Function pio_to_format 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_to_format(
    net: *const PioNetwork,
    to: *const c_char,
    out_warnings: *mut *mut c_char,
    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. Also accepts powerio-json as an alias for pio_to_json. Model JSON cannot represent a non-finite f64 (Inf/NaN): it writes null, records the field in out_warnings, and fails validation when read back.

Returns the text as an owned C string (free with pio_string_free), NULL on error (message into errbuf). Fidelity warnings, if any, are published through out_warnings as one owned C string (free it with pio_string_free), or NULL when there are none; a returned string has no handle to attach them to. Pass NULL to discard them.