使用SDL

• 创建简单的线程

把一个函数作为SDL_CreateThread()的参数就可以创建一个线程。如果调用成功，该函数就 开始并行运行了，使用自己的栈、寄存器等上下文（context），并能象程序其他部分一样访问内存和文件句柄。

#include "SDL_thread.h"

int global_data = 0;

int thread_func(void *unused)
{
    int last_value = 0;

    while ( global_data != -1 ) {
        if ( global_data != last_value ) {
            printf("数据改变为 %d\n", global_data);
            last_value = global_data;
        }
        SDL_Delay(100);
    }
    printf("线程退出\n");
    return(0);
}

{
    SDL_Thread *thread;
    int i;

    thread = SDL_CreateThread(thread_func, NULL);
    if ( thread == NULL ) {
        fprintf(stderr, "无法创建线程： %s\n", SDL_GetError());
        return;
    }

    for ( i=0; i<5; ++i ) {
        printf("更改数据为 %d\n", i);
        global_data = i;
        SDL_Delay(1000);
    }

    printf("通知线程退出\n");
    global_data = -1;
    SDL_WaitThread(thread, NULL);
}

• 对资源访问进行同步

通过创建互斥体（mutex），并将资源用lock (SDL_mutexP())和unlock (SDL_mutexV())保护起来，就可以防止多个线程同时访问该资源。

#include "SDL_thread.h"
#include "SDL_mutex.h"

int potty = 0;
int gotta_go;

int thread_func(void *data)
{
    SDL_mutex *lock = (SDL_mutex *)data;
    int times_went;

    times_went = 0;
    while ( gotta_go ) {
        SDL_mutexP(lock);    /* 锁住potty */
        ++potty;
        printf("线程%d 正在使用potty\n", SDL_ThreadID());
        if ( potty > 1 ) {
            printf("哦，有人在用potty!\n");
        }
        --potty;
        SDL_mutexV(lock);
        ++times_went;
    }
    printf("好了\n");
    return(times_went);
}

{
    const int progeny = 5;
    SDL_Thread *kids[progeny];
    SDL_mutex  *lock;
    int i, lots;

    /* 创建同步锁 */
    lock = SDL_CreateMutex();

    gotta_go = 1;
    for ( i=0; i<progeny; ++i ) {
        kids[i] = SDL_CreateThread(thread_func, lock);
    }

    SDL_Delay(5*1000);
    SDL_mutexP(lock);
    printf("都完成了吗？\n");
    gotta_go = 0;
    SDL_mutexV(lock);

    for ( i=0; i<progeny; ++i ) {
        SDL_WaitThread(kids[i], &lots);
        printf("线程 %d 用过potty %d 次了\n", i+1, lots);
    }
    SDL_DestroyMutex(lock);
}