Set up your computer

/* Open a window and fill it with one color. */
#define SDL_MAIN_USE_CALLBACKS
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>

static SDL_Window *window = NULL;
static SDL_Renderer *renderer = NULL;

/* SDL calls this once, when the program starts. */
SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
{
    if (!SDL_Init(SDL_INIT_VIDEO)) {
        SDL_Log("SDL_Init failed: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    if (!SDL_CreateWindowAndRenderer("Hello, window", 640, 480, 0,
                                     &window, &renderer)) {
        SDL_Log("SDL_CreateWindowAndRenderer failed: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    SDL_SetRenderVSync(renderer, 1);
    return SDL_APP_CONTINUE;
}

/* SDL calls this for each event, such as a click on the close button. */
SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
{
    if (event->type == SDL_EVENT_QUIT) {
        return SDL_APP_SUCCESS;
    }
    return SDL_APP_CONTINUE;
}

/* SDL calls this again and again, once for each frame. */
SDL_AppResult SDL_AppIterate(void *appstate)
{
    SDL_SetRenderDrawColor(renderer, 32, 96, 160, SDL_ALPHA_OPAQUE);
    SDL_RenderClear(renderer);
    SDL_RenderPresent(renderer);
    return SDL_APP_CONTINUE;
}

/* SDL calls this once, when the program ends. */
void SDL_AppQuit(void *appstate, SDL_AppResult result)
{
    /* SDL destroys the window and the renderer for you. */
}

cmake_minimum_required(VERSION 3.24)
project(hello C)

# Use an installed SDL 3.4 or later if there is one. If not, download and build it.
include(FetchContent)
FetchContent_Declare(
    SDL3
    GIT_REPOSITORY https://github.com/libsdl-org/SDL.git
    GIT_TAG release-3.4.18
    GIT_SHALLOW TRUE
    FIND_PACKAGE_ARGS 3.4 CONFIG
)
FetchContent_MakeAvailable(SDL3)

add_executable(hello WIN32 main.c)
target_link_libraries(hello PRIVATE SDL3::SDL3)

/* Open a window and fill it with one color. */
#include <SDL.h>
#include <stdbool.h>

#ifdef __EMSCRIPTEN__
#include <emscripten.h>
#endif

static SDL_Window *window = NULL;
static SDL_Renderer *renderer = NULL;
static bool running = true;

/* The loop calls this again and again, once for each frame. */
static void frame(void)
{
    SDL_Event event;

    while (SDL_PollEvent(&event)) {
        if (event.type == SDL_QUIT) {
            running = false;
        }
    }

    SDL_SetRenderDrawColor(renderer, 32, 96, 160, SDL_ALPHA_OPAQUE);
    SDL_RenderClear(renderer);
    SDL_RenderPresent(renderer);
}

int main(int argc, char *argv[])
{
    if (SDL_Init(SDL_INIT_VIDEO) != 0) {
        SDL_Log("SDL_Init failed: %s", SDL_GetError());
        return 1;
    }

    if (SDL_CreateWindowAndRenderer(640, 480, 0, &window, &renderer) != 0) {
        SDL_Log("SDL_CreateWindowAndRenderer failed: %s", SDL_GetError());
        SDL_Quit();
        return 1;
    }

    SDL_SetWindowTitle(window, "Hello, window");
    SDL_RenderSetVSync(renderer, 1);

#ifdef __EMSCRIPTEN__
    /* A browser does not let a program stay in its own loop. */
    emscripten_set_main_loop(frame, 0, 1);
#else
    while (running) {
        frame();
    }
#endif

    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();
    return 0;
}

cmake_minimum_required(VERSION 3.16)
project(hello C)

find_package(SDL2 REQUIRED CONFIG)

add_executable(hello WIN32 main.c)

# Windows needs SDL2main. The other systems do not have this library.
if(TARGET SDL2::SDL2main)
    target_link_libraries(hello PRIVATE SDL2::SDL2main)
endif()
target_link_libraries(hello PRIVATE SDL2::SDL2)

emcc main.c -sUSE_SDL=3 -o index.html
emrun index.html

emcc main.c -sUSE_SDL=2 -o index.html
emrun index.html