RealtimeSanitizer

RealtimeSanitizer (a.k.a. RTSan) is a real-time safety testing tool for C and C++ projects. RTSan can be used to detect real-time violations, i.e. calls to methods that are not safe for use in functions with deterministic runtime requirements.

RTSan is part of the LLVM project, starting from v20.0.0. Like other popular sanitizers, it uses a lightweight dynamic library that can detect real-time unsafe system library calls. Calls to functions such as malloc, free and pthread_mutex_lock (along with anything else RTSan believes may have a nondeterministic execution time) will cause RTSan to error, but only if they are called within a real-time context, as defined by the user. Real-time contexts are defined by the user simply by marking functions with the [[clang::nonblocking]] attribute.

Usage

Using RealtimeSanitizer requires LLVM 20's clang and two actions:

1. Mark a real-time function with the [[clang::nonblocking]] attribute:

void process(processing_data const & data) [[clang::nonblocking]]
{
    auto x = std::vector<float> (16); // oops!
}

2. Add -fsanitize=realtime to your compile and link flags (for CMake see below):

clang -fsanitize=realtime main.cpp

At run-time, real-time violations are presented with a stack trace:

> ./a.out
==1==ERROR: RealtimeSanitizer: unsafe-library-call
Intercepted call to real-time unsafe function `malloc` in real-time context!
    #0 0x591265778cb8 in malloc /root/llvm-project/compiler-rt/lib/rtsan/rtsan_interceptors_posix.cpp:444:3
    #1 0x73a58271117b in operator new(unsigned long) (/test/lib/libstdc++.so.6+0xc517b)
    #2 0x59126579f576 in std::__new_allocator<float>::allocate(unsigned long, void const*) /test/include/c++/15.0.0/bits/new_allocator.h:151:27
    #3 0x59126579f576 in std::allocator_traits<std::allocator<float>>::allocate(std::allocator<float>&, unsigned long) /test/include/c++/15.0.0/bits/alloc_traits.h:614:20
    #4 0x59126579f576 in std::_Vector_base<float, std::allocator<float>>::_M_allocate(unsigned long) /test/include/c++/15.0.0/bits/stl_vector.h:384:20
    #5 0x59126579f576 in std::_Vector_base<float, std::allocator<float>>::_M_create_storage(unsigned long) /test/include/c++/15.0.0/bits/stl_vector.h:402:33
    #6 0x59126579f576 in std::_Vector_base<float, std::allocator<float>>::_Vector_base(unsigned long, std::allocator<float> const&) /test/include/c++/15.0.0/bits/stl_vector.h:338:9
    #7 0x59126579f576 in std::vector<float, std::allocator<float>>::vector(unsigned long, std::allocator<float> const&) /test/include/c++/15.0.0/bits/stl_vector.h:584:9
    #8 0x59126579f4ed in process(processing_data const&) /app/example.cpp:10:14
    #9 0x59126579f535 in main /app/example.cpp:15:5
    #10 0x73a582229d8f  (/lib/x86_64-linux-gnu/libc.so.6+0x29d8f)
    #11 0x73a582229e3f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e3f)
    #12 0x591265777434 in _start (/app/output.s+0x6434)

Table Of Contents

1. Getting RTSan
   1. Docker
   2. Building LLVM 20 from source
   3. Windows
2. Documentation
3. CMake
4. How it works
5. Using RTSan Standalone (with compilers other than clang 20)
6. Contact

Getting RTSan

Until LLVM 20 is shipped with your OS, you'll need to download or build it yourself to use RealtimeSanitizer. The easiest way to experiment with RTSan is with our Docker image, for which more details can be found below.

Docker

The fastest way to try RealtimeSanitizer is to pull the pre-built docker image, which has clang (and other llvm tooling) with RTSan readily installed.

docker pull realtimesanitizer/rtsan-clang

You can quickly experiment in your own repository using a shared-volume:

docker run -v $(pwd):/my_repo -w /my_repo -it realtimesanitizer/rtsan-clang /bin/bash

which mounts the host's current working directory at path /my_repo in the container, and initialises the working directory to it. Alternatively, you may prefer to use the RTSan Docker image as a parent image for your own development or CI environment:

FROM realtimesanitizer/rtsan-clang:latest
RUN apt-get update && apt-get install -y git cmake vim

Building LLVM 20 from source

RTSan is now part of LLVM 20; please see the build instructions here for more information.

Windows

Apologies, RealtimeSanitizer does not yet support Windows. We very much welcome contributions, so please contact us if you're interested.

Documentation

A full list of features and instructions for configuring RealtimeSanitizer can be found in the official docs here.

We have given various talks about RTSan at software engineering conferences. Note that the older the talk is, the more likely it is to have stale info in it.

• CppCon 2024
• CppCast Podcast

CMake

LLVM 20 clang is installed to /usr/local in the RTSan Docker image, and CMake will automatically detect it. However, if you've built the latest LLVM from source, you'll need to instruct CMake to use the right version of clang by either i) setting the CC and CXX environment variables or ii) passing the CMAKE_C_COMPILER and CMAKE_CXX_COMPILER options to CMake as arguments.

Adding the compile and link flag -fsanitize=realtime can be done however works best for your project. One unintrusive option is to pass them as arguments to CMake:

cmake \
  -DCMAKE_CXX_FLAGS="-fsanitize=realtime" \
  -DCMAKE_C_FLAGS="-fsanitize=realtime" \
  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=realtime" ...

How it works

RTSan's algorithm consists of two parts that work together:

• A compilation step that
  • signals real-time context entry and exit for the duration of [[clang::nonblocking]] functions, and
• A lightweight runtime library that
  • keeps track of the real-time context,
  • detects calls to system library functions that are known to block, and
  • raises an error if a blocking library function is called in a real-time context.

Using RTSan Standalone (with compilers other than clang 20)

The recommended way to use RTSan is to use it with LLVM 20 directly, as described elsewhere in this document. The rest of this section describes a hack which may or may not continue to work in the future.

If you are in a position where you cannot use this compiler and instead rely on AppleClang or GCC, you can still use RTSan by directly linking in the runtime in directly.

First, build the RTSan runtime by following the instructions in the official docs.

From there, find the RTSan runtime library and link it to your binary. This differs based on system:

> cd $BUILD_DIR_MAC
> find . -name "*rtsan_osx*dylib"
./lib/clang/20/lib/darwin/libclang_rt.rtsan_osx_dynamic.dylib

...
> cat your_proj/CMakeLists.txt
...
target_link_libraries(helloWorld PRIVATE
  libclang_rt.rtsan_osx_dynamic.dylib
)

> cd $BUILD_DIR_LINUX
> find . -name "libclang_rt.rtsan.a"
./lib/clang/20/lib/aarch64-unknown-linux-gnu/libclang_rt.rtsan.a

...
> cat your_proj/CMakeLists.txt
...
target_link_libraries(helloWorld PRIVATE
  libclang_rt.rtsan.a
  pthread
  dl
)

In your code, you must #include "rtsan_standalone/rtsan_standalone.h", provided in this repo. Initialize RTSan, and put __rtsan::ScopedSanitizeRealtime() in places where you would normally use [[clang::nonblocking]] (in the top level of your real-time callback).

#include "rtsan_standalone/rtsan_standalone.h"

int main() {
    __rtsan::Initialize();
    ...
}

void my_real_time_function() {
    __rtsan::ScopedSanitizeRealtime ssr;
    ...
}

To indicate that a function is not real-time safe, add the __RTSAN_NOTIFY_BLOCKING_CALL() macro to the first line of that function. This is analogous to marking the function [[clang::blocking]]

void my_unsafe_spinlock() {
    __RTSAN_NOTIFY_BLOCKING_CALL();
    ...
}

To "enable" the sanitizer, you must compile your code defining __SANITIZE_REALTIME, e.g.

clang++ main.cpp -D__SANITIZE_REALTIME

Without this flag, each of these aforementioned constructs will compile to a no-op, and the sanitizer will be disabled.

This header also defines __rtsan::ScopedDisabler(), which allows for disabling the sanitizer in a specific scope. Please see the official docs for more information.

Contact

RealtimeSanitizer was invented by David Trevelyan and Ali Barker, and the upstream integration was authored by Chris Apple and David Trevelyan, who continue its maintenance. We welcome further contributions to make this tool more helpful to a wider group of developers. For all comments, suggestions and queries, please contact us by:

1. joining the Discord server,
2. sending an email to realtime.sanitizer@gmail.com, or
3. raising an issue in this GitHub repository.