多亏了@rustyx，我发现了如何使用xcb + shm来完成这一点。下面是我的一个示例和评论，供那些也在努力寻找它如何工作的人参考：

#include <xcb/shm.h>
#include <xcb/xcb.h>

#include <sys/shm.h>

int main() {

    // Connect to X server
    auto conn = xcb_connect(nullptr, nullptr);

    if (xcb_connection_has_error(conn)) { /* do something on error*/
    }

    // Is needed to access screen/visual data
    const auto setup = xcb_get_setup(conn);

    // Getting screen from xcb
    auto screen = xcb_setup_roots_iterator(setup).data;

    // Generating id for our future shared memory
    auto seg = xcb_generate_id(conn);

    // Allocating shared memory for 1920x1080 screen
    auto shmid = shmget(IPC_PRIVATE, 1920 * 1080 * 4, IPC_CREAT | 0777);

    if (shmid == -1) { /* do something on error*/
    }

    // Attaching our shared memory to xcb, so X server will know what shared
    // memory we are talking about
    xcb_shm_attach(conn, seg, shmid, false);

    // Retrieving pointer to the shared memory and setting it to our buffer
    std::uint8_t *buffer = static_cast<std::uint8_t *>(
        shmat(shmid, nullptr, 0));

    xcb_shm_get_image_cookie_t cookie;

    while (true) {

        // Creating cookie for our image request. 0,0,1920,1080 are x-y
        // coordinates for our screen and width-height for our screen
        cookie = xcb_shm_get_image_unchecked(conn, screen->root, 0, 0, 1920,
                                             1080, ~0,
                                             XCB_IMAGE_FORMAT_Z_PIXMAP, seg, 0);

        // freeing image reply. Needs to be done as xcb says that we need to
        // free it ourselves to avoid memory leaks
        free(xcb_shm_get_image_reply(conn, cookie, nullptr));

        // At this moment, we can do what we want with our image. Image is
        // written into 'buffer' and in our case has length 1920*1080*4. Image
        // is encoded in bgr0, so for every pixel there's 4 memory bytes, 3 for
        // data and 1 for align
    }

    // Detaching shared memory
    shmdt(buffer);
}