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I don't use the CAPS LOCK functionality in OS X, so I disabled the key under the system preferences. Now that key is completely useless to me - ok, I could map the CMD/CTRL/ALT key to that key, but I already got keys for that…

So what I have in mind is to use the LED on the key (I use the recent Apple wireless keyboard) to annouce if I've got mails waiting… something like that. Is it possible to control the LED directly? Controlling it via an objective-c cocoa app is okay, I'll write that app by myself, but until now, I can find anything useful from Apples cocoa documentation.

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I guess that's more or less hardwired into the keyboard firmware, so you'd have to crack that first. –  slhck May 18 '11 at 7:49
    
I use Seil (formerly PCKeyboardHack) to not only disable Caps Lock's default functionality (which is useless) but also to set it to another key, and in turn I use this key heavily in the command line as a generic command key (a sort of overloaded context switching functionality). It's really useful. 2 years later, I started wondering how to control this green LED light that's also on this key that I had forgotten about. –  Steven Lu Jul 21 '14 at 23:52

3 Answers 3

up vote 4 down vote accepted

One Python example: http://www.psychicorigami.com/2009/03/01/5k-morse-code-app-using-capslock-led/

It is possible. I am trying to find an app that I found years ago, that could toggle the num lock and caps lock LEDs on my old iBook (10.4). Found that while looking today.

Found it.

Needs to be compiled though.

Oh, and HID LED test tool – Xcode example :D

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Thanks. As a note: Only the last example (Xcode) works on the current Mac OS X 10.6. –  DASKAjA May 24 '11 at 7:35
    
Well, the last example is listed as 10.5 on Apple's page. Yet the morse code app is based on that code and won't work on 10.6. Don't know about the keyboard_leds one, but I sure hope it does. –  cde May 24 '11 at 7:53

Here's another tool to control keyboard leds under OSX: https://github.com/busyloop/maclight

MacLight lets you control the keyboard-LEDs (capslock, numlock) on your Mac.

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You can manipulate Mac keyboard LEDs with this..

/*
 * keyboard_leds.c
 * Manipulate keyboard LEDs (capslock and numlock) programmatically.
 *
 * gcc -Wall -o keyboard_leds keyboard_leds.c -framework IOKit
 *     -framework CoreFoundation
 *
 * Copyright (c) 2007,2008 Amit Singh. All Rights Reserved.
 *
 *  Redistribution and use in source and binary forms, with or without
 *  modification, are permitted provided that the following conditions
 *  are met:
 *  1. Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *  2. Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *     
 *  THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 *  ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 *  ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
 *  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 *  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 *  OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 *  HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 *  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 *  OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 *  SUCH DAMAGE.
 */

#define PROGNAME "keyboard_leds"
#define PROGVERS "0.1"

#include <stdio.h>
#include <getopt.h>
#include <stdlib.h>
#include <sysexits.h>
#include <mach/mach_error.h>

#include <IOKit/IOCFPlugIn.h>
#include <IOKit/hid/IOHIDLib.h>
#include <IOKit/hid/IOHIDUsageTables.h>

static IOHIDElementCookie capslock_cookie = (IOHIDElementCookie)0;
static IOHIDElementCookie numlock_cookie  = (IOHIDElementCookie)0;
static int capslock_value = -1;
static int numlock_value  = -1;

void         usage(void);
inline void  print_errmsg_if_io_err(int expr, char* msg);
inline void  print_errmsg_if_err(int expr, char* msg);

io_service_t find_a_keyboard(void);
void         find_led_cookies(IOHIDDeviceInterface122** handle);
void         create_hid_interface(io_object_t hidDevice,
                                  IOHIDDeviceInterface*** hdi);
int          manipulate_led(UInt32 whichLED, UInt32 value);

void
usage(void)
{
    fprintf(stderr, "%s (version %s)\n"
      "Copyright (c) 2007,2008 Amit Singh. All Rights Reserved.\n"
      "Manipulate keyboard LEDs\n\n"
      "Usage: %s [OPTIONS...], where OPTIONS is one of the following\n\n"
      "  -c[1|0], --capslock[=1|=0] get or set (on=1, off=0) caps lock LED\n"
      "  -h,      --help            print this help message and exit\n"
      "  -n[1|0], --numlock[=1|=0]  get or set (on=1, off=0) num lock LED\n",
    PROGNAME, PROGVERS, PROGNAME);
}

inline void
print_errmsg_if_io_err(int expr, char* msg)
{
    IOReturn err = (expr);

    if (err != kIOReturnSuccess) {
        fprintf(stderr, "*** %s - %s(%x, %d).\n", msg, mach_error_string(err),
                err, err & 0xffffff);
        fflush(stderr);
        exit(EX_OSERR);
    }
}

inline void
print_errmsg_if_err(int expr, char* msg)
{
    if (expr) {
        fprintf(stderr, "*** %s.\n", msg);
        fflush(stderr);
        exit(EX_OSERR);
    }
}

io_service_t
find_a_keyboard(void)
{
    io_service_t result = (io_service_t)0;

    CFNumberRef usagePageRef = (CFNumberRef)0;
    CFNumberRef usageRef = (CFNumberRef)0;
    CFMutableDictionaryRef matchingDictRef = (CFMutableDictionaryRef)0;

    if (!(matchingDictRef = IOServiceMatching(kIOHIDDeviceKey))) {
        return result;
    }

    UInt32 usagePage = kHIDPage_GenericDesktop;
    UInt32 usage = kHIDUsage_GD_Keyboard;

    if (!(usagePageRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType,
                                        &usagePage))) {
        goto out;
    }

    if (!(usageRef = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType,
                                    &usage))) {
        goto out;
    }

    CFDictionarySetValue(matchingDictRef, CFSTR(kIOHIDPrimaryUsagePageKey),
                         usagePageRef);
    CFDictionarySetValue(matchingDictRef, CFSTR(kIOHIDPrimaryUsageKey),
                         usageRef);

    result = IOServiceGetMatchingService(kIOMasterPortDefault, matchingDictRef);

out:
    if (usageRef) {
        CFRelease(usageRef);
    }
    if (usagePageRef) {
        CFRelease(usagePageRef);
    }

    return result;
}

void
find_led_cookies(IOHIDDeviceInterface122** handle)
{
    IOHIDElementCookie cookie;
    CFTypeRef          object;
    long               number;
    long               usage;
    long               usagePage;
    CFArrayRef         elements;
    CFDictionaryRef    element;
    IOReturn           result;

    if (!handle || !(*handle)) {
        return;
    }

    result = (*handle)->copyMatchingElements(handle, NULL, &elements);

    if (result != kIOReturnSuccess) {
        fprintf(stderr, "Failed to copy cookies.\n");
        exit(1);
    }

    CFIndex i;

    for (i = 0; i < CFArrayGetCount(elements); i++) {

        element = CFArrayGetValueAtIndex(elements, i);

        object = (CFDictionaryGetValue(element, CFSTR(kIOHIDElementCookieKey)));
        if (object == 0 || CFGetTypeID(object) != CFNumberGetTypeID()) {
            continue;
        }
        if (!CFNumberGetValue((CFNumberRef) object, kCFNumberLongType,
                              &number)) {
            continue;
        }
        cookie = (IOHIDElementCookie)number;

        object = CFDictionaryGetValue(element, CFSTR(kIOHIDElementUsageKey));
        if (object == 0 || CFGetTypeID(object) != CFNumberGetTypeID()) {
            continue;
        }
        if (!CFNumberGetValue((CFNumberRef)object, kCFNumberLongType,
                              &number)) {
            continue;
        }
        usage = number;

        object = CFDictionaryGetValue(element,CFSTR(kIOHIDElementUsagePageKey));
        if (object == 0 || CFGetTypeID(object) != CFNumberGetTypeID()) {
            continue;
        }
        if (!CFNumberGetValue((CFNumberRef)object, kCFNumberLongType,
                              &number)) {
            continue;
        }
        usagePage = number;

        if (usagePage == kHIDPage_LEDs) {
            switch (usage) {

            case kHIDUsage_LED_NumLock:
                numlock_cookie = cookie;
                break;

            case kHIDUsage_LED_CapsLock:
                capslock_cookie = cookie;
                break;

            default:
                break;
            }
        }
    }

    return;
}

void
create_hid_interface(io_object_t hidDevice, IOHIDDeviceInterface*** hdi)
{
    IOCFPlugInInterface** plugInInterface = NULL;

    io_name_t className;
    HRESULT   plugInResult = S_OK;
    SInt32    score = 0;
    IOReturn  ioReturnValue = kIOReturnSuccess;

    ioReturnValue = IOObjectGetClass(hidDevice, className);

    print_errmsg_if_io_err(ioReturnValue, "Failed to get class name.");

    ioReturnValue = IOCreatePlugInInterfaceForService(
                        hidDevice, kIOHIDDeviceUserClientTypeID,
                        kIOCFPlugInInterfaceID, &plugInInterface, &score);
    if (ioReturnValue != kIOReturnSuccess) {
        return;
    }

    plugInResult = (*plugInInterface)->QueryInterface(plugInInterface,
                     CFUUIDGetUUIDBytes(kIOHIDDeviceInterfaceID), (LPVOID)hdi);
    print_errmsg_if_err(plugInResult != S_OK,
                        "Failed to create device interface.\n");

    (*plugInInterface)->Release(plugInInterface);
}

int
manipulate_led(UInt32 whichLED, UInt32 value)
{
    io_service_t           hidService = (io_service_t)0;
    io_object_t            hidDevice = (io_object_t)0;
    IOHIDDeviceInterface **hidDeviceInterface = NULL;
    IOReturn               ioReturnValue = kIOReturnError;
    IOHIDElementCookie     theCookie = (IOHIDElementCookie)0;
    IOHIDEventStruct       theEvent;

    if (!(hidService = find_a_keyboard())) {
        fprintf(stderr, "No keyboard found.\n");
        return ioReturnValue;
    }

    hidDevice = (io_object_t)hidService;

    create_hid_interface(hidDevice, &hidDeviceInterface);

    find_led_cookies((IOHIDDeviceInterface122 **)hidDeviceInterface);

    ioReturnValue = IOObjectRelease(hidDevice);
    if (ioReturnValue != kIOReturnSuccess) {
        goto out;
    }

    ioReturnValue = kIOReturnError;

    if (hidDeviceInterface == NULL) {
        fprintf(stderr, "Failed to create HID device interface.\n");
        return ioReturnValue;
    }

    if (whichLED == kHIDUsage_LED_NumLock) {
        theCookie = numlock_cookie;
    } else if (whichLED == kHIDUsage_LED_CapsLock) {
        theCookie = capslock_cookie;
    }

    if (theCookie == 0) {
        fprintf(stderr, "Bad or missing LED cookie.\n");
        goto out;
    }

    ioReturnValue = (*hidDeviceInterface)->open(hidDeviceInterface, 0);
    if (ioReturnValue != kIOReturnSuccess) {
        fprintf(stderr, "Failed to open HID device interface.\n");
        goto out;
    }

    ioReturnValue = (*hidDeviceInterface)->getElementValue(hidDeviceInterface,
                                               theCookie, &theEvent);
    if (ioReturnValue != kIOReturnSuccess) {
        (void)(*hidDeviceInterface)->close(hidDeviceInterface);
        goto out;
    }

    fprintf(stdout, "%s\n", (theEvent.value) ? "on" : "off");
    if (value != -1) {
        if (theEvent.value != value) {
            theEvent.value = value;
            ioReturnValue = (*hidDeviceInterface)->setElementValue(
                                hidDeviceInterface, theCookie,
                                &theEvent, 0, 0, 0, 0);
            if (ioReturnValue == kIOReturnSuccess) {
                fprintf(stdout, "%s\n", (theEvent.value) ? "on" : "off");
            }
        }
    }

    ioReturnValue = (*hidDeviceInterface)->close(hidDeviceInterface);

out:
    (void)(*hidDeviceInterface)->Release(hidDeviceInterface);

    return ioReturnValue;
}

static const char *options = "c::hn::";
static struct option
long_options[] = {
    { "capslock", optional_argument, 0, 'c' },
    { "help",     no_argument,       0, 'h' },
    { "numlock",  optional_argument, 0, 'n' },
    { 0, 0, 0, 0 },
};

int
main (int argc, char **argv)
{
    UInt32 whichLED = (UInt32)0;
    int c, ctr = 0;
    int target_value = -1;

    while ((c = getopt_long(argc, argv, options, long_options, NULL)) != -1) {

        switch (c) {

        case 0:
            break;

        case 'c':
            ctr++;
            whichLED = kHIDUsage_LED_CapsLock;
            if (optarg) {
                capslock_value = atoi(optarg);
                target_value = (capslock_value);
            }
            break;

        case 'h':
            usage();
            exit(0);
            break;

        case 'n':
            ctr++;
            whichLED = kHIDUsage_LED_NumLock;
            if (optarg) {
                numlock_value = atoi(optarg);
                target_value = (numlock_value);
            }
            break;

        default:
            usage();
            exit(1);
            break;
        }
    }

    if (ctr != 1) {
        fprintf(stderr, "Missing options or invalid combination of options. "
                        "Try -h for help.\n");
        exit(1);
    }

    return manipulate_led(whichLED, target_value);
}

To compile and use this script

gcc -Wall -o keyboard_leds keyboard_leds.c \
    -framework IOKit -framework CoreFoundation
./keyboard_leds -h
./keyboard_leds -c # query caps lock LED state
./keyboard_leds -c1 # turn caps lock LED on
./keyboard_leds -c0 # turn caps lock LED off
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