CGEventPost - possible bug when simulating keyboard events?

I have found a reliable way to post modified keyboard events - it does not follow the example in Apple's documentation (which doesn't work) but seems to make sense, and most importantly, WORKS.

Rather than sending 'shift key down' and 'shift key up' messages (as instructed in the docs), you need to set a modifier flag on the keypress. Here's how to output an uppercase Z.

CGEventRef event1, event2;
event1 = CGEventCreateKeyboardEvent(NULL, (CGKeyCode)6, true);//'z' keydown event
CGEventSetFlags(event1, kCGEventFlagMaskShift);//set shift key down for above event
CGEventPost(kCGSessionEventTap, event1);//post event

I'm then releasing the 'z' key for completeness (also setting the shift-flag on, though not sure if this is correct).

event2 = CGEventCreateKeyboardEvent(NULL, (CGKeyCode)6, false);
CGEventSetFlags(event2, kCGEventFlagMaskShift);
CGEventPost(kCGSessionEventTap, event2);

Finally (and bizarrely) you DO need to send the 'key up' event for the shift key:

  e5 = CGEventCreateKeyboardEvent(NULL, (CGKeyCode)56, false);
CGEventPost(kCGSessionEventTap, e5);

Don't forget to release your events once you're done with them.

I hope this is useful to someone - it took me a long time to get this to work.

The cleanest way for this is bitwise OR'ing the current modifier flags with the flag of your desired modifier(s) , e.g.:

CGEventFlags flags = kCGEventFlagMaskShift;
CGEventRef ev;
CGEventSourceRef source = CGEventSourceCreate (kCGEventSourceStateCombinedSessionState);

//press down            
ev = CGEventCreateKeyboardEvent (source, keyCode, true);    
CGEventSetFlags(ev,flags | CGEventGetFlags(ev)); //combine flags                        
CGEventPost(kCGHIDEventTap,ev);
CFRelease(ev);              

//press up                                  
ev = CGEventCreateKeyboardEvent (source, keyCode, false);                       
CGEventSetFlags(ev,flags | CGEventGetFlags(ev)); //combine flags                        
CGEventPost(kCGHIDEventTap,ev); 
CFRelease(ev);              

CFRelease(source);

Like another comment referring to other solutions here, after using uppercase with the Shift mask, a successive call would cause any further intended non-shifted character to be turned into an shifted characters. I figured the call to CGEventCreateKeyboardEvent was somehow saving previous masks, so I purposefully clear the mask of the modifier:

CGEventRef event1, event2;

CGEventSourceRef source = CGEventSourceCreate (kCGEventSourceStateCombinedSessionState);
CGEventFlags flags;

event1 = CGEventCreateKeyboardEvent (source, keyCode, true);
if (upper)
    flags = kCGEventFlagMaskShift | CGEventGetFlags(event1);
else
    flags = ~kCGEventFlagMaskShift & CGEventGetFlags(event1);

CGEventSetFlags(event1, flags);
CGEventPost(kCGHIDEventTap,event1);

event2 = CGEventCreateKeyboardEvent (source, keyCode, false);
if (upper)
    flags = kCGEventFlagMaskShift | CGEventGetFlags(event2);
else
    flags = ~kCGEventFlagMaskShift & CGEventGetFlags(event2);
CGEventSetFlags(event2, flags);
CGEventPost(kCGHIDEventTap,event2);
CFRelease(event1);
CFRelease(event2);
CFRelease(source);

I had a similar problem recently. Instead of passing NULL as the first argument of CGEventCreateKeyboardEvent, create an event source like this:

CGEventSourceRef eventSource = CGEventSourceCreate(kCGEventSourceStateHIDSystemState);

and use it while you are creating and posting events. CFRelease it when you are done with it.