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Case 57: Enum Underlying Size Changed

Field Value
Verdict 🔴 BREAKING
Category Breaking
Platforms Linux, macOS
Flags ABI break, API break
Detected ChangeKinds enum_underlying_size_changed
Source files browse on GitHub

Category: Type Layout | Verdict: BREAKING

What this case is about

v1 defines Color enum with values 0-2 (fits in int, 4 bytes on LP64). v2 adds a sentinel value 0x100000000LL that exceeds INT_MAX, forcing the compiler to use a 64-bit underlying type.

This changes sizeof(Color) from 4 to 8, which breaks any struct containing the enum.

What breaks at binary level

  • Enum size doubles: 4 → 8 bytes. All structs containing Color change layout.
  • Struct Pixel grows: alpha moves from offset 4 to offset 8.
  • Function ABI changes: Color is now passed in a 64-bit register/slot.
  • Arrays break: Color arr[N] has different stride.

What abicheck detects

  • ENUM_UNDERLYING_SIZE_CHANGED: Detected via DWARF DW_AT_byte_size on the enumeration type.
  • TYPE_SIZE_CHANGED: Structs containing the enum also change size.

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

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: ENUM_UNDERLYING_SIZE_CHANGED

Real-world examples

  • Adding a large sentinel to a public enum is a common mistake in C libraries.
  • C++11's enum class Color : uint64_t makes the underlying type explicit, but changing it between releases still breaks ABI.

References

Source files

See also: Examples overview · All BREAKING cases · Category: Breaking.