Parse JSON directly from decimal uint8 vector

[NicolaBlasuttigh]

Hi!
I am working in Simulink on a TCP connection between client (my computer) and server. Through standard Ethernet blocks, I send an HTTP request like the following:

GET /status HTTP/1.1
Host:...
Accept: application/json

To send it, I convert it with the uint8('GET /status...') command in Simulink. The server responds, always with a uint8 array (decimal bytes) like this :

resp=[72 84 84 80 47 49 32 50 48...] //(HTTP/1.1 200 OK... ).

In the reply "resp" content, always in decimal characters, there is a text in JSON format like {"VarA":1000, "VarB":2000,...}.

My purpose is to create an S-function (but more generally a code in C++) that taking in input the vector "resp", returns the values of VarA and VarB (1000 and 2000).

My idea was to convert "resp" into a string and then pass it to the json:parse functions. Is this correct? Can it be done?

More important: I also wonder if it is right to convert it into a string and then work on it...does anyone know if json::parse() can work directly on the uint_8 vector without the string conversion?

Thank you everyone!

[nlohmann]

You can pass an iterator range:

#include <iostream>
#include "json.hpp"

using json = nlohmann::json;

int main()
{
    std::vector<std::uint8_t> resp = {'f', 'o', 'o', '{', '"', 'V', 'a', 'r', '"', ':', '1', '0', '}'};
    json j = json::parse(resp.begin() + 3, resp.end());
    std::cout << j << std::endl;
}

Output:

{"Var":10}

[nlohmann]

You can still pass two iterators, which would be pointers in your case. For the example above:

json j = json::parse(resp.data() + 3, resp.data() + resp.size());

Here, we take the std::uint8_t* array via resp.data() and use pointer arithmetic to access the 4th value to start. The second pointer resp.data() + resp.size() points to one value after the last one. If you know the size of your resp8 array, then you can do the same, assuming you know when the headers end.

[NicolaBlasuttigh]

Thank you very much for your reply @nlohmann ! You really thought of everything!

However, it keeps giving me error... the problem is that Simulink creates an InputUInt8PtrsType pointer rather than std::uint8_t.
The function data() belongs to std::vector so I can't use it with my pointer type.
However, I "solved" (I don't think in the most elegant way) by creating an std::vector and copying all the values of the pointer inside it.
In this way:

InputPtrsType u = ssGetInputPortSignalPtrs(S,0); 
InputUInt8PtrsType resp8 = (InputUInt8PtrsType)u; 

std::vector<std::uint8_t> resp_vec; 
    
for ( int i=0; i<=3000; i++ ){
     resp_vec.push_back(*resp8[i]); 
    }

Now I have my resp_vec std::vector which I can work with using begin(), end() and everything else.
It's probably possible to convert the pointer from one type to another but I don't know how and I've done it this way.

About the end of the headers, they always end with \r\n\r\n (13 10 13 10 in decimal). Do you think it's possible to find those 4 bytes and start the parse() from the next number? (which will be "{" (123). It seems laborious but it's the only way to know where they end up since the headers could change their size! I was hoping to look for the first "{" but the cookie also uses "{".

Thank you again!

[nlohmann]

• Did you try passing the pointers of type InputUInt8PtrsType directly to parse? Making a copy seems so wasteful.
• I'm afraid there is no function in the library to skip until \r\n\r\n is read. You could write a function that just advances a pointer until you see the pattern and then pass that pointer as first argument to parse.

[NicolaBlasuttigh]

• Yes! That's exactly what I did and wrote before!

But if I write:
json j = json::parse(*resp8);
an error comes up:
Unexpected exception ([json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - invalid literal; last read: 'H') was caught.

And if I write json j = json::parse(resp8);
Building Error Reason:
impossible to convert from 'InputUInt8PtrsType' to 'nlohmann::detail::input_adapter'.

Actually, json j = json::parse(*resp8); would work, but given that it's not a std::vector type I cannot use begin(), end(), etc... And I didn't find similar functions to work with pointers!
Anyway, you gave me a lot of help! I think that when I'll move everything in C++ (and not in S-function) will be easier!

• Fine! I'll try to write something and then I'll share here!

[nlohmann]

What I meant is not passing a single pointer, but two pointer like I described in #2321 (reply in thread). You do not need begin()/end(), but can use offsets. Something like

json j = json::parse(resp8, resp8 + size);