Case 36 -- Anonymous Struct/Union Change¶

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

Verdict: 🔴 BREAKING abicheck verdict: BREAKING

What changes¶

Version	Definition
v1	`struct Variant { int tag; union { int i; float f; }; };` (size 8)
v2	`struct Variant { int tag; union { int i; double d; }; };` (size 16)

Why this is a binary ABI break¶

Changing float f to double d inside the anonymous union increases the union's size from 4 bytes to 8 bytes. This changes the overall struct size from 8 to 16 bytes (with alignment padding). When the app allocates a Variant on the stack using v1's sizeof (8 bytes), but the v2 library's functions expect 16-byte objects, the mismatch can cause:

Stack corruption if the library writes beyond the 8-byte allocation
Wrong field offsets if the i member moves due to alignment changes
Subtle data corruption in arrays of Variant

The demo makes this offset mismatch deterministic by embedding the Variant in a buffer filled with sentinel bytes (0xAA). When v2's variant_get_int() reads v->i at offset 8 instead of offset 4, it reads the sentinel bytes instead of the actual value 42, producing a visibly wrong result every run.

Code diff¶

 struct Variant {
     int tag;
     union {
         int    i;
-        float  f;    /* 4 bytes -> union size = 4 */
+        double d;    /* 8 bytes -> union size = 8, struct gains padding */
     };
 };

Real Failure Demo¶

Severity: CRITICAL

# 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
# -> sizeof(Variant) = 8 (compiled against v1)
# -> tag = 1, i = 42
# -> variant_get_int() = 42

# Swap to v2
gcc -shared -fPIC -g v2.c -o libv1.so
./app
# -> sizeof(Variant) = 8 (compiled against v1)    <-- app still uses v1's size!
# -> tag = 1, i = 42
# -> variant_get_int() = -1431655766               <-- WRONG! (0xAAAAAAAA sentinel)
# -> ERROR: expected 42, got -1431655766 — ABI layout mismatch!
#      v2's variant_get_int() read 'i' at offset 8 instead of 4
#      (sentinel bytes 0xAA were read instead of the actual value)

Why CRITICAL: The struct size changed from 8 to 16 bytes. The double member in v2's anonymous union forces 8-byte alignment, shifting v->i from offset 4 to offset 8. Reading v->i via variant_get_int() is already undefined behavior due to the layout change — the demo makes this visible by using sentinel-filled buffers so the wrong offset read produces a deterministic incorrect value.

Case 36 -- Anonymous Struct/Union Change¶

What changes¶

Why this is a binary ABI break¶

Code diff¶

Real Failure Demo¶

Why runtime result may differ from verdict¶

Runtime note¶

References¶

Source files¶