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Case 214: Convention Macro Redefined After Use (Negative Control)

Field Value
Verdict ✅ NO_CHANGE
Category No Change
Classification Rule
Platforms Linux
Flags —
Detected ChangeKinds —
Source files catalog/cases/case214_calling_convention_macro_redefined_after_use/
Rule family calling-convention-macro-redefined-after-use
Subject Safe changes correctly not flagged

Category: Calling Convention | Verdict: ✅ NO_CHANGE

Verdict and consumer impact

Negative control for case212. vector_dot is declared through MATHLIB_CALL = __attribute__((ms_abi)) in both versions. After the declaration the header #undefs and redefines MATHLIB_CALL for later use, and only that later definition differs (sysv_abi in v1, ms_abi in v2). Both libraries are ms_abi, so an old consumer works against either.

A reader that uses the final macro table expands the later definition into the earlier declaration and reports a false calling_convention_changed; the 2026-10-11 re-audit saw exactly that on the CastXML lane.

Old/new diff

v1.h v2.h
#define MATHLIB_CALL __attribute__((ms_abi)) same
MATHLIB_CALL double vector_dot(...); same
#undef MATHLIB_CALL / #define MATHLIB_CALL __attribute__((sysv_abi)) #undef MATHLIB_CALL / #define MATHLIB_CALL __attribute__((ms_abi))

abicheck command

gcc -shared -fPIC -g v1.c -o libv1.so
gcc -shared -fPIC -g v2.c -o libv2.so
abicheck compare libv1.so libv2.so --header old=v1.h --header new=v2.h

Expected abicheck finding

Verdict: NO_CHANGE (exit 0)
  (no calling_convention_changed, no func_return_changed)

Same result with the CastXML and the Clang header backend (ABICHECK_AST_FRONTEND=castxml|clang).

Minimum evidence

L2 (headers): the case is only meaningful where conventions are read from headers; without them there is nothing to misread.

Why abicheck catches it

Each declaration is read from its own preprocessed line, so a definition that comes after it cannot change it, exactly as for the compiler.

Runtime failure demonstration

gcc app.c -L. -l:libv1.so -Wl,-rpath,. -lm -o app && ./app   # 32.0, exit 0
cp libv2.so libv1.so && ./app                                 # 32.0, exit 0

Safe redesign

Nothing to fix: this is a compatible change.

Source files

  • CMakeLists.txt
  • app.c
  • v1.c
  • v1.h
  • v2.c
  • v2.h

See also: Compatibility Catalog · All NO_CHANGE cases · Category: No Change · Rule: Calling convention macro redefined after use · Subject: Safe changes correctly not flagged.