如何通过xcb RandR扩展获得RandR输出的分辨率？

我正在开发一个已经使用xcb的项目,需要获得单个输出的分辨率而不是组合屏幕的分辨率.我可以用xcb的RandR扩展吗？如果是这样,我如何使用我的xcb_connection_t对象来执行此操作.

解决方法:

我也在寻找答案.我不知道你是否成功,但既然你没有回答这个问题,我猜你没有.

对于那些正在寻找如何做到这一点的人,您必须了解XCB通常如何运作.这对RandR XCB扩展尤为重要.您可能知道要从X服务器获取一些信息,您需要先使用请求进行查询.如果在发送请求后XCB,您将获得一个存储已发送请求的ID的cookie.你为什么需要这样的饼干？原因很简单也很明显 – 要从X服务器获得回复,我们需要使用我们发送的请求的ID来询问它(因此服务器知道您想要答案的请求).

为了使它更清晰,这是日常生活中的一个简单例子 – 想象一个记忆极短的酒保,所以它是这样的：

client   : "I'd like to order a beer."
bartender: "OK, I'll get you a beer."
*bartender handles a ticket with an ID of the beer order to the client*
*client waits and waits and then he approaches bartender*
client   : "Dude, give me my beer finally, here's your damn ticket"
*client handles the ticket to bartender, who looks for the order from the 
 ticket in his order book and when he finds it he replies*
bartender: "Here's your beer, enjoy."

好的,那么为什么XCB不能简单地发送请求并一次性得到答案呢？嗯,这正是Xlib的工作原理,事实证明,在某些条件下,XCB的速度提高了117倍.如果您想了解更多信息,请阅读XCB教程中的Basic XCB Notions.

回到问题 – 如何获得输出(或更确切地说CRTC)的分辨率？这是我如何做的最小化版本：

#include <cstdio>
#include <xcb/xcb.h>
#include <xcb/randr.h>

int main()
{
    //Open connection to X server
    xcb_connection_t* XConnection = xcb_connect(0, 0);

    //Get the first X screen
    xcb_screen_t* XFirstScreen = xcb_setup_roots_iterator(
                               xcb_get_setup(XConnection)).data;
    //Generate ID for the X window
    xcb_window_t XWindowDummy = xcb_generate_id(XConnection);

    //Create dummy X window
    xcb_create_window(XConnection, 0, XWindowDummy, XFirstScreen->root,
                      0, 0, 1, 1, 0, 0, 0, 0, 0);

    //Flush pending requests to the X server
    xcb_flush(XConnection);

    //Send a request for screen resources to the X server
    xcb_randr_get_screen_resources_cookie_t screenResCookie = {};
    screenResCookie = xcb_randr_get_screen_resources(XConnection, 
                                                     XWindowDummy);

    //Receive reply from X server
    xcb_randr_get_screen_resources_reply_t* screenResReply = {};
    screenResReply = xcb_randr_get_screen_resources_reply(XConnection,
                     screenResCookie, 0);

    int crtcs_num = 0;
    xcb_randr_crtc_t* firstCRTC;

    //Get a pointer to the first CRTC and number of CRTCs
    //It is crucial to notice that you are in fact getting
    //an array with firstCRTC being the first element of
    //this array and crtcs_length - length of this array
    if(screenResReply)
    {
        crtcs_num = xcb_randr_get_screen_resources_crtcs_length(screenResReply);

        firstCRTC = xcb_randr_get_screen_resources_crtcs(screenResReply);
    }
    else
        return -1;

    //Array of requests to the X server for CRTC info
    xcb_randr_get_crtc_info_cookie_t* crtcResCookie = new   
               xcb_randr_get_crtc_info_cookie_t[crtcs_num];
    for(int i = 0; i < crtcs_num; i++)
        crtcResCookie[i] = xcb_randr_get_crtc_info(XConnection, 
                                            *(firstCRTC+i), 0);
    //Array of pointers to replies from X server
    xcb_randr_get_crtc_info_reply_t** crtcResReply = new 
               xcb_randr_get_crtc_info_reply_t*[crtcs_num];
    for(int i = 0; i < crtcs_num; i++)
        crtcResReply[i] = xcb_randr_get_crtc_info_reply(XConnection,
                                             crtcResCookie[i], 0);
    //Self-explanatory
    for(int i = 0; i < crtcs_num; i++)
    {
        if(crtcResReply[i])
        {
            printf("CRTC[%i] INFO:\n", i);
            printf("x-off\t: %i\n", crtcResReply[i]->x);
            printf("y-off\t: %i\n", crtcResReply[i]->y);
            printf("width\t: %i\n", crtcResReply[i]->width);
            printf("height\t: %i\n\n", crtcResReply[i]->height);
        }
    }

    xcb_disconnect(XConnection);

    return 0;
}

示例输出：

CRTC[0] INFO:
x-off   : 1920
y-off   : 0
width   : 1920
height  : 1200

CRTC[1] INFO:
x-off   : 0
y-off   : 0
width   : 1920
height  : 1200

CRTC[2] INFO:
x-off   : 0
y-off   : 0
width   : 0
height  : 0

CRTC[3] INFO:
x-off   : 0
y-off   : 0
width   : 0
height  : 0

如果你想要的话,可以使用vector和whatnot来提高代码的可读性,而不是使用指针进行动态分配,但我认为这并不难理解.

我还是XCB编程的新手,而且PROBABLY是一种更好的方法,但这种方法看起来并不差.我花了很多时间试图理解这一点,所以我希望它会帮助别人.

有用的链接：

> XCB Tutorial(建议找到你的路)
> XCB API(非常有用,在这种情况下特别是RandR API)

标签：c-3,linux,x11,xcb,xrandr
来源： https://codeday.me/bug/20190528/1174760.html