Case 45: Multi-Dimensional Array Element Type Change¶

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

Category: Struct Layout | Verdict: 🔴 BREAKING

What breaks¶

The Matrix struct contains a 2D array member data[ROWS][COLS]. Its element type changes from float (4 bytes) to double (8 bytes), so sizeof(Matrix) doubles from 72 → 136 bytes (4×4×4 + 8 = 72 → 4×4×8 + 8 = 136). Any consumer compiled against v1 headers will read/write the struct at the wrong offsets. Return types of matrix_get()/matrix_set() also change.

Why abidiff catches it¶

abidiff reports Subrange_Change in multi-dim array and exits 4. abicheck detects: TYPE_SIZE_CHANGED, TYPE_FIELD_TYPE_CHANGED, FUNC_RETURN_CHANGED.

Code diff¶

v1.h	v2.h
`float data[4][4];` — 64 bytes	`double data[4][4];` — 128 bytes
`float matrix_get(...)`	`double matrix_get(...)`

Real Failure Demo¶

Severity: 🔴 CRITICAL

gcc -shared -fPIC -g v1.c -o libv1.so
gcc -shared -fPIC -g v2.c -o libv2.so

abidw --out-file v1.abi libv1.so
abidw --out-file v2.abi libv2.so
abidiff v1.abi v2.abi
echo "exit: $?"   # → 4 (TYPE_SIZE_CHANGED + FUNC_RETURN_CHANGED)

Reproduce manually¶

gcc -shared -fPIC -g v1.c -o libv1.so
gcc -shared -fPIC -g v2.c -o libv2.so
abidw --headers-dir . --out-file v1.abi libv1.so
abidw --headers-dir . --out-file v2.abi libv2.so
abidiff v1.abi v2.abi

How to fix¶

Abstract the element type via a typedef: typedef float mat_elem_t; — change the typedef, not the struct directly.
Version via SONAME — element type changes in matrix structs require a major version bump.
Provide both matrix_float_get() and matrix_double_get() during a deprecation window.