Can't Close Window | 易学教程

问题

I'm following the Lazy Foo tutorial on C++ and SDL2. I'm trying to learn it using regular C and noticed something interesting when following instructions on adding events to detect a close window event.

Here is the code.

#include <stdio.h>
#include <stdbool.h>
#include "SDL2/SDL.h"

bool init();
bool loadMedia();
void close();

const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;

SDL_Window *gWindow = NULL;
SDL_Surface *gScreenSurface = NULL;
SDL_Surface *gHelloWorld = NULL;

int main(int argc, char *argv[])
{
    if(!init()) {
        printf("Failed to initialize!\n");
    }
    else {
        if(!loadMedia()) {
            printf("Failed to load media!\n");
        }
        else {
            bool quit = false;
            SDL_Event e;

            while(!quit) {
                printf("%d", SDL_PollEvent(&e));

                while(SDL_PollEvent(&e) != 0) {
                    if(e.type == SDL_QUIT) {
                        quit = true;
                    }
                }   

                SDL_BlitSurface(gHelloWorld, NULL, gScreenSurface, NULL);
                SDL_UpdateWindowSurface(gWindow);
            }
        }
    }       

    close();

    return 0;
}

bool init()
{
    bool success = true;

    if(SDL_Init(SDL_INIT_VIDEO) < 0) {
        printf("SDL could not initialize! SDL_Error: %s\n", SDL_GetError());
        success = false;
    } 
    else {
        gWindow = SDL_CreateWindow("SDL Tutorial 03", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN);

        if(gWindow == NULL) {
            printf("Window could not be created! SDL_Error: %s\n", SDL_GetError());
            success = false;
        } 
        else {
            gScreenSurface = SDL_GetWindowSurface(gWindow);
        }
    }

    return success;
}

bool loadMedia()
{
    bool success = true;

    gHelloWorld = SDL_LoadBMP("images/hello_world.bmp");

    if(gHelloWorld == NULL) {
        printf("Unable to load image %s! SDL_Error: %s\n", "images/hello_world.bmp", SDL_GetError());
        success = false;
    }

    return success;
}

void close()
{
    SDL_FreeSurface(gHelloWorld);
    gHelloWorld = NULL;

    SDL_DestroyWindow(gWindow);
    gWindow = NULL;

    SDL_Quit();
}

If I compile this with a ".c" extension, it compiles without errors, but selecting the "X" on the window title bar does nothing. If I change said extension to ".cpp", the "X" works as intended.

I'm using the following command to compile the code.

gcc main.c -w -lSDL2 -o main

Any ideas why this may work with C++, but not with C?

回答1:

The function SDL_PollEvent will remove the event from the internal event queue if an address of an SDL_event object is passed to it.

The printf call, that also calls the function SDL_PollEvent, before the event loop which will remove the quit event from the queue. This means the event loop won't find this event:

printf("%d", SDL_PollEvent(&e));

while(SDL_PollEvent(&e) != 0) {
    if(e.type == SDL_QUIT) {
        quit = true;
    }
}

If you only want to check if there are events pending in the queue, then use the function SDL_PollEvent with a NULL argument:

printf("%d", SDL_PollEvent(NULL));

来源：https://stackoverflow.com/questions/39562704/cant-close-window

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c++

sdl