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