Cap’n Proto C++ serialize to char array (or any byte array)

My goal is to send serialized data through MPI. I have done this with ProtoBuf but I would like to try to use a faster serialize method such as Cap’n Proto (I will try others as well but here I am stuck). With ProtoBuf I use the function SerializeToArray(void * data, int size) function which works just fine.

Now, I want to do the same thing but with Cap’n Proto but I cannot find anywhere how to do this (if you have a link please send it). Since Cap’n Proto claims to be a faster substitute for ProtoBuf I find this surprising. Maybe, I am going about this the completely wrong way.

So my question becomes:

How do I serialize to char array (or any byte array) with Cap’n Proto (if it is at all possible)? Or how do I serialize in a way which could easily be sent over MPI using C++?

For the message given in this AddrssBook example from documentation, you can do something like this:

// Documentation: https://capnproto.org/cxx.html
// AddressBook example

void sendMessage( const char* data, const std::size_t size ); 

void writeAddressBook()
{
    ::capnp::MallocMessageBuilder message;

    auto addressBook = message.initRoot<AddressBook>();
    auto people = addressBook.initPeople(1);

    auto alice = people[0];
    alice.setId(123);
    alice.setName("Alice");
    alice.setEmail("alice@example.com");

    auto alicePhones = alice.initPhones(1);
    alicePhones[0].setNumber("555-1212");
    alicePhones[0].setType(Person::PhoneNumber::Type::MOBILE);
    alice.getEmployment().setSchool("MIT");

    // get char array and send

    const auto m = capnp::messageToFlatArray( message );
    const auto c = m.asChars();
    std::cout << c.size() << '\n';

    sendMessage( c.begin(), c.size() ); // pass as char array
}