Skip to content

Instantly share code, notes, and snippets.

What would you like to do?
OSX global keyboard hook. Requires root privileges.
// alterkeys.c
// 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.
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.
event, kCGKeyboardEventKeycode, (int64_t)keycode);
// We must return the event for it to be useful.
return event;
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");
// Create a run loop source.
runLoopSource = CFMachPortCreateRunLoopSource(
kCFAllocatorDefault, eventTap, 0);
// Add to the current run loop.
CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource,
// Enable the event tap.
CGEventTapEnable(eventTap, true);
// Set it all running.
// In a real program, one would have arranged for cleaning up.
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment