Case 132: Thread-Safe Statics Mode Flip (-fno-threadsafe-statics)¶
| Field | Value |
|---|---|
| Verdict | 🟡 COMPATIBLE_WITH_RISK |
| Category | Risk |
| Platforms | Linux |
| Flags | — |
Detected ChangeKinds |
threadsafe_statics_mode_changed |
| Source files | examples/case132_threadsafe_statics_flip/ |
Category: Build mode | Verdict: 🟡 COMPATIBLE_WITH_RISK
Same source, same symbols; v1 built with thread-safe local-static initialization (
-fthreadsafe-statics, the default), v2 with-fno-threadsafe-statics. The generated CMake L3 build context reveals the flip →threadsafe_statics_mode_changed.
What this demonstrates¶
-fno-threadsafe-statics omits the __cxa_guard acquire/release around a
function-local static's first-use initialization. A public inline holding a
function-local static, compiled in different modes across translation units, has
mismatched guard expectations — a data race or double-init on concurrent first use.
Why COMPATIBLE_WITH_RISK¶
A build-mode signal, not a proven binary break (ADR-028 D3); the artifact diff proves any concrete break, this localizes the risk.
How abicheck detects it¶
The CMake fixture builds v1 with -fthreadsafe-statics and v2 with
-fno-threadsafe-statics; the generated build-dir compile_commands.json
carries those flags. The L3 diff normalizes to the canonical
threadsafe_statics option and reports the flip.
Reproduce manually¶
cmake -S examples -B /tmp/abicheck-examples-build -DCMAKE_BUILD_TYPE=Release -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
cmake --build /tmp/abicheck-examples-build --target case132_threadsafe_statics_flip_v1 case132_threadsafe_statics_flip_v2
abicheck dump /tmp/abicheck-examples-build/case132_threadsafe_statics_flip/libv1.so --build-info /tmp/abicheck-examples-build/compile_commands.json -o v1.abi.json
abicheck dump /tmp/abicheck-examples-build/case132_threadsafe_statics_flip/libv2.so --build-info /tmp/abicheck-examples-build/compile_commands.json -o v2.abi.json
abicheck compare v1.abi.json v2.abi.json # → threadsafe_statics_mode_changed
How to fix¶
Keep thread-safe statics enabled for any public inline holding a function-local static, or guarantee consumers never rely on cross-TU first-use ordering.
Source files¶
CMakeLists.txtapp.cppv1.cppv2.cpp
See also: Examples overview · All COMPATIBLE_WITH_RISK cases · Category: Risk.