Case 73: Typedef Underlying Type Changed

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Classification	Rule
Platforms	Linux, macOS, Windows
Flags	ABI break, API break
Detected ChangeKinds	typedef_base_changed
Source files	catalog/cases/case73_typedef_underlying_changed/
Rule family	typedef-underlying-changed
Subject	Opaque types, typedefs, and contract identifiers

Category: Type ABI | Verdict: 🔴 BREAKING

Verdict and consumer impact

The typedef handle_t changes its underlying type from int (4 bytes) to void* (8 bytes on x86-64). Every function using handle_t — parameters and the return value — inherits the break from this single typedef change: handle_open()'s return value grows from 4 to 8 bytes, and the h parameters of handle_read()/handle_close() do too. A binary compiled against v1 truncates the returned pointer to 32 bits and passes that truncated value back into the library, which dereferences a corrupted pointer. Recompilation is mandatory.

Old/new diff

v1.h	v2.h
typedef int handle_t;	typedef void *handle_t;

abicheck command

gcc -shared -fPIC -g v1.c -o libfoo_v1.so
gcc -shared -fPIC -g v2.c -o libfoo_v2.so
abicheck compare libfoo_v1.so libfoo_v2.so

Expected abicheck finding

Verdict: BREAKING (exit 4)

- typedef_base_changed: Typedef base type changed: handle_t (int -> void *)
  > Underlying type changed; old code using the typedef operates on wrong
    representation.
  2 derived change(s) collapsed
  Affected symbols: handle_close, handle_open, handle_read
- param_pointer_level_changed: Parameter pointer level changed: handle_close
  param h (depth 0 -> 1)
- param_pointer_level_changed: Parameter pointer level changed: handle_read
  param h (depth 0 -> 1)
- return_pointer_level_changed: Return pointer level changed: handle_open
  (depth 0 -> 1)

Deployment risk:
- imported_symbol_added: New imported symbol: free@GLIBC_2.2.5, malloc@GLIBC_2.2.5

Minimum evidence

min_evidence: L1 — DWARF's typedef DIE for handle_t records its underlying type for both versions; abicheck compares that plus every using-function's parameter/return DWARF types directly from debug info (-g), no public headers required.

Why abicheck catches it

abicheck resolves handle_t's DWARF typedef target (int vs void *) and reports the root typedef_base_changed finding, then collapses the per-function pointer-level changes it implies (handle_open's return, handle_read/handle_close's h parameter) under it — the underlying type comparison needs only debug info, no header parsing.

Runtime failure demonstration

Severity: CRITICAL

Scenario: compile app against v1, swap in v2 .so without recompile.

# Build v1 and app
gcc -shared -fPIC -g v1.c -o libhandle.so
gcc -g app.c -L. -lhandle -Wl,-rpath,. -o app
./app
# → handle = 1
# → read 4 bytes
# → Done.

# Swap in v2 (handle_t is now void*, no recompile)
gcc -shared -fPIC -g v2.c -o libhandle.so
./app
# → Segmentation fault (core dumped)

Why CRITICAL: the app was compiled to treat handle_t as a 4-byte int, so it truncates v2's 8-byte pointer return value. Passing that truncated value into handle_read()/handle_close() — now void* parameters — makes the library dereference or free() a corrupted pointer, producing the segfault observed above.

Safe redesign

Design handles as opaque pointers from the start, or use a fixed-width integer that is already the maximum size ever needed:

/* Option 1: opaque pointer from the start */
typedef struct handle_impl *handle_t;

/* Option 2: use a large enough integer from day one */
#include <stdint.h>
typedef uintptr_t handle_t;  /* always 8 bytes on 64-bit */

If the change is unavoidable, bump the SONAME and provide a migration path.

Real-world example: this is a common pattern when handles evolve — POSIX pid_t has varied width across platforms, Windows HANDLE is void* but was historically int in some SDK versions, and database client libraries sometimes widen handle types to support larger connection pools.

Cross-tool comparison

abidw --out-file v1.xml libfoo_v1.so
abidw --out-file v2.xml libfoo_v2.so
abidiff v1.xml v2.xml

References

• ABICC: Typedef base type change detection
• System V AMD64 ABI: parameter passing for integer types

---

Source files

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

See also: Compatibility Catalog · All BREAKING cases · Category: Breaking · Rule: Typedef underlying type changed · Subject: Opaque types, typedefs, and contract identifiers.