Case 35 -- Field Rename¶

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

Verdict: 🔴 BREAKING (policy escalated source break) abicheck verdict: BREAKING

Compatibility classification¶

Binary ABI impact: Compatible (layout and offsets unchanged).
Source compatibility impact: BREAKING (field identifiers removed/renamed).
Runtime behavior impact: Existing binaries typically keep working; recompilation fails.
Policy severity: BREAKING in ground_truth.json (source_break category escalated by policy).

What changes¶

Version	Definition
v1	`struct Point { int x; int y; };`
v2	`struct Point { int col; int row; };`

Why this is NOT a binary ABI break¶

Renaming struct fields does not change the binary layout. The fields x/col are at offset 0 and y/row are at offset 4 in both versions. Field names are not encoded in the compiled binary -- they are resolved to offsets at compile time. A binary compiled against v1 will continue to work correctly with v2's shared library because make_point() returns the same struct layout.

However, source code referencing p.x or p.y will fail to compile against v2's header, making this a source-level break.

Code diff¶

 struct Point {
-    int x;      /* offset 0 */
-    int y;      /* offset 4 */
+    int col;    /* offset 0 -- was x */
+    int row;    /* offset 4 -- was y */
 };

Real Failure Demo¶

Severity: API_BREAK (binary compatible)

# Build v1 lib + app
gcc -shared -fPIC -g v1.c -o libv1.so
gcc -g app.c -I. -L. -lv1 -Wl,-rpath,. -o app
./app
# -> p.x = 10
# -> p.y = 20

# Swap to v2 .so (do NOT recompile app)
gcc -shared -fPIC -g v2.c -o libv1.so
./app
# -> p.x = 10       <-- still correct! binary layout unchanged
# -> p.y = 20       <-- still correct!

# But recompiling against v2 header FAILS:
sed 's/#include "v1.h"/#include "v2.h"/' app.c > /tmp/app_v2_test.c
gcc -g /tmp/app_v2_test.c -I. -L. -lv1 -Wl,-rpath,. -o app
# -> error: 'struct Point' has no member named 'x'
rm -f /tmp/app_v2_test.c

Why API_BREAK: The struct layout is bit-for-bit identical between v1 and v2. Only the field names changed, which are a compile-time concept. Existing binaries are fully compatible.

Why runtime result may differ from verdict¶

Field rename: binary compat (field exists), source break (name changed)