How do you manage end of a message in a protocol ? I use msgpack-c and the only solution I found is to send the header before the payload (separately).

Send the header to client :

// header
{
  "message_type": "hello",
  "payload_size": 10
}

The client received the header, unpack it, and allocate a buffer of "payload_size", receive data from stream, and if the buffer is complete the message is finish.

I want to send header and body succinctly

{
  "header": { "message_type":"hello", "payload_size": 10},
  "payload": {...} // can come in multiple frame
}

My problem is that I don't know if it's possible to partially unpack the header for knowing the size before receiving the full message (splitted if > 4096kb due to libevent restriction)

How would you do that ? I am open to all solutions.

C++

Using unpack() function

You can use offset parameter of unpack() function. See https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_unpacker#client-controls-a-buffer

Here is a code example:

#include <iostream>
#incluee <msgpack.hpp>

int main() {
    msgpack::sbuffer buf;
    msgpack::pack(buf, std::make_tuple("first message", 123, 56.78));
    msgpack::pack(buf, std::make_tuple("second message", 42));
    std::size_t off = 0; // cursor of buf
    {
        // unpack() function starts parse from off (0)
        auto oh = msgpack::unpack(buf.data(), buf.size(), off);
        // off is updated to 25. 25 is MessagePack formatted byte size
        // of ["first message",123,56.78] 
        // (I use JSON notation but actual format is MessagePack)
        std::cout << "off:" << off << std::endl;
        std::cout << *oh << std::endl;
    }
    {
        // unpack() function starts parse from off (25)
        auto oh = msgpack::unpack(buf.data(), buf.size(), off);
        // off is updated to 42. 
        // 42 - 25 = 17. 17 is MessagePack formatted byte size
        // of ["second message",42] 
        // (I use JSON notation but actual format is MessagePack)
        std::cout << "off:" << off << std::endl;
        std::cout << *oh << std::endl;
    }
}

Output is

off:25
["first message",123,56.78]
off:42
["second message",42]

msgpack-c unpack() manage the position of buffer internally. You don't need to pass payload_size.

In addition you can mix non-msgpack format data in the buffer.

+--------------------+-----------------------------+--------------------+
| MessagePackBytes1  | Any format user knows size  | MessagePackBytes2  |
+--------------------+-----------------------------+--------------------+

Let's say user knows the data structure that contains MessgePackBytes1(unknown size), any format data (known size), and MessgePackBytes1(unknown size).

#include <iostream>
#incluee <msgpack.hpp>

int main() {
    msgpack::sbuffer buf;
    msgpack::pack(buf, std::make_tuple("first message", 123, 56.78));
    std::string non_mp = "non mp format data";
    buf.write(non_mp.data(), non_mp.size());
    msgpack::pack(buf, std::make_tuple("second message", 42));
    std::size_t off = 0; // cursor of buf
    {
        auto oh = msgpack::unpack(buf.data(), buf.size(), off);
        std::cout << "off:" << off << std::endl;
        std::cout << *oh << std::endl;
    }
    {
        std::string extracted{buf.data() + off, non_mp.size()};
        std::cout << extracted << std::endl;
        off += non_mp.size();
    }
    {
        auto oh = msgpack::unpack(buf.data(), buf.size(), off);
        std::cout << "off:" << off << std::endl;
        std::cout << *oh << std::endl;
    }
}

Output is

off:25
["first message",123,56.78]
non mp format data
off:60
["second message",42]

Using unpacker

It is a little advanced but it might fit streaming usecases. https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_unpacker#msgpack-controls-a-buffer Here is an example that unpacking MessagePack from continuous and scattered receiving message. https://github.com/msgpack/msgpack-c/blob/700167995927f0348fb08ae2579440c1bc135480/example/boost/asio_send_recv.cpp#L41-L64

C

C version is basically similar to C++.

Using unpack() function

C version has the similar unpack function. Here is the prototype:

msgpack_unpack_return
msgpack_unpack_next(msgpack_unpacked* result,
        const char* data, size_t len, size_t* off);

You can pass off as offset similar to C++ version. C doesn't have reference so you need to pass the address of off using &off. See https://github.com/msgpack/msgpack-c/wiki/v2_0_c_overview#using-unpack-function

If you want to know individual variable length field size such as stirng, you can access size member variable of unpacked object. For example:

typedef struct {
    uint32_t size;
    struct msgpack_object* ptr;
} msgpack_object_array;

typedef struct {
    uint32_t size;
    const char* ptr;
} msgpack_object_str;

typedef struct {
    uint32_t size;
    const char* ptr;
} msgpack_object_bin;

typedef struct {
    int8_t type;
    uint32_t size;
    const char* ptr;
} msgpack_object_ext;

See https://github.com/msgpack/msgpack-c/wiki/v2_0_c_overview#object

Using unpacker

See https://github.com/msgpack/msgpack-c/wiki/v2_0_c_overview#using-unpacker