Function croaring_sys::ra_portable_serialize

pub unsafe extern "C" fn ra_portable_serialize(
    ra: *const roaring_array_t,
    buf: *mut c_char
) -> usize

write a bitmap to a buffer. This is meant to be compatible with the Java and Go versions. Return the size in bytes of the serialized output (which should be ra_portable_size_in_bytes(ra)).