OpenNHP/opennhp · error

data must be a pointer

Error message

data must be a pointer

What it means

toStructure is the file-to-struct deserialization entry point and requires its data argument to be a pointer, because unmarshal writes into the destination via reflection and must be able to address it. Passing a non-pointer makes this impossible, so the function fails fast with this error before any reading occurs.

Solutions

1. Pass a pointer: toStructure(f, &myStruct).
2. If you have an `any`, assert it holds a non-nil pointer before calling.
3. Fix wrapper functions to require pointer parameters in their signatures.
4. Document the pointer requirement in the wrapper's API to catch this at the call site.

Example fix

// before
var hdr Header
toStructure(f, hdr) // "data must be a pointer"
// after
var hdr Header
err := toStructure(f, &hdr)

func ensurePtr(v any) bool { return reflect.ValueOf(v).Kind() == reflect.Pointer }

if v := reflect.ValueOf(data); v.Kind() != reflect.Pointer || v.IsNil() {
    return errors.New("toStructure requires a non-nil pointer")
}

if err := toStructure(f, data); err != nil {
    if strings.Contains(err.Error(), "data must be a pointer") {
        return fmt.Errorf("call toStructure(f, &value), not toStructure(f, value)")
    }
    return err
}

When it happens

Trigger: Calling toStructure(f, someStruct) with a struct value instead of &someStruct; passing a non-pointer map/slice value; wrapper functions that accept `any` and forward a value instead of a pointer.

Common situations: Forgetting the & when passing a local struct; helper wrappers that accept `any` and forward without dereferencing; porting code from JSON APIs where non-pointer targets are sometimes allowed.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• data must be a struct
• unsupported field type
• unsupported field type
• unsupported element type
• unknown remote provider

AI-assisted analysis of OpenNHP/opennhp@6e04ca5ff0 (2026-09-07). Data as JSON: /api/errors/922a38e0ea1c1dfc. Report an issue: GitHub.

Appendix: source

Thrown at nhp/core/ztdo/ztdo.go:628

		}
	}

	return nil
}

// toBuffer provides unified way to serialize a Go struct into bytes buffer
func toBuffer(data any) *bytes.Buffer {
	buf := bytes.NewBuffer(nil)
	_ = marshal(buf, data)
	return buf
}

// toStructure provides unified way to deserialize bytes from a file into a Go struct
func toStructure(f *os.File, data any) error {
	lengthMap = make(map[string]uint32)
	rValues := reflect.ValueOf(data)
	if rValues.Kind() != reflect.Pointer {
		return fmt.Errorf("data must be a pointer")
	}
	return unmarshal(f, data)
}

// setBytes provides unified way to set bytes to a slice or array
func setBytes(rvalue reflect.Value, dst []byte) {
	if rvalue.Kind() == reflect.Slice {
		rvalue.SetBytes(dst)
	} else if rvalue.Kind() == reflect.Array {
		len := rvalue.Len()
		elType := rvalue.Type().Elem()

		arrayType := reflect.ArrayOf(len, elType)
		newArray := reflect.New(arrayType).Elem()

		for i := range len {
			newArray.Index(i).SetUint(uint64(dst[i]))
		}

View on GitHub (pinned to 6e04ca5ff0)