This is a race condition. There is no reason to believe that because a file did not exist when the previous instruction was executed that it still does not exist (your operating system could have run another proccess in between). So when your
loop ends there is no guarantee that the file does not exist (infact, even if there were no race condition, you have a time limit on this loop but never check to see if the loop terminated because time was up or if it terminated because the file doesn't exist -- but like I said, checking to see if the file doesn't exist would still be a bug). If you are unable to use flock(), you could see if you're able to use sysopen with the O_EXCL flag which will guarantee that the file does not exist when opened (if it does exist, the sysopen call will fail and you'll probably want to handle this rather than simply returning from the function)