https://developer.apple.com/library/archive/qa/qa1133/_index.html clang -framework CoreFoundation -framework SystemConfiguration ./console_user.c

console_user.c

#include <assert.h>

#include <SystemConfiguration/SystemConfiguration.h>

static CFStringRef CopyCurrentConsoleUsername(SCDynamicStoreRef store)
    // Returns the name of the current console user, or NULL if there is 
    // none. store may be NULL, in which case a transient dynamic store 
    // session is used.
{
    CFStringRef result;

    result = SCDynamicStoreCopyConsoleUser(store, NULL, NULL);

    // If the current console user is "loginwindow", treat that as equivalent 
    // to none.
    
    if ( (result != NULL) && CFEqual(result, CFSTR("loginwindow")) ) {
        CFRelease(result);
        result = NULL;
    }

    return result;
}

static CFStringRef gCurrentConsoleUser;

static void MyNotificationProc(
    SCDynamicStoreRef	store,
    CFArrayRef          changedKeys,
    void *              info
)
    // Called out of our runloop when the current console user value 
    // changes in the dynamic store. It's possible to get multiple 
    // redundant notifications, so we debounce the notification by checking 
    // for changes relative to gCurrentConsoleUser.
{
    #pragma unused(changedKeys)
    #pragma unused(info)
    CFStringRef         currentConsoleUser;
    Boolean             didChange;

    // Get the current console user.

    currentConsoleUser = CopyCurrentConsoleUsername(store);

    // See if it changed.

    didChange = (gCurrentConsoleUser != NULL) != (currentConsoleUser != NULL);
    if ( ! didChange && (gCurrentConsoleUser != NULL) ) {
        assert(currentConsoleUser != NULL); // because if it was NULL, 
                                            // didChange would already be true
        didChange = ! CFEqual(gCurrentConsoleUser, currentConsoleUser);
    }

    // If it did, log that fact and remember the current value.

    if (didChange) {
        if (gCurrentConsoleUser != NULL) {
            CFRelease(gCurrentConsoleUser);
        }
        gCurrentConsoleUser = currentConsoleUser;
        if (gCurrentConsoleUser != NULL) {
            CFRetain(gCurrentConsoleUser);
        }

        CFShow(gCurrentConsoleUser);
    }
}

int main(int argc, char **argv)
{
    #pragma unused(argc)
    #pragma unused(argv)
    Boolean             success;
    SCDynamicStoreRef   store;
    CFStringRef         key;
    CFArrayRef          keys;
    CFRunLoopSourceRef  rls;

    // Set up our connection to the dynamic store so that notifications are 
    // delivered by calling MyNotificationProc.

    store = SCDynamicStoreCreate(
        NULL, 
        CFSTR("com.apple.dts.ConsoleUser"), 
        MyNotificationProc, 
        NULL
    );
    assert(store != NULL);

    // Set it up to notify us when the console user value changes.

    key = SCDynamicStoreKeyCreateConsoleUser(NULL);
    assert(key != NULL);

    keys = CFArrayCreate(NULL, (const void **) &key, 1, &kCFTypeArrayCallBacks);
    assert(keys != NULL);

    success = SCDynamicStoreSetNotificationKeys(store, keys, NULL);
    assert(success);

    // Add it to the runloop.

    rls = SCDynamicStoreCreateRunLoopSource(NULL, store, 0);
    assert(rls != NULL);

    CFRunLoopAddSource(CFRunLoopGetCurrent(), rls, kCFRunLoopDefaultMode);

    // Print the current value.

    gCurrentConsoleUser = CopyCurrentConsoleUsername(store);
    CFShow(gCurrentConsoleUser);

    // Run forever printing any changes.

    CFRunLoopRun();

    // Clean up code. This simple tool will never execute this code 
    // (because CFRunLoopRun will never return), but I've left it here 
    // in case you adapt the code for a more complex situation.

    if (gCurrentConsoleUser != NULL) {
        CFRelease(gCurrentConsoleUser);
    }
    CFRunLoopSourceInvalidate(rls);
    CFRelease(rls);
    CFRelease(keys);
    CFRelease(key);
    CFRelease(store);

    return EXIT_SUCCESS;
}