# Clang 15 documentation

## ThreadSanitizer

## Introduction

ThreadSanitizer is a tool that detects data races. It consists of a compiler instrumentation module and a run-time library. Typical slowdown introduced by ThreadSanitizer is about 5x-15x. Typical memory overhead introduced by ThreadSanitizer is about 5x-10x.

## How to build

Build LLVM/Clang with CMake.

## Supported Platforms

ThreadSanitizer is supported on the following OS:

- Android aarch64, x86_64

- Darwin arm64, x86_64

- FreeBSD

- Linux aarch64, x86_64, powerpc64, powerpc64le

- NetBSD

Support for other 64-bit architectures is possible, contributions are welcome. Support for 32-bit platforms is problematic and is not planned.

## Usage

Simply compile and link your program with `-fsanitize=thread`. To get a reasonable performance add `-O1` or higher. Use `-g` to get file names and line numbers in the warning messages.

Example:

    % cat projects/compiler-rt/lib/tsan/lit_tests/tiny_race.c
    #include <pthread.h>
    int Global;
    void *Thread1(void *x) {
      Global = 42;
      return x;
    }
    int main() {
      pthread_t t;
      pthread_create(&t, NULL, Thread1, NULL);
      Global = 43;
      pthread_join(t, NULL);
      return Global;
    }

    $ clang -fsanitize=thread -g -O1 tiny_race.c

If a bug is detected, the program will print an error message to stderr. Currently, ThreadSanitizer symbolizes its output using an external `addr2line` process (this will be fixed in future).

    % ./a.out
    WARNING: ThreadSanitizer: data race (pid=19219)
      Write of size 4 at 0x7fcf47b21bc0 by thread T1:
        #0 Thread1 tiny_race.c:4 (exe+0x00000000a360)

      Previous write of size 4 at 0x7fcf47b21bc0 by main thread:
        #0 main tiny_race.c:10 (exe+0x00000000a3b4)

      Thread T1 (running) created at:
        #0 pthread_create tsan_interceptors.cc:705 (exe+0x00000000c790)
        #1 main tiny_race.c:9 (exe+0x00000000a3a4)

## `__has_feature(thread_sanitizer)`

In some cases one may need to execute different code depending on whether ThreadSanitizer is enabled. __has_feature can be used for this purpose.

    #if defined(__has_feature)
    #  if __has_feature(thread_sanitizer)
    // code that builds only under ThreadSanitizer
    #  endif
    #endif

## `__attribute__((no_sanitize("thread")))`

Some code should not be instrumented by ThreadSanitizer. One may use the function attribute `no_sanitize("thread")` to disable instrumentation of plain (non-atomic) loads/stores in a particular function. ThreadSanitizer still instruments such functions to avoid false positives and provide meaningful stack traces. This attribute may not be supported by other compilers, so we suggest to use it together with `__has_feature(thread_sanitizer)`.

## `__attribute__((disable_sanitizer_instrumentation))`

The `disable_sanitizer_instrumentation` attribute can be applied to functions to prevent all kinds of instrumentation. As a result, it may introduce false positives and incorrect stack traces. Therefore, it should be used with care, and only if absolutely required; for example for certain code that cannot tolerate any instrumentation and resulting side-effects. This attribute overrides `no_sanitize("thread")`.

## Ignorelist

ThreadSanitizer supports `src` and `fun` entity types in Sanitizer special case list, that can be used to suppress data race reports in the specified source files or functions. Unlike functions marked with `no_sanitize("thread")` attribute, ignored functions are not instrumented at all. This can lead to false positives due to missed synchronization via atomic operations and missed stack frames in reports.

## Limitations

- ThreadSanitizer uses more real memory than a native run. At the default settings the memory overhead is 5x plus 1Mb per each thread. Settings with 3x (less accurate analysis) and 9x (more accurate analysis) overhead are also available.

- ThreadSanitizer maps (but does not reserve) a lot of virtual address space. This means that tools like `ulimit` may not work as usually expected.

- Libc/libstdc++ static linking is not supported.

- Non-position-independent executables are not supported. Therefore, the `fsanitize=thread` flag will cause Clang to act as though the `-fPIE` flag had been supplied if compiling without `-fPIC`, and as though the `-pie` flag had been supplied if linking an executable.

## Security Considerations

ThreadSanitizer is a bug detection tool and its runtime is not meant to be linked against production executables. While it may be useful for testing, ThreadSanitizer’s runtime was not developed with security-sensitive constraints in mind and may compromise the security of the resulting executable.

## Current Status

ThreadSanitizer is in beta stage. It is known to work on large C++ programs using pthreads, but we do not promise anything (yet). C++11 threading is supported with llvm libc++. The test suite is integrated into CMake build and can be run with `make check-tsan` command.

We are actively working on enhancing the tool — stay tuned. Any help, especially in the form of minimized standalone tests is more than welcome.

## More Information

https://github.com/google/sanitizers/wiki/ThreadSanitizerCppManual