This paper describes the design and implementation of a tool for C programs that provides run-time checks based on type information. The tool instruments a program to monitor the type stored in each memory location. Whenever a value is written into a location, the location's run-time type tag is updated to match the type of the value. Also, the location's static type is compared with the value's type; if there is a mismatch, a warning message is issued. Whenever the value in a location is used, its run-time type tag is checked, and if the type is inappropriate in the context in which the value is being used, an error message is issued.

The tool has been used to pinpoint the cause of bugs in several Solaris utilities and Olden benchmarks, usually providing information that is succinct and precise.

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