1 ThreadSanitizer 2 =============== 3 4 Introduction 5 ------------ 6 7 ThreadSanitizer is a tool that detects data races. It consists of a compiler 8 instrumentation module and a run-time library. Typical slowdown introduced by 9 ThreadSanitizer is about **5x-15x**. Typical memory overhead introduced by 10 ThreadSanitizer is about **5x-10x**. 11 12 How to build 13 ------------ 14 15 Follow the `Clang build instructions <../get_started.html>`_. CMake build is 16 supported. 17 18 Supported Platforms 19 ------------------- 20 21 ThreadSanitizer is supported on Linux x86_64 (tested on Ubuntu 12.04). 22 Support for other 64-bit architectures is possible, contributions are welcome. 23 Support for 32-bit platforms is problematic and is not planned. 24 25 Usage 26 ----- 27 28 Simply compile and link your program with ``-fsanitize=thread``. To get a 29 reasonable performance add ``-O1`` or higher. Use ``-g`` to get file names 30 and line numbers in the warning messages. 31 32 Example: 33 34 .. code-block:: c++ 35 36 % cat projects/compiler-rt/lib/tsan/lit_tests/tiny_race.c 37 #include <pthread.h> 38 int Global; 39 void *Thread1(void *x) { 40 Global = 42; 41 return x; 42 } 43 int main() { 44 pthread_t t; 45 pthread_create(&t, NULL, Thread1, NULL); 46 Global = 43; 47 pthread_join(t, NULL); 48 return Global; 49 } 50 51 $ clang -fsanitize=thread -g -O1 tiny_race.c 52 53 If a bug is detected, the program will print an error message to stderr. 54 Currently, ThreadSanitizer symbolizes its output using an external 55 ``addr2line`` process (this will be fixed in future). 56 57 .. code-block:: bash 58 59 % ./a.out 60 WARNING: ThreadSanitizer: data race (pid=19219) 61 Write of size 4 at 0x7fcf47b21bc0 by thread T1: 62 #0 Thread1 tiny_race.c:4 (exe+0x00000000a360) 63 64 Previous write of size 4 at 0x7fcf47b21bc0 by main thread: 65 #0 main tiny_race.c:10 (exe+0x00000000a3b4) 66 67 Thread T1 (running) created at: 68 #0 pthread_create tsan_interceptors.cc:705 (exe+0x00000000c790) 69 #1 main tiny_race.c:9 (exe+0x00000000a3a4) 70 71 ``__has_feature(thread_sanitizer)`` 72 ------------------------------------ 73 74 In some cases one may need to execute different code depending on whether 75 ThreadSanitizer is enabled. 76 :ref:`\_\_has\_feature <langext-__has_feature-__has_extension>` can be used for 77 this purpose. 78 79 .. code-block:: c 80 81 #if defined(__has_feature) 82 # if __has_feature(thread_sanitizer) 83 // code that builds only under ThreadSanitizer 84 # endif 85 #endif 86 87 ``__attribute__((no_sanitize_thread))`` 88 ----------------------------------------------- 89 90 Some code should not be instrumented by ThreadSanitizer. 91 One may use the function attribute 92 :ref:`no_sanitize_thread <langext-thread_sanitizer>` 93 to disable instrumentation of plain (non-atomic) loads/stores in a particular function. 94 ThreadSanitizer still instruments such functions to avoid false positives and 95 provide meaningful stack traces. 96 This attribute may not be 97 supported by other compilers, so we suggest to use it together with 98 ``__has_feature(thread_sanitizer)``. 99 100 Blacklist 101 --------- 102 103 ThreadSanitizer supports ``src`` and ``fun`` entity types in 104 :doc:`SanitizerSpecialCaseList`, that can be used to suppress data race reports in 105 the specified source files or functions. Unlike functions marked with 106 :ref:`no_sanitize_thread <langext-thread_sanitizer>` attribute, 107 blacklisted functions are not instrumented at all. This can lead to false positives 108 due to missed synchronization via atomic operations and missed stack frames in reports. 109 110 Limitations 111 ----------- 112 113 * ThreadSanitizer uses more real memory than a native run. At the default 114 settings the memory overhead is 5x plus 1Mb per each thread. Settings with 3x 115 (less accurate analysis) and 9x (more accurate analysis) overhead are also 116 available. 117 * ThreadSanitizer maps (but does not reserve) a lot of virtual address space. 118 This means that tools like ``ulimit`` may not work as usually expected. 119 * Libc/libstdc++ static linking is not supported. 120 * Non-position-independent executables are not supported. Therefore, the 121 ``fsanitize=thread`` flag will cause Clang to act as though the ``-fPIE`` 122 flag had been supplied if compiling without ``-fPIC``, and as though the 123 ``-pie`` flag had been supplied if linking an executable. 124 125 Current Status 126 -------------- 127 128 ThreadSanitizer is in beta stage. It is known to work on large C++ programs 129 using pthreads, but we do not promise anything (yet). C++11 threading is 130 supported with llvm libc++. The test suite is integrated into CMake build 131 and can be run with ``make check-tsan`` command. 132 133 We are actively working on enhancing the tool --- stay tuned. Any help, 134 especially in the form of minimized standalone tests is more than welcome. 135 136 More Information 137 ---------------- 138 `http://code.google.com/p/thread-sanitizer <http://code.google.com/p/thread-sanitizer/>`_. 139 140
