Case 130: Exceptions Mode Flip (-fno-exceptions)

Field	Value
Verdict	🟡 COMPATIBLE_WITH_RISK
Category	Risk
Platforms	Linux
Flags	—
Detected ChangeKinds	exceptions_mode_changed
Source files	examples/case130_exceptions_mode_flip/

Category: Build mode | Verdict: 🟡 COMPATIBLE_WITH_RISK

Same source, same exported symbols. The only difference is the build mode: v1 was built with C++ exceptions enabled (-fexceptions), v2 with -fno-exceptions. abicheck captures the per-side build context (L3) from a generated CMake compile_commands.json and reports exceptions_mode_changed.

What this demonstrates

The two modes are not link-compatible: an exception thrown in -fexceptions code that unwinds through a frame compiled -fno-exceptions is undefined behaviour, and -fno-exceptions changes the codegen / EH tables of every public inline that uses throw/try/catch. A symbol-only or even DWARF-only check sees nothing — the signal lives in the captured build flags.

Why COMPATIBLE_WITH_RISK

Per ADR-028 D3 a build-context finding never decides a shipped-ABI break on its own; the artifact diff proves any concrete break. This flags the elevated risk and localizes the cause for review.

How abicheck detects it

The CMake fixture builds v1 with -fexceptions and v2 with -fno-exceptions. The generated build-dir compile_commands.json carries those flags; the L3 build-evidence diff normalizes them to the canonical exceptions 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 case130_exceptions_mode_flip_v1 case130_exceptions_mode_flip_v2
abicheck dump /tmp/abicheck-examples-build/case130_exceptions_mode_flip/libv1.so --build-info /tmp/abicheck-examples-build/compile_commands.json -o v1.abi.json
abicheck dump /tmp/abicheck-examples-build/case130_exceptions_mode_flip/libv2.so --build-info /tmp/abicheck-examples-build/compile_commands.json -o v2.abi.json
abicheck compare v1.abi.json v2.abi.json   # → exceptions_mode_changed

How to fix

Ship one exception mode for the public ABI, or rebuild all consumers in the matching mode if the public API exposes exception types or throwing inlines.

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Source files

• CMakeLists.txt
• app.cpp
• v1.cpp
• v2.cpp

See also: Examples overview · All COMPATIBLE_WITH_RISK cases · Category: Risk.