OSX global keyboard hook. Requires root privileges.

Global keyboard hook for OSX

// alterkeys.c
// http://osxbook.com
//
// Complile using the following command line:
//     gcc -Wall -o alterkeys alterkeys.c -framework ApplicationServices
//
// You need superuser privileges to create the event tap, unless accessibility
// is enabled. To do so, select the "Enable access for assistive devices"
// checkbox in the Universal Access system preference pane.

#include <ApplicationServices/ApplicationServices.h>

// This callback will be invoked every time there is a keystroke.
//
CGEventRef
myCGEventCallback(CGEventTapProxy proxy, CGEventType type,
                  CGEventRef event, void *refcon)
{
    // Paranoid sanity check.
    if ((type != kCGEventKeyDown) && (type != kCGEventKeyUp))
        return event;
    
    // The incoming keycode.
    CGKeyCode keycode = (CGKeyCode)CGEventGetIntegerValueField(
                                                               event, kCGKeyboardEventKeycode);
    
    // Swap 'a' (keycode=0) and 'z' (keycode=6).
    if (keycode == (CGKeyCode)0)
        keycode = (CGKeyCode)6;
    else if (keycode == (CGKeyCode)6)
        keycode = (CGKeyCode)0;
    
    // Set the modified keycode field in the event.
    CGEventSetIntegerValueField(
                                event, kCGKeyboardEventKeycode, (int64_t)keycode);
    
    // We must return the event for it to be useful.
    return event;
}

int
main(void)
{
    CFMachPortRef      eventTap;
    CGEventMask        eventMask;
    CFRunLoopSourceRef runLoopSource;
    
    // Create an event tap. We are interested in key presses.
    eventMask = ((1 << kCGEventKeyDown) | (1 << kCGEventKeyUp));
    eventTap = CGEventTapCreate(kCGSessionEventTap, kCGHeadInsertEventTap, 0,
                                eventMask, myCGEventCallback, NULL);
    if (!eventTap) {
        fprintf(stderr, "failed to create event tap\n");
        exit(1);
    }
    
    // Create a run loop source.
    runLoopSource = CFMachPortCreateRunLoopSource(
                                                  kCFAllocatorDefault, eventTap, 0);
    
    // Add to the current run loop.
    CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource,
                       kCFRunLoopCommonModes);
    
    // Enable the event tap.
    CGEventTapEnable(eventTap, true);
    
    // Set it all running.
    CFRunLoopRun();
    
    // In a real program, one would have arranged for cleaning up.
    
    exit(0);
}

1. gcc -Wall -o alterkeys alterkeys.c -framework ApplicationServices
2. sudo ./alterkeys
3. input a or z to test it!

does not work on 10.13 High Sierra, pity.