Can't find where this std::logic_error is coming from

Question

My application works just fine when I have defined the CAMERA_NAME environment variable, but as soon as I remove that variable from my docker-compose file, the container will return a very useless error message that doesn't give me information such as what variable/line the error is occurring on:

basler-hd     | terminate called after throwing an instance of 'std::logic_error'
basler-hd     |   what():  basic_string::_M_construct null not valid

Here is the relevant code:

    try {
        char const* camera_name_ = std::getenv("CAMERA_NAME");
        if (!camera_name_)
            camera_name_ = generate_uuid_v4().c_str();
        
        init_loggers(camera_name_);
        SPDLOG_INFO("Camera name (found in environment or was generated): {}", camera_name_);

        SPDLOG_INFO("Starting Application.");
        PylonInitialize();

        mqtt::async_client client(DEFAULT_SERVER_ADDRESS, CLIENT_ID, PERSIST_DIRECTORY);
        
        auto willMessage = mqtt::message("nssams/camera", "Camera Disconnected", 1, true);
        auto connectionOptions = mqtt::connect_options_builder()
                                    .clean_session()
                                    .will(willMessage)
                                    .finalize();

        std::string ip_address = std::getenv("IP_ADDRESS") != NULL ? std::string(std::getenv("IP_ADDRESS")) : "192.168.0.1";
        if (ip_address.compare("192.168.0.1") == 0)
            SPDLOG_WARN("No IP address was found in environment, setting to '192.168.0.1'");

        std::string camera_name(camera_name_);
        // Instantiate Camera instance and connect. 
        Camera::MqttBaslerCamera camera( client, camera_name );
        try {
            camera.discover_and_create_device(ip_address);
        } catch (Camera::MqttBaslerCamera::DeviceNotFoundException & ex){
            SPDLOG_CRITICAL("No device found matching IP [{}] from environment", ip_address);
            PylonTerminate();
            return 1;
        }
        camera.connect();
        
        // Assign the Camera instance as the mqtt callback handler.
        client.set_callback(camera);

I am just struggling to see what the issue is. I am checking for NULL after I attempt to get a value from the CAMERA_NAME environment variable. If it's NULL, I am setting it to a value before I move on.

I just can't tell where it's crashing because the error message is so vague, and if I add any log/cout statements, they won't even run due to this error so I can't tell at what point it's crashing

what is `generate_uuid_v4()` ? Does it return a string by value? Are you storing the pointer to a temporary string in `camera_name_` ? — 463035818_is_not_an_ai, Mar 30 '22 at 06:43
Does this answer your question? [How do I find where an exception was thrown in C++?](https://stackoverflow.com/questions/2443135/how-do-i-find-where-an-exception-was-thrown-in-c) — infinitezero, Mar 30 '22 at 06:46

score 0 · Accepted Answer · answered Mar 30 '22 at 06:56

To see where the error is from you could use a debugger to step through the code line by line to find where something unexpected is happening.

If it's NULL, I am setting it to a value before I move on.

camera_name_ is a pointer. It is not a string. It can point to a c-string. Here:

camera_name_ = generate_uuid_v4().c_str();

You make it point to the internal buffer of the string returned by generate_uuid_v4(). If the function returns the string by value then this string is temporary and ceases to exist at the end of the line. The pointer you stored in camera_name_ is then dangling. If the function would return a reference to a string that is stored elsewhere, keeping a pointer to it would be fine.

Don't use a pointer to a string when you actually want to store a string:

#include <string>
#include <iostream>

const char* getenv() {
    return nullptr;
}

std::string generate_uid() {
    return "asd";
}

int main() {
    std::string name;
    if (auto env = getenv(); env == nullptr){
        name = generate_uid();
    } else {
        name = std::string(env);
    }
    std::cout << name;
}

So I have made some changes, and if I make sure there is a valid CAMERA_NAME env var, I just replaced it with the UUID to test it works, and it does work fine. However, as soon as I removed the env var (so getenv() returns NULL), I get a logic_error again immediately. No log statements get hit or anything so I can't tell where it's going wrong. Trying to get my debugger to work but there are issues locally, in docker its fine. Is there any way to narrow it down? I have wrapped my entire main function in a try/catch to try and narrow down what string is causing the problem, but it does nothing. — Ben Whitely, Mar 31 '22 at 00:09
Nevermind I figured out what it was. In other classes in my program, I was using getenv() on the same environment variable to get the camera name. I thought that if I had not defined that env variable, but in main I handle the scenario where it doesn't exist (set it to something else before the program continues), these other getenv() calls would be fine. But for some reason even though these classes get instantiated later in main after I would have already set the env var, it still crashes — Ben Whitely, Mar 31 '22 at 01:10

score 0 · Answer 2 · answered Mar 30 '22 at 07:12

0

Without more context, one can only guess:

I think this is a logical_error

        std::string camera_name(camera_name_);

if camera_name_ is a nullptr. Which might be what std::getenv returns if the system variable is not found.

If this is the problem, the fix depends on what you really want to do with that variable if the environment variable doesn't exists?

        std::string camera_name(camera_name_?camera_name_:"defaultName");

Or if you are too cool for C++ (non-standard)

        std::string camera_name(camera_name_?:"defaultName");

In fact you are already doing that for std::string ip_address.

answered Mar 30 '22 at 07:12

alfC

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OP is already handling the case of `getenv` returning nullptr, just wrong. `camera_name_ = generate_uuid_v4().c_str();` is most likely not working as they expect it to work – 463035818_is_not_an_ai Mar 30 '22 at 07:23
@463035818_is_not_a_number, you are probably right. Still the error probably comes from that line (string constructor). To your point, I bet `generate_uuid_v4()` is generating a temporary string and `camera_name_` points to garbage later. – alfC Mar 30 '22 at 11:52

Can't find where this std::logic_error is coming from

2 Answers2