Question: thread safety of read only accesses

[kimwalisch]

Hi,

Are nlohmann::json objects thread safe for read only operations? E.g. assuming that the json object is not modified anymore after load_json(), can I safely query the json object from multiple threads as in the example below?

// single threaded
auto json = load_json();

#pragma omp parallel for
for (int i = 0; i < threads; i++)
{
    // multi-threaded read only accesses
    double seconds = json["S2_easy"]["seconds"];
    int count = json["S2_easy"].count("s2_easy");
}

Thanks!

[nlohmann]

The library uses STL containers internally; that is, std::vector for JSON arrays and std::map for JSON objects. Very brief googling gives me the feeling that concurrent reads should be fine.

[kimwalisch]

Thanks!

[nlohmann]

If you experience any issues, please let me know! I had no reports so far about multithreading applications.

[kimwalisch]

If you experience any issues, please let me know! I had no reports so far about multi-threading applications.

My experiences:

I am using nlohmann::json in my math library to backup intermediate results of long running computations. In the resume step (used e.g. after a crash) multiple threads read values from a single nlohmann::json object (none of the threads modifies the object).

Everything works as expected, there are no issues. I don't use any mutexes or locks during the resume step. I have tested the code using valgrindand no issues have been detected.

Greetings,
Kim