Zero copy and zero allocation sends

[yb303]

Issue description

The zero-copy api is allocating content_t. It would be nice to avoid this malloc-free pair. Can you please make a new wrapper function that calls zmq::msg_t::init_external_storage ?

Environment

• libzmq version: any
• OS: any

[ljluestc]

#include <zmq.hpp>
#include <cstring>

// Hypothetical function to initialize a ZeroMQ message with external storage
void initMsgWithExternalStorage(zmq::message_t &msg, void *data, size_t size) {
    // Directly call the hypothetical zmq::msg_t::init_external_storage
    // Assuming this function initializes the message with provided external data without allocating new storage.
    msg.init_external_storage(data, size);
}

// Usage example
int main() {
    zmq::context_t context(1);
    zmq::socket_t socket(context, ZMQ_PUSH);
    socket.bind("tcp://*:5555");

    // Your external data buffer
    const char *data = "Hello, World!";
    size_t dataSize = std::strlen(data) + 1; // +1 for null terminator

    // Create a zmq message using the external data buffer
    zmq::message_t message;
    initMsgWithExternalStorage(message, (void*)data, dataSize);

    // Send the message
    socket.send(message, zmq::send_flags::none);

    return 0;
}