send and receive JSON over sockets in server and client application in C

Question

I want to send the data in JSON over sockets in a server-client application written in C.

I am using json-c / libjson library for handling JSON data

Answer 1

To use TCP socket messages you have the client and server learn how to interpret the messages.

HTTP, for example, uses the 2 characters \r\n as end of message Another alternative option is to send the size of the message before the message.

Here's a solution that does that using the JSON Jansson library. It adds the size as a sequence of characters (e.g "123", is later parsed as integer 123), adding the character "#" as end of size header then the JSON text representation of the JSON object (as defined by the Jansson library)

size_t socket_t::write(json_t *json)
{
  char *buf_json = NULL;
  std::string buf_send;
  size_t size_json;

  //get char* from json_t
  buf_json = json_dumps(json, JSON_PRESERVE_ORDER);
  size_json = strlen(buf_json);
  //construct send buffer, adding a header with size in bytes of JSON and # terminator
  buf_send = std::to_string(static_cast(size_json));
  buf_send += "#";
  buf_send += std::string(buf_json);

  this->write(buf_send.data(), buf_send.size());

  free(buf_json);
  return buf_send.size();
}

The reading is done with

json_t * socket_t::read()
{
  int recv_size; // size in bytes received or -1 on error 
  const int size_buf = 20;
  char buf[size_buf];

  //peek header
  if ((recv_size = recv(m_socket, buf, size_buf, MSG_PEEK)) == -1)
  {
    std::cout << "recv error: " << strerror(errno) << std::endl;
  }

  //get size of JSON message
  std::string str(buf);
  size_t pos = str.find("#");
  std::string str_header(str.substr(0, pos));

  //parse header
  if ((recv_size = recv(m_socket, buf, str_header.size() + 1, 0)) == -1)
  {
    std::cout << "recv error: " << strerror(errno) << std::endl;
  }

  //sanity check
  buf[recv_size - 1] = '\0';
  assert(str_header.compare(buf) == 0);

  size_t size_json = static_cast(std::stoull(str_header));
  std::string str_buf = read_all(size_json);
  //terminate buffer with size
  std::string str_json = str_buf.substr(0, size_json);

  //construct JSON
  json_error_t *err = NULL;
  json_t *json = json_loadb(str_json.data(), str_json.size(), JSON_PRESERVE_ORDER, err);
  return json;
}

This is implemented in

https://github.com/pedro-vicente/lib_netsockets