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

Remove or replace the fputs call in the code compiled into the module (use console_log! for diagnostics).

fputs is not supported

Error message

fputs is not supported

What it means

The `fputs` stub in the WASM libc shim panics unconditionally. `fputs` writes a string to a C file stream; since the sandbox exposes no real stream objects, the library exports the symbol purely for link compatibility and treats any runtime call as an unsupported operation.

Source

Thrown at backend/parsers/windmill-parser-java/src/wasm_libc.rs:155

}

/* --------------------------------- ctype.h -------------------------------- */

#[no_mangle]
pub unsafe extern "C" fn isprint(c: c_int) -> bool {
    c >= 32 && c <= 126
}

/* --------------------------------- stdio.h -------------------------------- */

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

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

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

#[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]

View on GitHub (pinned to e474e8803c)

Solutions

Remove or replace the fputs call in the code compiled into the module (use console_log! for diagnostics).
Check for a newer windmill-parser-java version where the triggering component avoids stdio.
Alter the parse input so the fputs-emitting path is not reached.
Request a stdout-shim implementation upstream if the call site is third-party and unavoidable.

Example fix

// before
fputs("done\n", stdout);

// after
console_log!("done");

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('fputs is not supported')) {
    throw new Error('native component attempted fputs; unsupported in parser WASM');
  }
  throw e;
}

Prevention

Route all native output through the shim's console_log! macro.
Grep bundled C/native sources for fputs before building the WASM module.
Keep parser inputs within the constructs the supported code paths handle.
Track wasm_libc.rs stubs to know which symbols are link-only.

When it happens

Trigger: Code inside the parser's WASM module calls `fputs(s, stream)` — typically plain-string output to stdout/stderr or to a file opened in the native component.

Common situations: A native/C dependency emitting unformatted output; transpiled Java/native helpers printing progress or results; code paths that open and write pseudo-files inside the sandbox.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/5ff1e53887e2363f. Report an issue: GitHub.