What causes "fwrite is not supported" in windmill-labs/windmill?

Replace the fwrite path with writes into a WASM-accessible memory buffer or a supported output channel.

fwrite is not supported

Error message

fwrite is not supported

What it means

The `fwrite` stub panics: binary block writes to a C stream are not implemented in the WASM libc shim. The symbol is exported for link compatibility only; any runtime call means the code is attempting to write data out through stdio, which the sandbox cannot do.

Source

Thrown at backend/parsers/windmill-parser-java/src/wasm_libc.rs:180

#[no_mangle]
pub unsafe extern "C" fn fdopen(_fd: c_int, _mode: *const c_void) -> *mut c_void {
    panic!("fdopen is not supported");
}

#[no_mangle]
pub unsafe extern "C" fn fclose(_file: *mut c_void) -> c_int {
    panic!("fclose is not supported");
}

#[no_mangle]
pub unsafe extern "C" fn fwrite(
    _ptr: *const c_void,
    _size: usize,
    _nmemb: usize,
    _stream: *mut c_void,
) -> usize {
    panic!("fwrite is not supported");
}

#[no_mangle]
pub unsafe extern "C" fn vsnprintf(
    _buf: *mut c_char,
    _size: usize,
    _format: *const c_char,
    _args: ...
) -> c_int {
    panic!("vsnprintf is not supported");
}

#[no_mangle]
pub extern "C" fn clock_gettime(ptr: usize, new_size: usize) {
    panic!("clock_gettime is not supported");
}

// int snprintf( char* restrict buffer, size_t bufsz, const char* restrict format, ... );

View on GitHub (pinned to e474e8803c)

Solutions

Replace the fwrite path with writes into a WASM-accessible memory buffer or a supported output channel.
Upgrade windmill-parser-java in case newer versions route output differently.
Adjust the parse input so the buffer-writing path is not reached.
Request a memory-backed stdio implementation upstream if the call site is a fixed dependency.

Example fix

// before
fwrite(out_buf, 1, out_len, stdout);

// after: return the buffer to the host instead of writing to a stream
set_result_buffer(out_buf, out_len); // exposed via wasm-bindgen

Defensive patterns

Strategy: try-catch

Validate before calling

if (!input || typeof input !== 'string') throw new Error('invalid parser input');

Try / catch

try {
  const result = parse(input);
} catch (e) {
  if (String(e).includes('fwrite is not supported')) {
    throw new Error('native component attempted buffered stream write; unsupported in parser WASM');
  }
  throw e;
}

Prevention

Return results via memory buffers or wasm-bindgen exports instead of stream writes.
Audit native serializers for fwrite before building the WASM artifact.
Keep output paths in the host, not inside the sandbox.
Add regression tests that exercise output-producing parse paths.

When it happens

Trigger: Code compiled into the parser's WASM module calls `fwrite(ptr, size, nmemb, stream)` — serializing results, dumping buffers, or writing parsed output to a file/stdout.

Common situations: Native serializers writing output blocks; debug dump routines in bundled components; transpiled code persisting intermediate data to temp files.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/2d373757b181b4e7. Report an issue: GitHub.