(From "Secure Programming with Static Analysis")
Time-of-check, time-of-use race conditions (TOCTOU) occur when a program checks a property and then makes a decision based on this property later on. The classic example is when a c program checks the access properties of a file using the file name and the access() method, and then tries to open and write to the file later. An attacker could change the meaning of the file name between the access and the open, and steal information.
