SDL Snake放慢速度？

Question

I've been working on a clone of Snake using SDL but I've found that as I run the game it slows down. The snake starts moving quite fast but after a few turn it slows down quite a bit. I've been trying to figure out why but can't. :( I assume it's got something to do with either the way I've tried to implement FPS or the update game function (where key handling takes place).

Here's where I'm trying to implement FPS:

void run(){
    int SKIP_TICKS;;
    long next_game_tick = time(0);
    long sleep_time = 0;
    std::cout<<next_game_tick<<std::endl;

    while (!quit){
        SKIP_TICKS = 1000 / fps;

        updateGame();
        render();

        next_game_tick += SKIP_TICKS;
        sleep_time = next_game_tick - time(0);
        sleep_time *= 10;
        usleep(sleep_time);
    }
}

And here is my update game function:

void updateGame(){
    SDL_Event event;

    if (!isFrutActive){
        doNewFruit();
    }

    while (SDL_PollEvent(&event)){
        if (event.type == SDL_QUIT){
            quit = true;
        }
        if (event.type == SDL_KEYDOWN){//Get keyboard input
            switch (event.key.keysym.sym){
                case SDLK_ESCAPE:   quit = true; break;

                case SDLK_UP:
                case SDLK_w:
                    ydir = -SPEED;
                    xdir = 0;
                    break;

                case SDLK_DOWN:
                case SDLK_s:
                    ydir = SPEED;
                    xdir = 0;
                    break;

                case SDLK_LEFT:
                case SDLK_a:
                    xdir = -SPEED;
                    ydir = 0;
                    break;

                case SDLK_RIGHT:
                case SDLK_d:
                    xdir = SPEED;
                    ydir = 0;
                    break;
            }
        }
    }

Any help would be much appreciated. Thanks.

Answer 1

I do believe that I've found a solution to the problem with sleep_time being set to a negative when I press the X. Here's my new code:

void run(){
    int SKIP_TICKS;
    long next_game_tick = SDL_GetTicks();
    long sleep_time = 0;

    while (!quit){
        SKIP_TICKS = 1000 / fps;

        updateGame();
        render();
        next_game_tick += SKIP_TICKS;
        sleep_time = next_game_tick - SDL_GetTicks();
        std::cout<<sleep_time<<std::endl;
        sleep_time *= 1000;
        if (sleep_time >= 0){
            usleep(sleep_time);
        }
        else{
            next_game_tick += SKIP_TICKS;
        }
    }
}

By using the if statement I avoid using usleep(sleep_time) if sleep_time is below zero. Thanks a lot to the people who helped me! :D