Case 116: _Atomic qualifier added (C11)¶

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Platforms	Linux
Flags	ABI break
Detected `ChangeKind`s	`atomic_qualifier_changed`
Source files	`examples/case116_atomic_qualifier_changed/`

Category: Binary ABI break / C11 | Verdict: 🔴 BREAKING

What changed¶

v1 exposes a struct counter whose value field is a plain (unqualified) int. v2 adds the C11 _Atomic qualifier to that field. The accessor get_count() returns a plain long in both versions — only the field changed.

Per WG14, the size and alignment of an _Atomic-qualified type may differ from the unqualified type and varies across implementations (some lock-free types carry extra padding/alignment). So the struct counter layout can diverge: a consumer built against v1 reads the field at the wrong offset/width against v2.

How abicheck catches it¶

atomic_qualifier_changed fires for each public slot (parameter, return, or field) where the _Atomic qualifier is added or removed. Layout-level findings (type_size_changed / struct_field_*) may also appear; the specialised kind names the _Atomic root cause.

Files¶

v1.h / v2.h — plain vs _Atomic declarations
v1.c / v2.c — the two library builds
app.c — consumer built against the plain interface

Source files¶

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