GunBox/__build-output/Ninja-MSVC-x64-Debug/CMakeFiles/CMakeTmp/cmTC_20743.exe -installedDir C:/SDKs/vcpkg/installed/x64-windows/debug/bin -OutVariable out””
    1> [CMake] LINK Pass 1: command “C:\PROGRA~2\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.24.28207\bin\Hostx64\x64\link.exe /nologo CMakeFiles\cmTC_20743.dir\testCCompiler.c.obj /out:cmTC_20743.exe /implib:cmTC_20743.lib /pdb:cmTC_20743.pdb /version:0.0 /machine:x64 /wholearchive:clang_rt.asan_dynamic-i386.lib /wholearchive:clang_rt.asan_dynamic_runtime_thunk-i386.lib /wholearchive:vcasan.lib /debug /INCREMENTAL /subsystem:console kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTFILE:CMakeFiles\cmTC_20743.dir/intermediate.manifest CMakeFiles\cmTC_20743.dir/manifest.res” failed (exit code 1356) with the following output:
    1> [CMake] LINK : fatal error LNK1356: cannot find library `clang_rt.asan_dynamic-i386.lib’ specified to be whole archived
    1> [CMake] ninja: build stopped: subcommand failed.

  • 이동익

    Hello 😀 I have a question. Is there a way to reduce the memory usage of ASAN?
    Too much memory used by ASAN to proceed with the test.

    Our program is using 2 gigabytes by default, but when used with the ASan , it quickly reaches 4 gigabytes, resulting in out_of_memory.

  • Матцал Коушек
    Матцал Коушек

    Is it able to detect all the things that clang/gcc ASAN detects?
    I mean all these stack out of bounds, use after scope, global buffer overflow, etc

    • Avatar
      Matthew McGovern

      Yes, this implementation shares a runtime with Clang on windows and it can detect the same categories of bugs.

  • Avatar
    Mike Stratoti

    Slight typo.

    Should be
    set ASAN_OPTIONS= windows_hook_rtl_allocators=true
    ASAN_OPTIONS=set windows_hook_rtl_allocators=true

  • Avatar
    Jeff Mears

    Windows has an ungodly number of ways that memory can be allocated. Some are even done by kernel drivers calling ZwMapViewOfSection on behalf of a user-mode application. How will such memory accesses be handled?

    Similarly, if you use standard free() to free memory, ntdll.dll might do NtFreeVirtualMemory because that whole heap area is no longer needed. Then you get a pointer to new kernel-allocated memory somehow, such as locking a graphics buffer. What happens if the address of this mapping matches the address range that unmapped? Does the ASan runtime monitor the internals of the ntdll.dll heap code to know when address space is allocated and freed?

    While I definitely would love ASan for Windows, I just worry that in a lot of use cases, it won’t be reliable.

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