Randr monitor property example

main.c

/* Compile with: gcc main.c -o dpi $(pkg-config --cflags --libs x11 xrandr) */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <assert.h>

#include <X11/Xlib.h>
#include <X11/Xatom.h>
#include <X11/extensions/Xrandr.h>

int main(int argc, char **argv) {
    if(argc != 3) {
        fprintf(stderr, "usage: dpi <monitor> <dpi>\n");
        return 1;
    }

    const char *target_monitor = argv[1];
    const int dpi = atoi(argv[2]);
    if(dpi <= 0) {
        fprintf(stderr, "Error: expected dpi to be an integer value and be above 0\n");
        return 1;
    }

    Display *dpy = XOpenDisplay(NULL);
    assert(dpy);
    const Atom xft_dpi_atom = XInternAtom(dpy, "Xft.dpi", False);
    bool monitor_found = false;
    
    int num_monitors = 0;
    XRRMonitorInfo *monitors = XRRGetMonitors(dpy, DefaultRootWindow(dpy), True, &num_monitors);
    for(int i = 0; i < num_monitors; ++i) {
        char *monitor_name = XGetAtomName(dpy, monitors[i].name);
        if(!monitor_name)
            continue;

        if(strcmp(monitor_name, target_monitor) == 0) {
            monitor_found = true;
            long values = dpi;
            for(int j = 0; j < monitors[i].noutput; ++j) {
                XRRConfigureOutputProperty(dpy, monitors[i].outputs[j], xft_dpi_atom, False, False, 0, NULL);
                XRRChangeOutputProperty(dpy, monitors[i].outputs[j], xft_dpi_atom, XA_INTEGER, 32, PropModeReplace, (const unsigned char*)&values, 1);
            }
        }
        XFree(monitor_name);
    }

    if(monitors)
        XRRFreeMonitors(monitors);

    if(!monitor_found)
        fprintf(stderr, "Error: monitor %s is not a valid active monitor\n", target_monitor);

    XFlush(dpy);
    XCloseDisplay(dpy);
    return 0;
}

Run with: dpi <monitor> <dpi>, for example dpi DP-1 96. The DPI value will then be visible for the monitor in xrandr --properties and can be set with xrandr --output DP-1 --set Xft.dpi 192