json.exception.parse_error.101 when parsing data received over a socket

[sonnyparlin]

What is the issue you have?

Getting a parse error when parsing data received via a read loop over a tcp socket.

The Error

[json.exception.parse_error.101]

Please describe the steps to reproduce the issue.

1. Set up a tcp socket and read data from the socket stream with a read loop
2. Set the buffer size to be smaller than the amount of data coming over the socket so the data has to be read in a loop
3. json parse the data after it's been read.

I have a string that's 95 chars long, I set my read buffer to 55 to make sure that all data gets read via the loop. I can confirm that I receive all of the data with a std::cout of the buffer, but when I try to parse the buffer, I get a parse error. The parse function thinks it only received 55 chars. Changing the buffer size to a number larger than the message text fixes the issue. But I need to be able to read the data in a loop as I will be transferring large json dumps. My code is below:

void SocketCommunication::receive_node_message(int sock) {
    const int BUFFER_SIZE = 55; // <--- problem is here
    char buffer[BUFFER_SIZE] = {0};
    ... 
    while (offset < sizetoread) {
        ssize_t reader = recv(sock, buffer+offset, sizetoread-offset, 0); // get message
        if(reader == 0) {
            break;
        }
        offset+=reader;
    }

    std::cout << buffer << std::endl; // <--- prints the entire string (all 95 characters) shown below
    // {"Message":{"Peers":null,"SocketConnector":{"ip":"127.0.0.1","port":10002},"Type":"DISCOVERY"}}

    auto j = nlohmann::json::parse(buffer); // <--- fails with parse error

What is the expected behavior?

The json should get parsed because I'm providing a full json string to the parse() method.

And what is the actual behavior instead?

libc++abi: terminating with uncaught exception of type nlohmann::detail::parse_error:
[json.exception.parse_error.101] parse error at line 1, column 56: syntax error while parsing
value - invalid string: missing closing quote; last read: '"127.0'

Which compiler and operating system are you using?

• Compiler: /usr/bin/g++ -Wall -std=c++17
• Operating system: OSX 12.1

Which version of the library did you use?

3.10.5

[nlohmann]

Indeed, the library will only read the buffer until its last character, so the error message makes sense here. Writing/reading to the buffer past its size should be undefined behavior - please run your code with Valgrind or ASAN.

[sonnyparlin]

I'm not sure I understand your answer (sorry I'm new to cpp) but it would seem that receiving large amounts of data in pieces via a socket read loop is well established doctrine in terms of network programming. Also, I'm giving nlohmann::json a full string to work with. The program reads data into a variable until the loop is done and the entire message has been delivered. Only after I've received the entire message do I try to json parse it. As you can see in the example above, I'm sending the entire string to be parsed.

[sonnyparlin]

After debugging, it turns out that buffer is indeed only 55 characters, I must be doing something wrong. Sorry for the trouble...