windmill-labs/windmill · error

fprintf is not supported

Error message

fprintf is not supported

What it means

The windmill-parser-r WASM libc shim stubs fprintf (stdio.h) with a panic, because the WASM sandbox has no file descriptors or output streams for the compiled C parser to write to. Any code path that tries to print (error messages, debug logging, scanner diagnostics via fprintf(stderr, ...)) hits this stub and panics with "fprintf is not supported".

Source

Thrown at backend/parsers/windmill-parser-r/src/wasm_libc.rs:236

/* --------------------------------- time.h --------------------------------- */

#[no_mangle]
pub unsafe extern "C" fn clock() -> u64 {
    panic!("clock is not supported");
}

/* --------------------------------- 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]

View on GitHub (pinned to e474e8803c)

Solutions

  1. Trace which call site invokes fprintf (instrument the shim to log the format string and then panic) and fix the input/state that reaches that error branch.
  2. Reimplement fprintf in the shim to format and route output to console.log via the existing console_log! macro instead of panicking, so error paths degrade to log output.
  3. Rebuild the grammar without debug instrumentation (disable debug flags) so fprintf is compiled out.
  4. Patch the grammar's scanner.c to remove or gate fprintf usage for WASM builds.

Example fix

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

// after — route output to the console instead of panicking
#[no_mangle]
pub unsafe extern "C" fn fprintf(_file: *mut c_void, _format: *const c_void, _args: ...) -> c_int {
    let msg = cstr(_format); // read the NUL-terminated format string
    console_log!("fprintf: {}", msg);
    msg.len() as c_int
}
Defensive patterns

Strategy: try-catch

Validate before calling

// At build time, confirm the compiled parser does not call fprintf:
//   grep -n 'fprintf' scanner.c src/parser.c  # should be absent or WASM-gated
// At runtime, validate input before parsing to avoid error branches:
function validateDoc(src) {
  return typeof src === 'string' && src.length > 0 && !src.includes('\u0000');
}

Type guard

function isCleanStringDoc(src) {
  return typeof src === 'string' && src.length > 0 && !src.includes('\u0000');
}

Try / catch

try {
  if (!validateDoc(doc)) throw new TypeError('invalid parser input');
  return parser.parse(doc);
} catch (e) {
  if (String(e).includes('fprintf is not supported')) {
    // scanner hit an error/log branch using stdio: recreate parser,
    // inspect the document, and route it to a manual-review queue
    parser = new Parser();
    return null;
  }
  throw e;
}

Prevention

When it happens

Trigger: The compiled tree-sitter C runtime or custom scanner calls fprintf — typically fprintf(stderr, ...) in an error/debug path: logging an unexpected token, a scanner error branch, or debug instrumentation left compiled in.

Common situations: Parsing R input that reaches a scanner error branch containing fprintf(stderr, ...); a grammar built with debug logging enabled; a grammar upgrade whose error handling now prints; reusing a native-targeted scanner that assumes stderr exists.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/25192f6e93a14775. Report an issue: GitHub.