C99 Designated Initializers Misplace Struct Array Elements in EmbeddedC

Answer to the Core Question

EmbeddedC 2.0 and later releases have been observed to generate correct offsets for struct members when designated initializers target array elements. The bug that caused misplacement in 1.9.x was fixed in the 2.0 series, but only after the release of the 2.0.1 patch, which corrected the offset calculation logic for array sub‑objects. If you are using 2.0.0 or earlier 2.0 releases, you should upgrade to at least 2.0.1 or newer.

Key Flags that Influence Code Generation

• -O0 – preserves the straightforward initializer sequence; offsets are emitted as expected.
• -O1/+O2/+Os – the compiler may replace the explicit initializer loop with a memset or memcpy that relies on the compiler’s internal layout tables. If those tables are stale (as in 1.9.x), offsets can be wrong.
• -mstrict-align – forces the compiler to generate stricter alignment checks. On Cortex‑M targets this can surface hidden mis‑offsets because the generated store might be split into multiple instructions.
• -mthumb vs -marm – the thumb mode can change the instruction size and the way offsets are encoded, but the underlying layout tables are the same. The bug is independent of the instruction set, so the same offset issue appears in both modes if the compiler version is < 2.0.1.

How to Verify Correct Offsets

1. Compile a minimal test case that uses a designated initializer on an array element inside a struct:

   typedef struct {
       int a;
       int arr[3];
       int b;
   } S;
   
   S s = { .a = 1, .arr[2] = 99, .b = 2 };
   

2. Generate assembly with -S and inspect the STR or STMIA instructions that write into arr[2]. The offset should be 8 bytes from the start of arr (since a occupies 4 bytes and arr[0] starts at offset 4).
3. Build the binary, load it into a debugger (e.g., GDB on QEMU or a real Cortex‑M target), and read the memory at s.arr[2]. Verify that the value is 99 and that s.arr[0] and s.arr[1] are zero-initialized as per the C99 rules.
4. Cross‑check the linker map or ELF symbol table to confirm that the struct’s symbol table lists the correct offsets for each member.

What to Do if the Offsets Still Look Wrong

1. Confirm you are using 2.0.1 or newer. Run embeddedc --version to get the exact build number. 2. Re‑run the test with -O0 and -mstrict-align to see if the problem disappears. If it does, the issue is tied to the optimization‑level code path. 3. If the problem persists, file a bug with the compiler vendor, including the exact compiler flags, target MCU, and the assembly snippet.

Missing Diagnostic Detail

To refine the recommendation, I need to know the exact compiler command line you used (including all flags) and the MCU family you target (e.g., Cortex‑M3 vs Cortex‑M4). This will help determine whether the mis‑offset is a compiler bug or a target‑specific alignment quirk.