Skip to content

Case 56: Struct Packing Changed (pragma pack)

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

Category: Type Layout / DWARF | Verdict: BREAKING

What this case is about

v1 defines Record with natural alignment (sizeof = 12, value at offset 4). v2 adds #pragma pack(1), eliminating all padding (sizeof = 6, value at offset 1).

This is a silent ABI break because every field (except tag) moves to a different offset, and the struct size shrinks.

What breaks at binary level

  • sizeof changes: 12 → 6. Stack allocations and arrays use wrong size.
  • Field offsets change: value moves from offset 4 to offset 1.
  • Alignment changes: value is no longer naturally aligned (may cause unaligned access faults on strict architectures like ARM).

What abicheck detects

  • STRUCT_PACKING_CHANGED: Detected via DWARF DW_AT_byte_size and field DW_AT_data_member_location changes. The packing attribute itself is recorded in DWARF as alignment metadata.

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 layout difference
pahole libbad.so  -C Record  # → size 12, value at offset 4
pahole libgood.so -C Record  # → size 6,  value at offset 1

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: field offsets changed

Real-world examples

  • Windows API headers use #pragma pack extensively. Mixing packed and unpacked struct definitions across DLL boundaries is a common source of crashes.
  • Network protocol libraries (e.g., pcap headers) often accidentally change packing when refactoring header includes.

References

Source files

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