Case 115: _BitInt(N) width change (C23 64 → 128)¶

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Platforms	Linux
Flags	ABI break
Detected `ChangeKind`s	`bit_int_width_changed`
Source files	`examples/case115_bit_int_width_changed/`

Category: Binary ABI break / C23 | Verdict: 🔴 BREAKING

What changed¶

v1 uses C23 _BitInt(64) for an accumulator's storage, parameter, and return type. v2 widens it to _BitInt(128). The bit width N is part of the type: it determines the storage size of the Accumulator struct and the calling-convention treatment of the by-value parameter and return.

A consumer built against v1 passes/reads a 64-bit value where the v2 library expects 128 bits, so arguments, the struct field, and the returned value are all miscompiled.

(128 bits is the maximum _BitInt width clang supports, so both versions stay buildable on every supported toolchain.)

How abicheck catches it¶

bit_int_width_changed fires for each public slot (parameter, return, or field) whose _BitInt(N) width changes — or that migrates to/from _BitInt.

Files¶

v1.h / v2.h — _BitInt(64) vs _BitInt(128) declarations
v1.c / v2.c — the two library builds
app.c — consumer built against the 64-bit interface

Source files¶

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