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

Avoid C-extension gems that rely on printf-family formatting inside the parser sandbox

vsnprintf is not supported

Error message

vsnprintf is not supported

What it means

`vsnprintf` (variadic formatted print into a buffer) is another libc function the WASM Ruby parser deliberately does not implement; its `#[no_mangle]` export panics unconditionally. Formatting machinery in the Ruby VM or linked C code that reaches vsnprintf aborts here.

Source

Thrown at backend/parsers/windmill-parser-ruby/src/wasm_libc.rs:276

#[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, ... );
#[no_mangle]
pub extern "C" fn snprintf() {
    panic!("snprintf is not supported");
}

#[no_mangle]
pub extern "C" fn __assert_fail(_: *const i32, _: *const i32, _: *const i32, _: *const i32) {
    panic!("oh no");
}

View on GitHub (pinned to e474e8803c)

Solutions

Avoid C-extension gems that rely on printf-family formatting inside the parser sandbox
Use pure-Ruby formatting (String#%, String#format equivalents) which does not descend into libc
Provide a working vsnprintf shim compiled for wasm32 if such gems must be supported

Defensive patterns

Strategy: validation

Validate before calling

// detect C-extension dependencies that may invoke printf-family libc calls
let risky = manifest.dependencies.iter()
    .any(|d| d.is_native_extension() && NATIVE_FORMAT_GEMS.contains(&d.name.as_str()));
if risky { warn("gem may call unsupported libc formatting in the wasm parser"); }

Try / catch

std::panic::catch_unwind(|| parse_ruby(source)).unwrap_or_else(|e| {
    Err(sandbox_unsupported_error("vsnprintf", e))
})

Prevention

Prefer pure-Ruby gems in parser-executed code
Avoid sprintf-heavy native gems inside the sandbox
Track which libc shims the parser implements before adding dependencies

When it happens

Trigger: Calling the exported `vsnprintf` symbol, or a code path in the VM/gems that formats strings through C stdio (`printf`-family internals).

Common situations: Ruby formatting (sprintf/String#%) routed into libc internals inside the wasm sandbox; typically triggered by gems with C extensions.

Related errors

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/00ab051cf644e925. Report an issue: GitHub.