Case 61: Global Variable Added¶

Field	Value
Verdict	🟢 COMPATIBLE
Category	Addition (Compatible)
Platforms	Linux, macOS
Flags	—
Detected `ChangeKind`s	`var_added`
Source files	`examples/case61_var_added/`

Category: Symbol API | Verdict: COMPATIBLE

What this case is about¶

v1 exports lib_version. v2 adds a new global variable lib_build_number. All existing symbols are unchanged — this is a purely additive change.

Real Failure Demo¶

Severity: COMPATIBLE - NO FAILURE EXPECTED

This is an additive ABI change. The old app does not reference the new global, so runtime substitution is safe.

cmake -S examples -B /tmp/abicheck-examples-build -DCMAKE_BUILD_TYPE=Debug
cmake --build /tmp/abicheck-examples-build --target case62_var_added_app case62_var_added_v2

tmp=$(mktemp -d)
cp /tmp/abicheck-examples-build/case62_var_added/app_v1 "$tmp/"
cp /tmp/abicheck-examples-build/case62_var_added/libv2.so "$tmp/libv1.so"
(cd "$tmp" && LD_LIBRARY_PATH=. ./app_v1)
# version = 2 / get_version() = 2

Why this is compatible¶

Existing binaries never reference lib_build_number, so it doesn't affect them.
New consumers can optionally use the new variable.
No layout, offset, or size changes.

What abicheck detects¶

VAR_ADDED: A new global variable symbol appeared in .dynsym.

Overall verdict: COMPATIBLE

How to reproduce¶

gcc -shared -fPIC -g bad.c  -o libbad.so
gcc -shared -fPIC -g good.c -o libgood.so

nm -D libgood.so | grep lib_build_number  # → D lib_build_number

python3 -m abicheck.cli dump libbad.so  -o /tmp/v1.json
python3 -m abicheck.cli dump libgood.so -o /tmp/v2.json
python3 -m abicheck.cli compare /tmp/v1.json /tmp/v2.json
# → COMPATIBLE: VAR_ADDED

References¶

ELF dynamic symbol table

Source files¶

CMakeLists.txt
app.c
bad.c
good.c