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

Suppress the code path in the WASM runtime that writes via fputs (disable the corresponding flag/feature).

fputs is not supported

Error message

fputs is not supported

What it means

fputs() is a deliberate stub in wasm_libc.rs that panics with "fputs is not supported". Writing a string to a FILE* stream is impossible in the WASM parser host because no file/stream objects exist. Any call to this libc symbol instantly aborts the running code.

Source

Thrown at backend/parsers/windmill-parser-csharp/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

Suppress the code path in the WASM runtime that writes via fputs (disable the corresponding flag/feature).
Route the output through the console_log shim instead.
Implement fputs in wasm_libc.rs as a no-op or console writer if the call is benign diagnostics.
Rebuild the offending native dependency without stdio output.

Example fix

// before
fputs("hello\n", stdout);
// after
console_log!("hello");

Defensive patterns

Strategy: validation

Validate before calling

// Ensure code destined for the wasm C# parser emits no fputs-style stream writes:
if (isWasmHost) {
  runtimeConfig.redirectOutput = "console"; // not stdout/stderr streams
}

Type guard

function usesStreamWrites(source) {
  return /\b(fputs|fprintf|fwrite|fputc)\s*\(/.test(source); // true => will panic in wasm
}

Try / catch

try {
  runInWasmParser(code);
} catch (e) {
  if (String(e?.message ?? e).includes("fputs is not supported")) {
    console.warn("fputs is unsupported in the wasm host; redirect output to the console shim.");
  } else {
    throw e;
  }
}

Prevention

Avoid native libs that print via fputs/puts when running under wasm.
Implement output through the console_log shim in wasm_libc.rs.
Consider making fputs a benign no-op shim if calls are purely diagnostic.
Test any new native dependency in the wasm parser before shipping.

When it happens

Trigger: C code or the .NET runtime compiled to WASM calls fputs(str, stream) to emit text to stdout/stderr or another stream.

Common situations: puts-style printf redirection implemented via fputs; native libraries printing status messages; runtime bootstrap code writing a banner or error string with fputs.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/e6b03758fb48051e. Report an issue: GitHub.