Case 46: Pointer Chain 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: Breaking | Verdict: 🔴 BREAKING

What breaks¶

Functions return int** and accept int*const* in v1. In v2, the ultimate pointee type changes from int to long. Callers that dereference the returned pointer chain and write int-sized values will corrupt memory — long is 8 bytes on 64-bit, int is 4 bytes, so every write is half the expected size.

Why abidiff catches it¶

abidiff reports Function_Return_Type_Change (indirect pointee type) and exits 4. abicheck detects: FUNC_RETURN_CHANGED, PARAM_TYPE_CHANGED.

Code diff¶

v1.h	v2.h
`int **get_matrix(void);`	`long **get_matrix(void);`
`void set_cell(int const matrix, int row, int col, int val);`	`void set_cell(long const matrix, int row, int col, long val);`
`int sum_row(int const matrix, int row, int cols);`	`long sum_row(long const matrix, int row, int cols);`

Real Failure Demo¶

Severity: 🔴 CRITICAL — silent memory corruption

Scenario: v1 caller writes int values into memory allocated for long — adjacent cells are overwritten.

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 (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¶

Opaque element type: typedef int mat_cell_t; at the API boundary — change only the typedef.
Never change primitive types deep in pointer chains — consumers are forced to cast through every level.
SONAME bump if the type change is unavoidable.