ThreadSanitizer — Clang 23.1.0 documentation

# Clang 23.1.0 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")`.

## Interaction of Inlining with Disabling Sanitizer Instrumentation

- A `no_sanitize` function will not be inlined heuristically by the compiler into a sanitized function.

- An `always_inline` function will adopt the instrumentation status of the function it is inlined into.

- Forcibly combining `no_sanitize` and `__attribute__((always_inline))` is not supported, and will often lead to unexpected results. To avoid mixing these attributes, use:

    // Note, __has_feature test for sanitizers is deprecated, and Clang will support __SANITIZE_<sanitizer>__ similar to GCC.
    #if __has_feature(thread_sanitizer) || defined(__SANITIZE_THREAD__) || ... <other sanitizers>
    #define ALWAYS_INLINE_IF_UNINSTRUMENTED
    #else
    #define ALWAYS_INLINE_IF_UNINSTRUMENTED __attribute__((always_inline))
    #endif

## Explicit Sanitizer Checks with `__builtin_allow_sanitize_check`

The `__builtin_allow_sanitize_check("thread")` builtin can be used to conditionally execute code depending on whether ThreadSanitizer checks are enabled and permitted by the current policy (after inlining). This is particularly useful for inserting explicit, sanitizer-specific checks around operations like syscalls or inline assembly, which might otherwise be unchecked by the sanitizer.

Example:

    void __tsan_read8(void *);

    inline __attribute__((always_inline))
    void my_helper(void *addr) {
      if (__builtin_allow_sanitize_check("thread"))
        __tsan_read8(addr);
      // ... actual logic, e.g. inline assembly ...
      asm volatile ("..." : : "r" (addr) : "memory");
    }

    void instrumented_function() {
      ...
      my_helper(&shared_data); // checks are active
      ...
    }

    __attribute__((no_sanitize("thread")))
    void uninstrumented_function() {
      ...
      my_helper(&shared_data); // checks are skipped
      ...
    }

## 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.

Example:

    # Ignore exactly this function (the names are mangled in C++)
    fun:_Z8MyFooBarv
    # Ignore all functions containing MyFooBar
    fun:*MyFooBar*
    # Ignore the whole file
    src:file_with_tricky_code.cc

## 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.

- ThreadSanitizer generally requires all code to be compiled with `-fsanitize=thread`. If some code (such as pre-compiled dynamic libraries) is not compiled with the flag, TSan may fail to detect races or may report false positives. Refer to the `ignore_interceptors_accesses` and `ignore_noninstrumented_modules` run-time options to work around issues arising from non-instrumented modules.

- On Linux, disabling ASLR may cause ThreadSanitizer to fail to allocate shadow memory, printing the error `FATAL: ThreadSanitizer can not mmap the shadow memory (something is mapped at ...)`. Note that GDB disables ASLR by default. To debug ThreadSanitizer binaries under GDB, configure it to preserve ASLR by running:

      $ gdb -ex 'set disable-randomization off' --args ./a.out

## 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.

## Run-time Flags

ThreadSanitizer supports a number of run-time flags, which can be specified in the `TSAN_OPTIONS` environment variable. Separate flags are separated with spaces.

Example:

    $ TSAN_OPTIONS="history_size=7 halt_on_error=1" ./myprogram

The most common run-time flags are:

- `detect_deadlocks` (default: `true`): Controls whether deadlock detection is enabled.

- `exitcode` (default: `66`): Override the exit status if an error is found. Note that for ThreadSanitizer the default exitcode is 66, unlike other sanitizers.

- `force_seq_cst_atomics` (default: `false`): If set, all atomic operations are treated as sequentially consistent (seq_cst). Useful for debugging relaxed atomic ordering bugs.

- `halt_on_error` (default: `false`): Exit after the first reported error.

- `history_size` (default: `0`): Controls per-thread history size, which determines how many previous memory accesses are remembered per thread. If you see “failed to restore the stack” reports, try increasing this flag (values from 0 to 7 are supported).

- `ignore_interceptors_accesses` (default: `false` on Linux/Windows, `true` on Apple platforms): Ignore reads and writes from all interceptors.

- `ignore_noninstrumented_modules` (default: `false` on Linux/Windows, `true` on Apple platforms): Ignore interceptors called from non-instrumented modules.

- `log_path` (default: `""`): If set, write logs to `log_path.pid` instead of stderr. Special values are `stdout` and `stderr`.

- `print_full_thread_history` (default: `false`): Print creation stack traces for all threads involved in the report and their ancestor threads back to the main thread.

- `report_atomic_races` (default: `true`): Report races between atomic and plain memory accesses.

- `suppressions` (default: `""`): Path to a suppressions file.

To see the complete list of flags and their descriptions, run an instrumented binary with the `help=1` option:

    $ TSAN_OPTIONS="help=1" ./myprogram

You can also refer to the source declarations in the LLVM repository under `compiler-rt/lib/tsan/rtl/tsan_flags.inc` (ThreadSanitizer-specific flags) and `compiler-rt/lib/sanitizer_common/sanitizer_flags.inc` (common sanitizer flags).

## Suppressions

If you have a data race or thread leak that you are already aware of but cannot fix right away (e.g., in a third-party library), you can suppress the reports at run-time using a suppressions file.

Specify the suppressions file path via the `suppressions` flag in the `TSAN_OPTIONS` environment variable:

    $ TSAN_OPTIONS="suppressions=/path/to/suppressions.supp" ./myprogram

Each non-empty line of the suppressions file represents one suppression of the form:

    suppression_type:suppression_pattern

The supported `suppression_type` values are:

- `race`: Suppresses data race reports. The pattern is matched against function names, source file names, or global variable names in the stacks of the report.

- `thread`: Suppresses thread leak reports. The pattern is matched against the name of the leaked thread.

- `called_from_lib`: Suppresses reports if the call originated from a specific non-instrumented library.

The pattern can contain the `*` wildcard, which matches any substring. By default, `*` is automatically prepended and appended to each pattern. You can use `^` and `$` to anchor patterns to the beginning and end of the string. Lines starting with `#` are treated as comments.

Example of a suppressions file:

    # Suppress data races in a third-party library 'foobar'
    race:foobar
    # Suppress data races in a specific function
    race:NuclearRocket::Launch
    # Suppress data races in a specific source file
    race:src/surgery/laser_scalpel.cc
    # Suppress data races on a specific global variable
    race:global_var
    # Suppress a leaked thread by name
    thread:MonitoringThread
    # Suppress warnings called from an uninstrumented library
    called_from_lib:libzmq.so

## Adaptive Delay

### Overview

Adaptive Delay is an optional ThreadSanitizer feature that injects delays at synchronization points to explore novel thread interleavings and increase the likelihood of exposing data races. By perturbing thread scheduling, adaptive delay creates more opportunities for concurrent accesses to shared data, improving race detection.

Adaptive delay is particularly useful for:

- Detecting races in rarely-executed thread interleavings or code paths

- Testing parallel data structures and algorithms

When enabled, adaptive delay maintains a configurable time budget to balance race exposure against performance overhead. The delays can be

> - random amount of spin cycles

> - a single yield to the OS scheduler

> - random usleep

The strategy prioritizes high-value synchronization points:

- Relaxed atomic operations receive cheap delays (spin cycles) with low sampling

- Synchronizing atomic operations (acquire/release/seq_cst) receive moderate delays with higher sampling

- Mutex and thread lifecycle operations receive the longest delays with highest sampling

The delays focus on synchronization points with clear happens-before relationships, as those are most likely to expose data races.

### Enabling Adaptive Delay

Adaptive delay is disabled by default. Enable it by setting the `enable_adaptive_delay` flag:

    $ TSAN_OPTIONS=enable_adaptive_delay=1 ./myapp

### Configuration Options

Adaptive Delay Options
Flag | Type | Default | Description
--- | --- | --- | ---
`enable_adaptive_delay` | bool | false | Enable adaptive delay injection to expose data races.
`adaptive_delay_aggressiveness` | int | 25 | Controls delay injection intensity for race detection. Higher values inject more delays to expose races. Value must be greater than 0. Suggested values: 10 (minimal), 50 (moderate), 200 (aggressive). This is a tuning parameter; actual overhead varies by workload and platform.
`adaptive_delay_relaxed_sample_rate` | int | 10000 | Sample 1 in N relaxed atomic operations for delay injection. Relaxed atomics have minimal synchronization, so sampling helps avoid excessive overhead.
`adaptive_delay_sync_atomic_sample_rate` | int | 100 | Sample 1 in N acquire/release/seq_cst atomic operations for delay injection. These synchronizing atomics are more likely to expose races, so are sampled more often.
`adaptive_delay_mutex_sample_rate` | int | 10 | Sample 1 in N mutex/condition variable operations for delay injection. Mutex ops are high-value synchronization points and are sampled frequently.
`adaptive_delay_max_atomic` | string | `"sleep_us=50"` | Maximum delay for atomic operations. Format: `"spin=N"` (N spin cycles, 1 <= N <= 10,000), `"yield"` (one yield to the OS), or `"sleep_us=N"` (up to N microseconds). The delay is randomly chosen up to the specified maximum N.
`adaptive_delay_max_sync` | string | `"sleep_us=500"` | Maximum delay for synchronization operations (mutex and thread lifecycle operations). Format: same as `adaptive_delay_max_atomic`. Typically set longer than atomic delays since these operations involve waking blocked threads and may be more likely to expose races.

### Examples

Enable adaptive delay with moderate aggressiveness:

    $ TSAN_OPTIONS=enable_adaptive_delay=1:adaptive_delay_aggressiveness=50 ./myapp

Enable aggressive delay injection:

    $ TSAN_OPTIONS=enable_adaptive_delay=1:adaptive_delay_aggressiveness=200 ./myapp

Increase sampling frequency for mutex operations:

    $ TSAN_OPTIONS=enable_adaptive_delay=1:adaptive_delay_mutex_sample_rate=5 ./myapp