How to publish JSON to a web server? - c++

Question

I am playing around with freeboard.io and trying to make a widget that pulls JSON data from a URL [TBD]. My original data source is from an iMX6-based Wandboard running Linux that is connected to the internet. I want to write a C++ program on the Wandboard that opens a socket to [TBD] and sends UDP packets, for example, containing my sensor data. My JSON data structure is like this:
{
"sensor_a": 1100,
"sensor_b": 247,
"sensor_c": 0
}
Can you help me put my JSON data structure into an IP packet using C++ on Ubuntu Linux? I know how to just serialize the data structure in ascii for example and build a buffer to stuff an IP packet but I'm wondering if there is a standard way to do this for cloud services, or will it be different for Azure vs AWS? Is some type of header info needed to "put" the data?

Answer 1

This is a very simple problem, like all simple problems no need for external libraries for serializing etc. Like #Galik said above your problem is how to send a string from client to server. Additionally for your case you need a JSON parser on the server (any C or C++ parser from the JSON page will do, I use gason because it's fast and simple).
In TCP/IP socket programming you have to make the other part know how many bytes (characters in your case) to read.
I faced a similar case: send JSON over the web.
here's the example, a JSON "message"
https://github.com/pedro-vicente/lib_netsockets/blob/master/examples/json_message.cc
in this case, the size of the message has this header format
nbr_bytes#json_string
where "json_string" is the JSON text, "nbr_bytes" is the number of characters "json_string" has and "#" is a separator character.
how does the server parse this?
By reading 1 character at a time until the "#" separator is found, then converting that string into a number;
then make the socket API read "nbr_bytes" characters and exit
example
100#{json_txt....}
in this case "json_txt" has 100 characters
here's the code for the parser
std::string read_response(socket_t &socket)
{
int recv_size; // size in bytes received or -1 on error
size_t size_json = 0; //in bytes
std::string str_header;
std::string str;
//parse header, one character at a time and look for for separator #
//assume size header lenght less than 20 digits
for (size_t idx = 0; idx < 20; idx++)
{
char c;
if ((recv_size = recv(socket.m_socket_fd, &c, 1, 0)) == -1)
{
std::cout << "recv error: " << strerror(errno) << std::endl;
return str;
}
if (c == '#')
{
break;
}
else
{
str_header += c;
}
}
//get size
size_json = static_cast<size_t>(atoi(str_header.c_str()));
//read from socket with known size
char *buf = new char[size_json];
if (socket.read_all(buf, size_json) < 0)
{
std::cout << "recv error: " << strerror(errno) << std::endl;
return str;
}
std::string str_json(buf, size_json);
delete[] buf;
return str_json;
}