Case 55: Type Kind Changed (struct → union)¶

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

Category: Type Layout | Verdict: BREAKING

What this case is about¶

v1 defines Data as a struct with two fields x and y laid out sequentially (sizeof = 8). v2 changes Data to a union where x and y overlap at offset 0 (sizeof = 4).

This is a fundamental ABI break: the memory layout, size, and semantics all change when a struct becomes a union.

What breaks at binary level¶

sizeof changes: sizeof(Data) shrinks from 8 to 4. Consumers that allocate Data on the stack or in arrays use the old size.
Field offsets change: y moves from offset 4 to offset 0 (overlapping x).
Semantic break: Writing to y no longer preserves x — they share storage.

What abicheck detects¶

TYPE_KIND_CHANGED: abicheck detects that Data changed from struct to union via DWARF DW_TAG_structure_type → DW_TAG_union_type.

Overall verdict: BREAKING

How to reproduce¶

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

# Verify size difference
pahole libbad.so  -C Data  # → struct, size 8
pahole libgood.so -C Data  # → union, size 4

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
# → BREAKING: TYPE_KIND_CHANGED

References¶

DWARF structure vs union tags