dotnet/runtime · critical · Error

Out of memory

Error message

Out of memory

What it means

mono_wasm_load_bytes_into_heap (and the persistent variant) allocates room for a byte buffer via Module._malloc / Module._sbrk; when the allocator returns <= 0 the wasm linear memory is exhausted and the runtime throws 'Out of memory'. The persistent variant retries sbrk once before failing.

Source

Thrown at src/mono/browser/runtime/memory.ts:343

export function withStackAlloc<T1, T2, T3, TResult> (bytesWanted: number, f: (ptr: VoidPtr, ud1?: T1, ud2?: T2, ud3?: T3) => TResult, ud1?: T1, ud2?: T2, ud3?: T3): TResult {
    const sp = Module.stackSave();
    const ptr = Module.stackAlloc(bytesWanted);
    try {
        return f(ptr, ud1, ud2, ud3);
    } finally {
        if (loaderHelpers.is_runtime_running()) Module.stackRestore(sp);

    }
}

// @bytes must be a typed array. space is allocated for it in the native heap
//  and it is copied to that location. returns the address of the allocation.
export function mono_wasm_load_bytes_into_heap (bytes: Uint8Array): VoidPtr {
    // pad sizes by 16 bytes for simd
    const memoryOffset = malloc(bytes.length + 16);
    if (<any>memoryOffset <= 0) {
        mono_log_error(`malloc failed to allocate ${(bytes.length + 16)} bytes.`);
        throw new Error("Out of memory");
    }
    const heapBytes = new Uint8Array(localHeapViewU8().buffer, <any>memoryOffset, bytes.length);
    heapBytes.set(bytes);
    return memoryOffset;
}

// @bytes must be a typed array. space is allocated for it in memory
//  and it is copied to that location. returns the address of the data.
// the result pointer *cannot* be freed because malloc is bypassed for speed.
export function mono_wasm_load_bytes_into_heap_persistent (bytes: Uint8Array): VoidPtr {
    // pad sizes by 16 bytes for simd
    const desiredSize = bytes.length + 16;
    // sbrk doesn't allocate whole pages so we can ask it for as many bytes as we want.
    let memoryOffset = Module._sbrk(desiredSize);
    if (<any>memoryOffset <= 0) {
        // sbrk failed. Due to identical bugs in v8 and spidermonkey, this isn't guaranteed to be OOM.
        // We use this function early during startup, when OOM shouldn't be possible anyway!
        // Log a warning, then retry.

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Increase EmccMaximumHeapSize (within the host's limit) so the heap can grow.
2. Reduce the size of the buffer being loaded; chunk large payloads and free intermediate buffers.
3. Free unmanaged allocations (free()) and release GCHandles/Span views to bring heap usage down.
4. If during startup, also check EmccInitialHeapSize is large enough for early allocations.

Example fix

// before
const ptr = mono_wasm_load_bytes_into_heap(hugeBytes); // throws when heap full
// after
// publish with a larger cap and chunk the payload
dotnet publish -p:EmccMaximumHeapSize=2147483648
// and in JS: process in smaller slices, freeing each

// Estimate available heap headroom before a large copy.
const bytesNeeded = bytes.length + 16;
// Module.HEAP8.byteLength vs MAXIMUM_MEMORY; this is a heuristic only.
if (Module.HEAP8 && (Module.HEAP8.buffer.byteLength + bytesNeeded) > maxMemoryBytes) {
  throw new Error('Likely out of memory: requested bytes would exceed the configured MAXIMUM_MEMORY.');
}

try {
  const ptr = mono_wasm_load_bytes_into_heap(bytes);
} catch (e) {
  if (String(e?.message) === 'Out of memory') {
    // free unused buffers / chunk the payload and retry each chunk
  }
  throw e;
}

When it happens

Trigger: Loading a large Uint8Array into the heap (mono_wasm_load_bytes_into_heap) when the wasm linear memory has reached MAXIMUM_MEMORY and cannot grow further; or persistent sbrk failing twice during early startup allocation.

Common situations: Streaming large payloads/files through interop; the heap already near EmccMaximumHeapSize; processing big binary assets; a memory leak growing the heap to the cap.

Related errors

• .NET runtime has failed to start, because too much memory wa
• value was not an integer
• value out of range
• Unsupported Webcil version: ${webcilVersion}
• Failed to initialize ICU data

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/224b40a6b517ea83. Report an issue: GitHub.