Possible Duplicate:
access() Security Hole
I quote from man page access(2):
Warning: Using access() to check if a user is authorized to, for example, open a file before actually doing so using open(2) creates a security hole, because the user might exploit the short time interval between checking and opening the file to manipulate it. For this reason, the use of this system call should be avoided.
What does this mean, and in what situation would it be a concern?