How can I append to a string many times?

So I have a loop that generates a sting every time and I want to append this string to an existing variable.

char fullString[] = "start: ";

while(x < 200){

    char someVar[] = "test "

    //append someVar to fullString

    x++;

}

So I would like to end up with a string like this:

start: test test test test test ...

I can do it easily on any other language, just not sure how to do it in c, any ideas?