SDK 4.17 emulator firmware ignores `ModdableCreationRecord` memory sizes, causing apps to crash on launch

Watchapp/Face Developers Corner
1

Note: bug report created with help of Claude for clarity. I’ve since fixed this issue locally for my own testing.

Summary: JavaScript (Alloy/Moddable) apps that pass custom stack/slot/chunk sizes to moddable_createMachine() abort at launch with xsPlatform.c:125> fxAbort memory full: on the SDK 4.17 emulator, because the firmware silently ignores the requested sizes and always uses the default 32 KB static machine. The root cause is already fixed on PebbleOS main, but SDK 4.17 still ships emulator firmware built from v4.17.0, which predates the fix. Requesting an SDK release with emulator firmware ≥ v4.29.0.

Environment

  • pebble-tool v5.0.39, SDK 4.17 (active), macOS (Apple Silicon)
  • pebble install --emulator emery (emulator firmware identifies as v4.17.0)
  • App: JS mod (mcrun/XSA) using the PIU framework, with a C bootstrap (mdbl.c) passing:
ModdableCreationRecord creation = {
  .recordSize = sizeof(ModdableCreationRecord),
  .stack = 4096,
  .slot  = 32768,
  .chunk = 32768
};
moddable_createMachine(&creation);

Symptom

  • App builds cleanly and installs, then immediately logs fxAbort memory full and hangs the emulator firmware (subsequent pebble ping / pebble screenshot time out until the QEMU process is killed and storage wiped).
  • The same app binary and creation record work correctly on the SDK 4.9.127 emulator, where the requested pool sizes are honored.
  • Enabling kModdableCreationFlagLogInstrumentation on the 4.17 emulator confirms the record is ignored — at the abort the VM reports the default pools despite the explicit record: Chunk available: 8192, Slot available: 8176, Stack available: 6144 (i.e. the default creation: chunk 8192, heap 512 slots, stack 384 slots, 32 KB static). This app’s real peak usage (measured on 4.9.127) is ~21.7 KB slot + ~16.9 KB chunk, so it can never fit the 32 KB static default.

Root cause

In firmware v4.17.0, src/fw/applib/moddable/moddable.c (moddable_createMachine syscall) declares a shadowing local inside the size-override branch:

struct xsCreationRecord creation = *defaultCreation;   // outer
if (NULL != cr) {
    ...
    if (stack || slot || chunk) {
        ...
        struct xsCreationRecord creation = *defaultCreation;   // <-- shadows the outer variable
        creation.stackCount = stack / sizeof(xsSlot);          //     all writes discarded
        creation.initialHeapCount = slot / sizeof(xsSlot);
        creation.initialChunkSize = chunk;
        ...
    }
    ...
}
xsMachine *the = modCloneMachine(&creation, NULL);     // uses the untouched outer defaults

All customizations are applied to the inner copy, which goes out of scope; modCloneMachine always receives the defaults. (A secondary issue in the same version: the fxBuildFFI branch resizes the initial pools to fill staticSize exactly without zeroing staticSize, leaving no headroom for machine overhead — also already fixed on main.)

This was fixed upstream in coredevices/pebbleos commit 76cd7328 (“moddable: fix ffi memory crash and memory allocation”, 2026-07-03), first included in the v4.29.0 firmware release (2026-07-17). Real watches on current firmware are therefore fine —
only the SDK-bundled emulator firmware is affected.

Request

Publish an SDK update whose emulator images (qemu_micro_flash.bin) are built from firmware ≥ v4.29.0, so that JS apps needing more than the 32 KB default can be developed in the emulator.

Workaround (for anyone else hitting this)

The default txCreation for app machines is plain ROM data in the emulator image. In
SDKs/4.17/sdk-core/pebble/emery/qemu/qemu_micro_flash.bin the struct {8192, 1024, 512, 64, 384, 32, 32, 53, 3, 1024, 17, 32768} (twelve little-endian int32s) is at offset 0x117230; patching the final value (staticSize, offset 0x117230 + 44) from 32768 to 65536 raises the cap and lets larger apps run (growth happens within the static region).

Values much larger (e.g. 96 KB) starve the kernel heap the machine is allocated from and hang the emulator; 64 KB works. Back up the image first.