File locks for linux

I tried using temp files:

char *temp = tempnam(NULL, "myapp_");
printf("Tempname: %s", temp)     // Prints /tmp/myapp_random
while (1) { }

But when I check /tmp (while the app is still running), the myapp_random is not there!

As for using File Locks, I can't get a good grasp on it, I tried looking at but it seems to focus on locks in specified portions of a file. I just want to use the file entirely as a lock (which is why I prefer trying the temp file approach).

Any ideas?