# ThreadSanitizer

- Introduction
- How to Build
- Supported Platforms
- Usage
- Limitations
- Current Status
- More Information

## 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 5x-15x (TODO: these numbers are approximate so far).

## How to build

Follow the clang build instructions.
Note: CMake build does not work yet. See bug 12272.

## Supported Platforms

ThreadSanitizer is supported on Linux x86_64 (tested on Ubuntu 10.04).
Support for MacOS 10.7 (64-bit only) is planned for late 2012.
Support for 32-bit platforms is problematic and not yet planned.

## Usage

Simply compile your program with `-fthread-sanitizer -fPIE` and link it with `-fthread-sanitizer -pie`.
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/output_tests/tiny_race.c
    #include
    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 -fthread-sanitizer -g -O1 tiny_race.c -fPIE -pie

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).

    % TSAN_OPTIONS=strip_path_prefix=`pwd`/  # Don't print full paths.
    % ./a.out 2> log
    % cat log
    WARNING: ThreadSanitizer: data race (pid=19219)
      Write of size 4 at 0x7fcf47b21bc0 by thread 1:
        #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 1 (running) created at:
        #0 pthread_create ??:0 (exe+0x00000000c790)
        #1 main tiny_race.c:9 (exe+0x00000000a3a4)

## Limitations

- ThreadSanitizer uses more real memory than a native run. At the default settings the memory overhead is 9x plus 9Mb per each thread. Settings with 5x and 3x overhead (but less accurate analysis) 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.
- Static linking is not supported.
- ThreadSanitizer requires `-fPIE -pie`

## Current Status

ThreadSanitizer is in alpha stage. It is known to work on large C++ programs using pthreads, but we do not promise anything (yet).
C++11 threading is not yet supported. 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

http://code.google.com/p/thread-sanitizer.