monitors.c

/**
 * Compile it: gcc monitors.c -lX11 -lXrandr -Wall -Wextra -Werror -std=c89 -O3 -pedantic-errors -o monitors
 * Run it: ./monitors
 */

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

int main(void)
{
    char    *dpy;
    Display *xdpy;
    Window   root;
    XRRMonitorInfo *monitors;
    int total;
    int i;
    char *m_name;

    dpy = getenv("DISPLAY");
    xdpy = XOpenDisplay(dpy);

    if (!xdpy) {
        fprintf(stderr, "Unable to open X display '%s': %s\n",
                                        dpy, strerror(errno));
        return 1;
    }
    root = DefaultRootWindow(xdpy);
    monitors = XRRGetMonitors(xdpy, root, True, &total);

    if (total == -1) {
        fprintf(stderr, "XRRGetMonitors() failed: %s\n", strerror(errno));
        XCloseDisplay(xdpy);
        return 1;
    }
    printf("Number of connected monitors: %d\n", total);

    for (i = 0; i < total; i++) {
        m_name = XGetAtomName(xdpy, monitors[i].name);
        printf("Monitor %d: %s%s\n", i + 1, m_name,
                             monitors[i].primary ? " (Primary)" : "");
        XFree(m_name);
    }
    XRRFreeMonitors(monitors);
    XCloseDisplay(xdpy);

    return 0;
}