dotnet/runtime · critical · Error

posix_memalign failed for Webcil payload

Error message

posix_memalign failed for Webcil payload

What it means

Thrown by allocWebcilPayload() (src/native/libs/Common/JavaScript/host/assets.ts:93) when _posix_memalign fails to allocate the 16-byte-aligned buffer that backs a Webcil image. This buffer holds the image payload passed as the imageBase import and must outlive the instantiate call, so it is heap-allocated rather than stack-allocated.

Source

Thrown at src/native/libs/Common/JavaScript/host/assets.ts:98

    }
}

async function checkWebcilResponse(webcilPromise: Promise<Response>, virtualPath: string): Promise<Response> {
    const res = await webcilPromise;
    if (!res || res.ok === false) {
        throw new Error(`Failed to load Webcil module '${virtualPath}'. HTTP status: ${(res as Response)?.status} ${(res as Response)?.statusText}`);
    }
    return res;
}

// Allocates a 16-byte-aligned buffer for the Webcil payload. The pointer is heap memory that
// outlives the stack frame, so it can be passed as the imageBase import.
function allocWebcilPayload(payloadSize: number): number {
    const sp = _ems_.stackSave();
    try {
        const ptrPtr = _ems_.stackAlloc(sizeOfPtr);
        if (_ems_._posix_memalign(ptrPtr as any, 16, payloadSize)) {
            throw new Error("posix_memalign failed for Webcil payload");
        }
        return _ems_.HEAPU32[ptrPtr as any >>> 2];
    } finally {
        _ems_.stackRestore(sp);
    }
}

// Builds the `webcil` import object. For R2R images (tableSize > 0) the module imports the runtime's
// stack pointer, exception tag, indirect-call table and base globals; this also grows the table.
// These import names and the webcilVersion/getWebcilPayload/fillWebcilTable handshake in
// finishWebcilInstance are the R2R Webcil-in-Wasm host ABI defined by crossgen's WasmObjectWriter
// (src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs, CreateDefaultGlobalImports/
// WriteExports). Keep in sync with the corerun host
// (src/coreclr/hosts/corerun/wasm/libCorerun.js, BrowserHost_ExternalAssemblyProbe). Unlike corerun,
// which parses data segment 0 for payloadSize/tableSize, this loader receives them from boot config.
function buildWebcilImports(memory: WebAssembly.Memory, payloadPtr: number, tableSize: number): Record<string, WebAssembly.ImportValue> {
    const webcilImports: Record<string, WebAssembly.ImportValue> = { memory };
    if (tableSize > 0) {

View on GitHub (pinned to 60108ba66e)

Solutions

1. Increase the runtime's maximum WebAssembly.Memory so the heap can hold the largest webcil image plus the rest of the working set.
2. Reduce the size or number of R2R webcil assemblies loaded concurrently (lazy-load large assemblies).
3. If it appears after earlier successes, look for a heap memory leak in the host.

Example fix

// before: default memory cap, large R2R assembly -> posix_memalign failed for Webcil payload

// after
{ "memory": { "initial": 1, "maximum": 4294 } }

// approximate headroom before instantiating a large webcil image
function hasRoomFor(payloadSize: number): boolean {
  const max = 2 * 1024 * 1024 * 1024; // typical wasm memory ceiling
  const used = (globalThis as any).Module?.HEAP8?.buffer.byteLength ?? 0;
  return used + payloadSize < max;
}

try {
  await instantiateWebcilModule(webcilPromise, memory, virtualPath, tableSize, payloadSize);
} catch (err) {
  if (/posix_memalign failed for Webcil payload/.test(String((err as Error).message))) {
    // raise maximum memory or lazy-load the oversized R2R assembly
  }
  throw err;
}

When it happens

Trigger: instantiateWebcilModule computes payloadSize from the boot config and calls allocWebcilPayload(payloadSize); posix_memalign returns non-zero because the wasm heap cannot accommodate payloadSize bytes (would exceed maximum memory).

Common situations: An R2R/large webcil assembly larger than remaining memory headroom; a runtime whose maximum memory is too small; many concurrent webcil instantiations exhausting the heap.

Related errors

• posix_memalign failed
• posix_memalign failed for ICU data
• Unsupported Webcil version: ${webcilVersion}
• Failed to load ICU data
• .NET runtime has failed to start, because too much memory wa

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/8cc9e90a9e83f831. Report an issue: GitHub.