Dmitry Vyukov edited this page Apr 24, 2018 · 5 revisions


ThreadSanitizer (aka TSan) is a data race detector for C/C++. Data races are one of the most common and hardest to debug types of bugs in concurrent systems. A data race occurs when two threads access the same variable concurrently and at least one of the accesses is write. C++11 standard officially bans data races as undefined behavior.

Here is an example of a data race that can lead to crashes and memory corruptions:

#include <pthread.h>
#include <stdio.h>
#include <string>
#include <map>

typedef std::map<std::string, std::string> map_t;

void *threadfunc(void *p) {
  map_t& m = *(map_t*)p;
  m["foo"] = "bar";
  return 0;

int main() {
  map_t m;
  pthread_t t;
  pthread_create(&t, 0, threadfunc, &m);
  printf("foo=%s\n", m["foo"].c_str());
  pthread_join(t, 0);

There are a lot of various ways to trigger a data race in C++, see ThreadSanitizerPopularDataRaces, TSan detects all of them and more -- ThreadSanitizerDetectableBugs.

Obtaining ThreadSanitizer

ThreadSanitizer is part of clang 3.2 and gcc 4.8. To build the freshest version see ThreadSanitizerDevelopment page.

Supported Platforms

TSan is supported on Linux x86_64, tested on Ubuntu 12.04.


Simply compile your program with -fsanitize=thread and link it with -fsanitize=thread. To get a reasonable performance add -O2. Use -g to get file names and line numbers in the warning messages.

When you run the program, TSan will print a report if it finds a data race. Here is an example:

$ cat
#include <pthread.h>
#include <stdio.h>

int Global;

void *Thread1(void *x) {
  return NULL;

void *Thread2(void *x) {
  return NULL;

int main() {
  pthread_t t[2];
  pthread_create(&t[0], NULL, Thread1, NULL);
  pthread_create(&t[1], NULL, Thread2, NULL);
  pthread_join(t[0], NULL);
  pthread_join(t[1], NULL);
$ clang++ -fsanitize=thread -fPIE -pie -g
$ ./a.out 
WARNING: ThreadSanitizer: data race (pid=26327)
  Write of size 4 at 0x7f89554701d0 by thread T1:
    #0 Thread1(void*) (exe+0x000000006e66)

  Previous write of size 4 at 0x7f89554701d0 by thread T2:
    #0 Thread2(void*) (exe+0x000000006ed6)

  Thread T1 (tid=26328, running) created at:
    #0 pthread_create (exe+0x00000001108b)
    #1 main (exe+0x000000006f39)

  Thread T2 (tid=26329, running) created at:
    #0 pthread_create (exe+0x00000001108b)
    #1 main (exe+0x000000006f63)
ThreadSanitizer: reported 1 warnings

Refer to ThreadSanitizerReportFormat for explanation of reports format.

There is a bunch of runtime and compiler flags to tune behavior of TSan -- see ThreadSanitizerFlags.

Suppressing Reports

Sometimes you can't fix the race (e.g. in third-party code) or don't want to do it straight away. There are several options how you can suppress known reports:

  • Suppressions files (runtime mechanism).
  • Blacklist files (compile-time mechanism).
  • Exclude problematic code/test under TSan with #if defined(__has_feature) && __has_feature(thread_sanitizer).

How To Test

To start, run your tests using ThreadSanitizer. The race detector only finds races that happen at runtime, so it can't find races in code paths that are not executed. If your tests have incomplete coverage, you may find more races by running a binary built with -fsanitize=thread under a realistic workload.

Runtime Overhead

The cost of race detection varies by program, but for a typical program, memory usage may increase by 5-10x and execution time by 2-20x.

Non-instrumented code

ThreadSanitizer generally requires all code to be compiled with -fsanitize=thread. If some code (e.g. dynamic libraries) is not compiled with the flag, it can lead to false positive race reports, false negative race reports and/or missed stack frames in reports depending on the nature of non-instrumented code. To not produce false positive reports ThreadSanitizer has to see all synchronization in the program, some synchronization operations (namely, atomic operations and thread-safe static initialization) are intercepted during compilation (and can only be intercepted during compilation). ThreadSanitizer stack trace collection also relies on compiler instrumentation (unwinding stack on each memory access is too expensive).

There are some precedents of making ThreadSanitizer work with non-instrumented libraries. Success of this highly depends on what exactly these libraries are doing. Besides suppressions, one can use ignore_interceptors_accesses flag which ignores memory accesses in all interceptors, or ignore_noninstrumented_modules flag which makes ThreadSanitizer ignore all interceptors called from the given set of (non-instrumented) libraries.


  • Q: When I run the program, it says: FATAL: ThreadSanitizer can not mmap the shadow memory (something is mapped at 0x555555554000 < 0x7cf000000000). What to do? You need to enable ASLR:
$ echo 2 >/proc/sys/kernel/randomize_va_space

This may be fixed in future kernels, see

  • Q: When I run the program under gdb, it says: FATAL: ThreadSanitizer can not mmap the shadow memory (something is mapped at 0x555555554000 < 0x7cf000000000). What to do? Run as:
$ gdb -ex 'set disable-randomization off' --args ./a.out
  • Q: Does it actually use hundreds of GBs of memory (as I see in top)? No, it does not. TSan maps (but does not reserve) a lot of virtual address space. This means that tools like ulimit may not work as expected.

  • Q: I want to link libc/libstdc++ statically into my program. With ThreadSanitizer it produces either link-time or run-time errors. Libc/libstdc++ static linking is not supported.

  • Q: What synchronization primitives are supported? TSan supports pthread synchronization primitives, built-in compiler atomic operations (sync/atomic), C++ <atomic> operations are supported with llvm libc++ (not very throughly tested, though).

  • Q: I see what looks like a false report inside the libstdc++ (libc++) code. This may happen in c++11 mode. One such case is discussed here. Note that ThreadSanitizer is not yet heavily tested on code that uses c++11 threading. The solution is be to rebuild libstdc++ (libc++) with ThreadSanitizer (this might be tricky, and the process changes periodically; contact us for details). It is also possible that libstdc++ (libc++) has a bug, we've seen at least one such before.

  • Q: My code with C++ exceptions does not work with tsan. Tsan does not support C++ exceptions.


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