iokit / applicationservices cli to change the display brightness

displayBrightness.c

/*
 display-brightness.c
 gcc -o display-brightness display-brightness.c \
 -framework IOKit -framework ApplicationServices
 */

#include <stdio.h>
#include <IOKit/graphics/IOGraphicsLib.h>
#include <ApplicationServices/ApplicationServices.h>

#define PROGNAME "display-brightness"

FILE *displayData;

void
usage(void)
{
	fprintf(stderr, "usage: %s <brightness>\n"
			"    where 0.0 <= brightness <= 1.0\n", PROGNAME);
	exit(1);
}

int
main(int argc, char **argv) 
{
	CGDisplayErr      dErr;
	io_service_t      service;
	CGDirectDisplayID targetDisplay;
	
	CFStringRef key = CFSTR(kIODisplayBrightnessKey);
	float brightness = HUGE_VALF;
	
	switch (argc) {
		case 1:
			break;
			
		case 2:
			brightness = strtof(argv[1], NULL);
			break;
			
		default:
			usage();
			break;
	}
	
	targetDisplay = CGMainDisplayID();
	service = CGDisplayIOServicePort(targetDisplay);
	
	if (brightness != HUGE_VALF) { // set the brightness, if requested
		dErr = IODisplaySetFloatParameter(service, kNilOptions, key, brightness);
	}
	
	// now get the brightness
	dErr = IODisplayGetFloatParameter(service, kNilOptions, key, &brightness);
	if (dErr != kIOReturnSuccess) {
		fprintf(stderr, "operation failed\n");
	} else {
		displayData = fopen("/Library/Application Support/EyeSaver/DisplayBrightness.txt", "w");
		fprintf(displayData, "%f", brightness, "");
		fclose(displayData);
	}
}