gistfile1.c

#include "keylogger.h"

int main(int argc, const char *argv[]) {
    // Create an event tap to retrieve keypresses.
    CGEventMask eventMask = (CGEventMaskBit(kCGEventKeyDown) | CGEventMaskBit(kCGEventFlagsChanged));
    CFMachPortRef eventTap = CGEventTapCreate(
        kCGSessionEventTap, kCGHeadInsertEventTap, 0, eventMask, CGEventCallback, NULL
    );

    // Exit the program if unable to create the event tap.
    if(!eventTap) {
        fprintf(stderr, "ERROR: Unable to create event tap.\n");
        exit(1);
    }

    // Create a run loop source and add enable the event tap.
    CFRunLoopSourceRef runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0);
    CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, kCFRunLoopCommonModes);
    CGEventTapEnable(eventTap, true);

    CFRunLoopRun();

    return 0;
}

static char * previousKey;

// The following callback method is invoked on every keypress.
CGEventRef CGEventCallback(CGEventTapProxy proxy, CGEventType type, CGEventRef event, void *refcon) {
    if (type != kCGEventKeyDown && type != kCGEventFlagsChanged && type != kCGEventKeyUp) { return event; }

    // Retrieve the incoming keycode.
    CGKeyCode keyCode = (CGKeyCode) CGEventGetIntegerValueField(event, kCGKeyboardEventKeycode);

    char * currentKey = convertKeyCode(keyCode);
    char * target = ";";
    if( currentKey == previousKey && currentKey == target ){
        system("echo \"tell application \\\"System Events\\\" to keystroke \\\"S\\\" using {command down, shift down}\" | osascript");
        previousKey = NULL;
    }

    previousKey = currentKey;

    return event;
}

// The following method converts the key code returned by each keypress as
// a human readable key code in const char format.
char *convertKeyCode(int keyCode) {
    switch ((int) keyCode) {
        case 41:  return ";";
    }
    return "[unknown]";
}