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: Addition | Verdict: 🟢 COMPATIBLE

Verdict and consumer impact¶

v1 exports lib_version. v2 adds a new global variable lib_build_number, leaving lib_version and get_version() untouched. Existing binaries never reference the new symbol, so they are completely unaffected — new consumers can opt in to reading lib_build_number once they recompile against v2's header.

Old/new diff¶

old/lib.c	new/lib.c
`int lib_version = 1;`	`int lib_version = 1;` `int lib_build_number = 1042;`

abicheck command¶

gcc -shared -fPIC -g old/lib.c -o libfoo_v1.so
gcc -shared -fPIC -g new/lib.c -o libfoo_v2.so
abicheck compare libfoo_v1.so libfoo_v2.so

Expected abicheck finding¶

Verdict: COMPATIBLE (exit 0)

- var_added: New public variable: lib_build_number
  > New variable available; existing binaries are unaffected.

Minimum evidence¶

min_evidence: L0 — lib_build_number appearing in v2's .dynsym (and absent from v1's) is enough on its own; no debug info or headers needed. (-g above is only there so the Runtime failure demonstration below can build a matching app.)

Why abicheck catches it¶

The dynamic symbol table is authoritative L0 evidence — abicheck diffs the exported-symbol sets directly and classifies a name present only in the new set as an addition.

Runtime failure demonstration¶

No observable effect on existing binaries — this is a pure addition.

# Build old library + app
gcc -shared -fPIC -g old/lib.c -o libfoo.so
gcc -g app.c -L. -lfoo -Wl,-rpath,. -o app
./app
# → version = 1
# → get_version() = 1

# Swap in new library (no recompile)
gcc -shared -fPIC -g new/lib.c -o libfoo.so
./app
# → version = 1
# → get_version() = 1   ← identical

The app never references lib_build_number, so the swap is transparent.

Safe redesign¶

This case is the safe pattern — adding a new exported symbol without touching existing ones is the textbook compatible ABI change. No redesign needed; just avoid ever repurposing lib_build_number's name or removing it once shipped.

References¶

ELF dynamic symbol table

Source files¶

CMakeLists.txt
app.c