gps.c阅读

最新推荐文章于 2022-10-15 10:58:31 发布

原创最新推荐文章于 2022-10-15 10:58:31 发布 · 494 阅读

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#gps #android

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本文详细解析了GPS定位服务的启动流程及运行机制，重点介绍了gps.c中线程的初始化、epoll机制的应用、socketpair的使用，以及如何通过控制线程实现定位数据的读取与应用层数据监听。

读懂gps.c首先需要理解如下几个方法

epoll机制：

https://blog.youkuaiyun.com/yusiguyuan/article/details/15027821

https://blog.youkuaiyun.com/outsinre/article/details/5669764

socketpair的用法和理解

https://blog.youkuaiyun.com/weixin_40039738/article/details/81095013

接下来看代码

static int
gps_state_init( GpsState*  state, GpsCallbacks* callbacks )
{
    ...
    // state->control[0]用于写，state->control[1]用于读
    if ( socketpair( AF_LOCAL, SOCK_STREAM, 0, state->control ) < 0 )  ;
    ...
    // state->monitor_control[0]用于写，state->monitor_control[1]用于读
    if ( socketpair( AF_LOCAL, SOCK_STREAM, 0, state->monitor_control ) < 0 )  ;
    ...
    // 创建并启动state thread
    state->thread = callbacks->create_thread_cb( "gps_state_thread", gps_state_thread, state );
    ...
    // 创建并启动monitor thread
    state->monitor_thread = callbacks->create_thread_cb( "gps_monitor_thread", gps_monitor_thread, state );
}

static void
gps_state_thread(void* arg)
{
    ...
    for (;;) {
        ...
        nevents = epoll_wait( epoll_fd, events, 2, -1 );
        ...
        for (ne = 0; ne < nevents; ne++) {
            ...
            int  fd = events[ne].data.fd;
            if (fd == control_fd) { // 接收父线程write的cmd
                ...
            }else
            if (fd == gps_fd) {
                for (;;) {
                    // 从串口读数据
                    // 将数据拆分并给相应测成员变量赋值
                    // 将数据丢给callback以供应用层数据监听
                    ...             
                }
            }
        }
    }
}

static void
gps_monitor_thread(void* arg)
{
    ...
     while(1)
     {
         ...
         // epoll_fd中有事件会立即返回，否则一直wait，直到timeout 返回值为0，0为超时
         nevents = epoll_wait( epoll_fd, events, 1, timeout);
         if(nevents == 0) //timeout
         {
             if(gps_state == GPS_STATE_ENABLED && (epoll_timeout_ctrl == 1) )
             {
                 chip_to_reset(); // 监控芯片是否有正常出数据，若长时间无数据则重启芯片
                 ALOGI("[%s %d]no nmea > 3s, chip reset", __FUNCTION__, __LINE__);
             }
         }
         ...
     }
}

static void
gps_state_start( GpsState*  s ) {
    ...
    do { ret=write( s->control[0], &cmd, 1 ); }
    ...
    send_cmd_to_monitor(s->monitor_control[0], CMD_START);
    ...
}

static void
gps_state_stop( GpsState*  s ) {
    ...
    send_cmd_to_monitor(s->monitor_control[0], CMD_STOP);
    do { ret=write( s->control[0], &cmd, 1 ); }
    ...
}