denoland/deno · error · TypeError

Cannot get pointer size, unsupported type

Error message

Cannot get pointer size, unsupported type: ${type}

What it means

getTypeSizeAndAlignment maps each FFI type token to a size/alignment pair. Supported tokens are u8 i8 u16 i16 u32 i32 f32 u64 i64 f64 pointer buffer function usize isize, plus { struct: [...] } objects. Any other value reaches the default branch and throws with the offending value interpolated, which makes the message double as a debugging hint.

Solutions

1. Replace C/ffi-napi names with Deno tokens: int32->i32, uint8->u8, float->f32, double->f64, size_t->usize, pointers->pointer
2. Use "void" only as a function result type, never inside struct fields or where a size must be computed
3. Read the interpolated value in the message to locate the offending field, then fix that exact token

Example fix

// before
const Point = { struct: ["float", "float"] }; // TypeError: unsupported type: float

// after
const Point = { struct: ["f32", "f32"] };

const FFI_SCALARS = new Set([
  "u8", "i8", "u16", "i16", "u32", "i32", "f32",
  "u64", "i64", "f64", "pointer", "buffer", "function", "usize", "isize",
]);

const isFfiType = (t) =>
  FFI_SCALARS.has(t) ||
  (typeof t === "object" && t !== null && Array.isArray(t.struct) &&
    t.struct.every(isFfiType));

if (!isFfiType(field)) throw new Error(`bad FFI type: ${String(field)}`);

When it happens

Trigger: Misspelled or non-Deno type names in struct fields or size-computed contexts: "uint8", "int", "float", "double", "size_t", "int32" (ffi-napi style), "bool", "void" inside a struct; passing a number, undefined, or an array where a type token is expected.

Common situations: Porting C headers verbatim; reusing type strings from Node's ffi-napi or napi bindings; generated bindings emitting C names; copy-paste edits that leave a placeholder value in a field list.

Related errors

• Cannot get pointer size: found recursive struct
• Cannot access pointer: expected 'ArrayBuffer'…
• Cannot construct UnsafeCallback: cannot be nonblocking
• Foreign symbol of type 'void' is not supported
• Initialize libloading failed

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/139d8c6aa32b68e7. Report an issue: GitHub.

Appendix: source

Thrown at ext/ffi/00_ffi.js:390

      return [1, 1];
    case "u16":
    case "i16":
      return [2, 2];
    case "u32":
    case "i32":
    case "f32":
      return [4, 4];
    case "u64":
    case "i64":
    case "f64":
    case "pointer":
    case "buffer":
    case "function":
    case "usize":
    case "isize":
      return [8, 8];
    default:
      throw new TypeError(`Cannot get pointer size, unsupported type: ${type}`);
  }
}

class UnsafeCallback {
  #refcount;
  // Internal promise only meant to keep Deno from exiting
  #refpromise;
  #rid;
  definition;
  callback;
  pointer;

  constructor(definition, callback) {
    if (definition.nonblocking) {
      throw new TypeError(
        "Cannot construct UnsafeCallback: cannot be nonblocking",
      );
    }

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