# 12.4 Sending a pre-built frame

_Peios / Developing for Peios / SDK Reference / rsi/response.h — Building Responses_

> Writing an already-built response frame to the source fd, for a source that encodes its own.

```c
ssize_t rsi_write_response(int fd, const void *frame, size_t len);
```

Writes one already-built response frame to the source fd — a thin `write(2)` wrapper returning the bytes written, or `-1` with `errno`. Most callers never need this; the `rsi_respond_*` helpers build *and* send. It exists for callers assembling frames by other means.