Makefile

LDFLAGS = -framework IOKit -framework ApplicationServices
test: test.m

test.m

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

const int kMaxDisplays = 256;

int get_display_count(CGDirectDisplayID displays[]) {
	CGDisplayErr err;
	CGDisplayCount count;
	err = CGGetActiveDisplayList(kMaxDisplays, displays, &count);
	if (err != CGDisplayNoErr) {
		return -1;
	}
	return count;
}

void print_displays(CGDirectDisplayID displays[], int count) {
	printf("display count:%d\n", count);
	for (CGDisplayCount i=0; i < count; ++i) {
		CGDirectDisplayID disp = displays[i];
		CFDictionaryRef origMode = CGDisplayCurrentMode(disp);
		if (origMode == NULL)
			printf("origMode == NULL\n");
		printf("display[%d]:", i);
		if (CGMainDisplayID() == disp)
			printf("(main display)");
		printf("ID 0x%x\n", (unsigned int)disp);
	}
}

float get_display_brightness(CGDirectDisplayID display) {
	float brightness = -1.0;
	io_service_t service = CGDisplayIOServicePort(CGMainDisplayID());
	CGDisplayErr err = IODisplayGetFloatParameter(service, kNilOptions,
			CFSTR(kIODisplayBrightnessKey), &brightness);
	if (err != kCGErrorSuccess) {
		fprintf(stderr, "error while get display birghtness: %d\n", err);
	}
	else
		printf("display 1 brightness: %f\n", brightness);

	return 0;
}

int main(int argc, char *argv[]) {
	CGDirectDisplayID displays[kMaxDisplays];
	int count = get_display_count(displays);
	if (count == -1) {
		fprintf(stderr, "error occured while get display count\n");
		return 1;
	}
	else
		print_displays(displays, count);
	get_display_brightness(displays[0]);

	return 0;
}