Case 02: Parameter Type Change¶

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Platforms	Linux
Flags	ABI break, API break
Detected `ChangeKind`s	`func_params_changed`
Source files	browse on GitHub

Category: Symbol API | Verdict: 🟡 ABI CHANGE (exit 4)

Note on abidiff 2.4.0: libabigail classifies parameter type changes as "indirect sub-type changes" with exit code 4 (ABI change detected, not flagged as symbol-removal breaking). The change is still real ABI drift and should be treated as BREAKING by policy.

What breaks¶

Callers compiled against v1 pass int a in a 32-bit register; v2 expects double a in an FP register (x86-64 SysV ABI). The argument is misread, producing wrong results or a crash. Re-compilation against v2 is mandatory.

Why abidiff catches it¶

Reports parameter 1 of type 'int' changed: type name changed from 'int' to 'double' with exit 4.

Code diff¶

v1.c	v2.c
`double process(int a, int b)`	`double process(double a, int b)`

Real Failure Demo¶

Severity: CRITICAL

Scenario: compile app against v1, swap in v2 .so without recompile.

# Build old library + app
gcc -shared -fPIC -g v1.c -o libfoo.so
gcc -g app.c -L. -lfoo -Wl,-rpath,. -o app
./app
# → process(3, 4) = 7.0
# → Expected: 7.0

# Swap in new library (no recompile)
gcc -shared -fPIC -g v2.c -o libfoo.so
./app
# → process(3, 4) = <garbage> (3 misread as double, wrong FP register)

Why CRITICAL: v2 expects double a in an FP register while the app passes an int in an integer register; the argument is silently misinterpreted, producing wrong output with no error or crash.

Reproduce manually¶

gcc -shared -fPIC -g v1.c -o libfoo_v1.so
gcc -shared -fPIC -g v2.c -o libfoo_v2.so
abidw --out-file v1.xml libfoo_v1.so
abidw --out-file v2.xml libfoo_v2.so
abidiff v1.xml v2.xml
echo "exit: $?"   # → 4

How to fix¶

Introduce a new function with the desired signature alongside the old one. Keep the old symbol with its original signature for at least one deprecation cycle.

Real-world example¶

Common in numerical libraries (BLAS, LAPACK) when precision is upgraded — float → double parameter changes require wrapper shims for backward compatibility.