Function pio_to_matpower 

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

Serialize net to MATPOWER .m text (byte-exact echo when parsed from MATPOWER). Returns an owned C string; free with pio_string_free. Returns NULL on error and writes the message into errbuf.