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X11 - Xrandr giving me false monitors

I was trying to find all monitors and their coordinates (width w, height h, x origin/top-left-most x, and y origin/top-left-most y) and was using this code, it works good on some systems. But on other systems I get false and duplicate entries. Would I be able to avoid these dupilcate/false monitor entries if I tested if monitor is mirror? How to test if its mirror?

So this is my code:

// start - get all monitor resolutions
var screen = XRRGetScreenResources(getXOpenDisplay(), getDefaultRootWindow(getXOpenDisplay()));

var noutputs = screen.noutput;

for (var i=noutputs-1; i>=0; i--) {
    var info = XRRGetOutputInfo(getXOpenDisplay(), screen, screen.outputs[i]);
    if (info.connection == RR_Connected) {
        var ncrtcs = info.ncrtc;
        for (var j=ncrtcs-1; j>=0; j--) {
            var crtc_info = XRRGetCrtcInfo(getXOpenDisplay(), screen, infoCrtcs[j]);
            console.info('screen #' + i + ' mon#' + j + ' details:', crtc_info.x, crtc_info.y, crtc_info.width, crtc_info.height);

            collMonInfos.push({
                x: crtc_info.x,
                y: crtc_info.y,
                w: crtc_info.width,
                h: crtc_info.height
            });

            XRRFreeCrtcInfo(crtc_info);
        }
    }
    XRRFreeOutputInfo(info);
}
XRRFreeScreenResources(screen);
console.info('JSON:', JSON.stringify(collMonInfos));
// end - get all monitor resolutions

And this outputs this to log:

"screen #4 mon#0 details:" 0 0 0 0
"screen #3 mon#1 details:" 0 0 1920 1200
"screen #3 mon#0 details:" 1920 469 1366 768
"screen #2 mon#1 details:" 0 0 1920 1200
"screen #2 mon#0 details:" 1920 469 1366 768
"screen #1 mon#1 details:" 0 0 1920 1200
"screen #1 mon#0 details:" 1920 469 1366 768
"screen #0 mon#1 details:" 0 0 1920 1200
"screen #0 mon#0 details:" 1920 469 1366 768

This is it in JSON format:

[{
    "x": 0,
    "y": 0,
    "w": 0,
    "h": 0
}, {
    "x": 0,
    "y": 0,
    "w": 1920,
    "h": 1200
}, {
    "x": 1920,
    "y": 469,
    "w": 1366,
    "h": 768
}, {
    "x": 0,
    "y": 0,
    "w": 1920,
    "h": 1200
}, {
    "x": 1920,
    "y": 469,
    "w": 1366,
    "h": 768
}, {
    "x": 0,
    "y": 0,
    "w": 1920,
    "h": 1200
}, {
    "x": 1920,
    "y": 469,
    "w": 1366,
    "h": 768
}, {
    "x": 0,
    "y": 0,
    "w": 1920,
    "h": 1200
}, {
    "x": 1920,
    "y": 469,
    "w": 1366,
    "h": 768
}]

I really only have 2 monitors, the 1920x1200 one and the 1366x768 one. How come all the other entries and how to test to avoid (rather then filter out in retrospect based on duplicates or 0 h/w)?

7 months ago · Santiago Trujillo

2 answers

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The accepted answer did not work for me. The correct way I found of doing this was this:

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/extensions/Xrandr.h>

int main(void) {
    Display *display = XOpenDisplay(NULL);

    if (NULL == display) {
        perror("No DISPLAY in environment!");
        exit(EXIT_FAILURE);
    }

    Window window = DefaultRootWindow(display);
    XRRScreenResources *screenr = XRRGetScreenResources(display, window);

    // This is the key right here. Use XRRScreenResources::noutput
    int output = screenr->noutput;

    for (int i = 0; i < output; ++i) {
        XRROutputInfo* out_info = XRRGetOutputInfo(display, screenr, screenr->outputs[i]);

        if (NULL != out_info && out_info->connection == RR_Connected) {
            XRRCrtcInfo* crt_info = XRRGetCrtcInfo(display, screenr, out_info->crtc);
            printf("%s\t%dx%d+%d+%d\n", out_info->name, 
                                        crt_info->width, 
                                        crt_info->height,
                                        crt_info->x,
                                        crt_info->y);
            XRRFreeCrtcInfo(crt_info);
        }

        XRRFreeOutputInfo(out_info);
    }

    XRRFreeScreenResources(screenr);
    XCloseDisplay(display);

    return 0;
}

As I said in the code comment, the trick is using XRRScreenResources::noutput instead of XRRScreenResources::ncrtc

7 months ago · Santiago Trujillo Report

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Accepted Answer · 2022-05-10T20:07:14.211Z

You are unnecessarily iterating over each output and then over each monitor. So you receive a duplicate entries. You do not have to call XRRGetOutputInfo for each output, since all data you need (number of monitors) can be found in structure returned by XRRGetScreenResources. Here is the C code that works (at least for me):

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/extensions/Xrandr.h>

int main(void) {
    Display *d = XOpenDisplay(getenv("DISPLAY"));
    Window   w = DefaultRootWindow(d);
    XRRScreenResources *xrrr = XRRGetScreenResources(d, w);
    XRRCrtcInfo *xrrci;
    int i;
    int ncrtc = xrrr->ncrtc;
    for (i = 0; i < ncrtc; ++i) {
        xrrci = XRRGetCrtcInfo(d, xrrr, xrrr->crtcs[i]);
        printf("%dx%d+%d+%d\n", xrrci->width, xrrci->height, xrrci->x, xrrci->y);
        XRRFreeCrtcInfo(xrrci);
    }
    XRRFreeScreenResources(xrrr);
    return 0;
}