How to manage global variables in C asynchronous programming?

Question

I'm using Vimba.c API, specifically the AsynchronousGrab example provided in the documentation.

Basically, AsynchronousGrab is a program that asynchronously captures an image. I've customized it to work on a user-sent trigger, which means I have my main code running "on the front", and AsynchronousGrab continuously streaming the camera feed "in the back". Whenever I send a trigger through the main, the asynchronous grab captures one frame, processes it, and continue with the streaming.

I already also edited the AsynchronousGrab.c to do a custom processing step to the captured frame, and then prints the result on the console.

The generic code from the program.c (main) is this:

        while (1) {
            f̶f̶l̶u̶s̶h̶(̶s̶t̶d̶i̶n̶)̶;̶ 
            printf("\n Insert <c> to capture a frame or <e> to stop acquisition and press ENTER\n\n");
            char_in = getchar();
            if (char_in == 'c') {
                err = VmbFeatureCommandRun(_cameraHandle, "TriggerSoftware");
                if (VmbErrorSuccess == err)
                {
                    printf("Software Trigger sent\n");
                }
                else
                {
                    printf("Error: %s\n", ErrorCodeToMessage(err));
                }
            }
            else if (char_in == 'e') {
                printf("Initializing closing routine\n");
                break;
            }
            else {
                printf("Invalid input\n");
            }
            char_in = getchar();    //consumes remaining '\n' from stdin
        }

Now, I'd like to update it to allow the image captured (and processed) to be saved.

            else if (char_in == 's') {
                /* some code to signal the save*/
                err = VmbFeatureCommandRun(_cameraHandle, "TriggerSoftware");
                if (VmbErrorSuccess == err)
                {
                    printf("Sotware Trigger sent. Frame %d will be saved.\n", frameCounter);
                }
                else
                {
                    printf("Error: %s\n", ErrorCodeToMessage(err));
                }

            }

My problem is, I'm unexperienced with this level of coding, and I cannot understand how do I send a flag to conditionally save to another .c file.

My first thought was "let's pass a variable by reference". But I don't know exactly how does VmbFeatureCommandRun() work. It's not a function from AsynchronousGrab.c (the file I edited to add my processing). It's a function from Vimba.c library instead. So I would have to edit both the library and the AsynchronousGrab.c.

Then I thought about "ok, if not by reference, let's make a global variable". I searched and found about extern declarations. Great, but it's an asynchronous program. If the processing of the first capture takes longer, I might be sending another trigger while the first one is still processing, so I might mess up the flag value for either of them. So I thought about having a buffer, i.e., an array of 3 flags, but then I should also have a global array of frame identifiers, to match the flag (and the procesing result, and the filename) to each identifier.

Now I'm thinking there might be an easier way of doing this, and I'm over-complicating it, and probably not even considering other factors that wouldn't allow my solution to work.

So, should I be creating two global arrays or not? If yes, do I just declare them as global in program.c and extern in AsynchronousGrab.c? And then find a way of always matching everything, on both sides, through the frame identifier?