Function pio_to_format 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_to_format(
    net: *const PioNetwork,
    to: *const c_char,
    warnbuf: *mut c_char,
    warnlen: usize,
    errbuf: *mut c_char,
    errlen: usize,
) -> *mut c_char

Serialize net to the named format to: the one text serializer; every format is named by a string. Accepts the pio_parse_str names: matpower is a byte-exact echo when the handle was parsed from MATPOWER, and powerio-json is the canonical snapshot (validated by pio_parse_str on the way back; the retained source text is the one field it omits). The snapshot is lossless except for a non-finite f64 (Inf/NaN), which JSON cannot represent: it is written as null, named in a fidelity warning, and then fails to read back; pass warnbuf to detect it.

Returns the text as an owned C string (free with pio_string_free), NULL on error (message into errbuf). Fidelity warnings, if any, are written \n-joined into warnbuf; a returned string has no handle to attach them to.