get Linux Monitor resolution with C

how can i get monitor resolution in Fedora Linux with C,
I need to know the screen size in my GTK3 application but I can't find a solution
Because I want to make a full screen software

And for this I need to know the screen size in pixels

How do desktops like gnome built with gtk find the screen size ?

I used this Code but it gave me an error :

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

unsigned short *get_screen_size(void)
{
    static unsigned short size[2];
    char *array[8];
    char screen_size[64];
    char* token = NULL;

    FILE *cmd = popen("xdpyinfo | awk '/dimensions/ {print $2}'", "r");

    if (!cmd)
        return 0;

    while (fgets(screen_size, sizeof(screen_size), cmd) != NULL);
    pclose(cmd);

    token = strtok(screen_size, "x\n");

    if (!token)
        return 0;

    for (unsigned short i = 0; token != NULL; ++i) {
        array[i] = token;
        token = strtok(NULL, "x\n");
    }
    size[0] = atoi(array[0]);
    size[1] = atoi(array[1]);
    size[2] = -1;

    return size;
}


int main(void)
{
    unsigned short *size = get_screen_size();

    printf("Screen resolution = %dx%d\n", size[0], size[1]);

    return 0;
}

when i run it :

sh: line 1: xdpyinfo: command not found
Segmentation fault (core dumped)

In GTK3 (GDK3 3.22+), gdk_monitor_get_geometry can be used to populate a GdkRectangle structure with the width and height of a monitor.

Alternatively, there is gdk_monitor_get_workarea, which functions much the same, but takes into consideration docks, panels, and other desktop components.

See also:

• gdk_window_fullscreen | gtk_window_fullscreen
• gdk_window_fullscreen_on_monitor | gtk_window_fullscreen_on_monitor
• gdk_window_unfullscreen | gtk_window_unfullscreen

#include <gtk/gtk.h>
#include <stdio.h>

static void
activate(GtkApplication* app, gpointer user_data)
{
	GdkDisplay *display = gdk_display_get_default();
	int n_monitors = gdk_display_get_n_monitors(display);

	for (int nth = 0; nth < n_monitors; nth++) {
		GdkMonitor *monitor = gdk_display_get_monitor(display, nth);

		GdkRectangle geo;
		gdk_monitor_get_geometry(monitor, &geo);

		printf("(#%d) %-5s %-10s | Scale: %2d | %5dx%-5d | Position: %6dx%-6d%s\n",
				nth,
				gdk_monitor_get_manufacturer(monitor),
				gdk_monitor_get_model(monitor),
				gdk_monitor_get_scale_factor(monitor),
				geo.width,
				geo.height,
				geo.x,
				geo.y,
				gdk_monitor_is_primary(monitor) ? " (Primary)" : "");
	}
}

int
main(int argc, char **argv)
{
	GtkApplication *app = gtk_application_new("com.example.monitorsize", G_APPLICATION_DEFAULT_FLAGS);
	g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);

	int status = g_application_run(G_APPLICATION(app), argc, argv);
	g_object_unref(app);

	return status;
}

Example output:

(#0) AOC   DVI-I-1    | Scale:  1 |  1920x1080  | Position:   1440x0      (Primary)
(#1) DEL   DP-1       | Scale:  1 |  1280x1024  | Position:   3360x0     
(#2) SAM   DVI-D-0    | Scale:  1 |  1440x900   | Position:      0x0

The (now deprecated) functions gdk_screen_width and gdk_screen_height retrieve the dimensions of the default screen, but this information is rather uninteresting when there are multiple monitors.

printf("%dx%d\n", gdk_screen_width(), gdk_screen_height());

4640x1080

The problem here is that the xdpyinfo command was not found ( not installed ).

The package that contains the xdpyinfo command on fedora is the xorg-x11-utils package.

To install the above package you can use the following command:

yum install xorg-x11-utils

You can use the X11 library if you're using linux.

// Declare includes.
#include <stdio.h>
#include <X11/Xlib.h>

int main (void)
{
// Declare variables.
    Display *dpy = XOpenDisplay(NULL);
    XWindowAttributes wa = {0};

// Get screen attributes.
    XGetWindowAttributes(dpy, DefaultRootWindow(dpy), &wa);

// Print values.
    printf("Width: %d\tHeight: %d\n", wa.width, wa.height);

// Exit program.
    return(0);
}

Keep in mind this is for systems with one screen only.

EDIT: You may need the package :libX11-devel