Allowed R6 · Within policy
{
"slug": "struct-padding",
"title": "Struct Padding",
"difficulty": "easy",
"tags": [
"coding",
"embedded",
"technical",
"swe"
],
"is_free": true,
"locked": false,
"frequency": 1,
"last_reported_at": "2026-07-15T18:54:56.479581Z",
"url": "https://www.mokabla.com/en/c/siemens/struct-padding",
"agent": {
"page": "https://www.mokabla.com/agent/en/c/siemens/struct-padding",
"question": "https://www.mokabla.com/agent/_call/get_question?company=siemens&question=struct-padding"
},
"company": {
"slug": "siemens",
"name_en": "Siemens (EDA / Digital Industries)"
},
"body_md": "## Problem\nI show you a C `struct` and ask for `sizeof` on a typical 32-bit target. Then I ask you to write a function that computes it: given the field types in declaration order, return the struct size with natural alignment padding.\n\nRules I state for the exercise: `char` is 1 byte / 1-aligned, `short` 2/2, `int` and `float` 4/4, `double` 8/8, pointers 4/4. Every field starts at an offset that is a multiple of its alignment, and the total size is rounded up to a multiple of the largest alignment in the struct.\n\n## Examples\n**Example 1**, `[\"char\", \"int\", \"char\"]`\n`char` at 0, then 3 bytes padding, `int` at 4-7, `char` at 8, tail padding to 12. Answer: `12`.\n\n**Example 2**, `[\"char\", \"char\", \"int\"]`\nSame fields, reordered: chars at 0 and 1, padding 2-3, `int` at 4-7. Answer: `8`. This is the point of the question: field order changes memory.\n\n**Example 3**, `[\"double\", \"char\"]` → `16`. `[\"char\"]` → `1`.\n\n## Constraints\n- 1 to 64 fields, only the types listed above.\n- No `#pragma pack`; ask me if you want to discuss what it would change.\n- Then explain: why does the compiler pad at all, and what happens on a Cortex-M if you read a misaligned `int` through a cast pointer.\n\n## What they look for\nCorrect offset arithmetic (`offset = roundUp(offset, align)`), remembering the tail padding, and being able to say **why** (bus access alignment, atomicity of aligned loads). Bonus points for mentioning `offsetof`, `_Alignof`, and that reordering fields by decreasing size minimises padding.",
"follow_ups_md": "- Add a nested struct as a field type: its alignment is its largest member's.\n- What does `#pragma pack(1)` do to size and to access speed?\n- Bit-fields: how does the compiler lay out `unsigned a:3; unsigned b:5;`?",
"access": "Question bodies are paid content: a person buys 30 days of access per company (200 EGP or 200 points) in the human app. Agents get titles, tags and difficulty; bodies only for free questions."
}