This gist is intended to be used with autorandr on X11 to automatically switch monitors when some display change event occurs

run_command_on_display_or_lid_change_linux.c

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

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

#include <unistd.h>

static XEvent xEvent;
static Display *XDisplay;
static Screen *XScreen;
static Window XRoot;

static Bool xavaSupportsRR;
static int xavaRREventBase;

static int XScreenNumber;

int init_window_x(void) {
	// connect to the X server
	XDisplay = XOpenDisplay(NULL);
	if(XDisplay == NULL) {
		fprintf(stderr, "cannot open X display\n");
		return 1;
	}

	XScreen = DefaultScreenOfDisplay(XDisplay);
	XScreenNumber = DefaultScreen(XDisplay);
	XRoot = RootWindow(XDisplay, XScreenNumber);

	XStoreName(XDisplay, XRoot, "Dumb wallpaper fixer 9000");
	XSelectInput(XDisplay, XRoot, RRScreenChangeNotifyMask | VisibilityChangeMask | StructureNotifyMask | ExposureMask | KeyPressMask | KeymapNotify);

	// query for the RR extension in X11
	int error;
	xavaSupportsRR = XRRQueryExtension(XDisplay, &xavaRREventBase, &error);
	if(xavaSupportsRR) {
		int rr_major, rr_minor;

		if(XRRQueryVersion(XDisplay, &rr_major, &rr_minor)) {
			int rr_mask = RRScreenChangeNotifyMask;
			if(rr_major == 1 && rr_minor <= 1) {
				rr_mask &= ~(RRCrtcChangeNotifyMask |
					RROutputChangeNotifyMask |
					RROutputPropertyNotifyMask);
			}

			// listen for display configure events only if enabled
			XRRSelectInput(XDisplay, XRoot, rr_mask);
		}
	}
	return 0;
}

void draw_graphical_x(void) {
	XSync(XDisplay, 0);
	return;
}

void cleanup_graphical_x(void) {
	// make sure that all events are dead by this point
	XSync(XDisplay, 1);

	XCloseDisplay(XDisplay);
	return;
}

#ifdef __linux__
char old_lid[128];

// VALUE MUST BE A STATIC 128-byte string
void get_lid_state__linux(char *value) {
	FILE *fp = fopen("/proc/acpi/button/lid/LID/state", "rb");
	if(fp == NULL) { // system has no lid or doesn't ACPI :(
		value[0] = '\0'; // empty string means error
		return;
	}

	int err = fscanf(fp, "state:      %127s", value);
	fclose(fp);

	if(err != 1) {
		value[0] = '\0'; // empty string means error
		return;
	}
}

bool lid_check__linux(void) {
	char result[128];
	get_lid_state__linux(result);

	if(!strcmp(old_lid, result))
		return false;

	strcpy(old_lid, result);
	return true;
}
#endif

void fire_command(int argc, char *argv[]) {
	size_t cmdLength = 0;
	cmdLength += argc;
	for(int i = 1; i<argc; i++) {
		cmdLength += strlen(argv[i]);
	}

	char *command = malloc(cmdLength);
	if(command == NULL) {
		printf("download mor WAM pls");
		exit(EXIT_FAILURE);
	}

	strcpy(command, argv[1]);
	for(int i = 2; i<argc; i++) {
		strcat(command, " ");
		strcat(command, argv[i]);
	}

	system(command);

	free(command);
}

int main(int argc, char *argv[]) {
	init_window_x();
	while(1) {
		if(!XPending(XDisplay)) {
			#ifdef __linux__
				if(lid_check__linux())
					fire_command(argc, argv);
			#endif
			usleep(1000000); // wait 1 second
			continue;
		}
		XNextEvent(XDisplay, &xEvent);
		if(xEvent.type == xavaRREventBase + RRScreenChangeNotify) {
			fire_command(argc, argv);
		}
		draw_graphical_x();
	}

	// this is never run, lol
	cleanup_graphical_x();
	return 0;
}