In computer programming, a sentinel value (also referred to as a flag value, rogue value, or signal value) is a special value that is used to terminate a loop that processes structured (especially sequential) data. The value should be selected in such a way that it will not be confused with legal data values.

Below are some examples of common sentinel values and their uses:

• Null character for indicating the end of a null-terminated string.
• Null pointer for indicating the end of a linked list.
• The empty list, (), in Lisp and its variants.

Sentinel values are often used when searching for something of importance.^[1]

In safe languages, most uses of sentinel values could be replaced with option types, which enforce explicit handling of the exceptional case.

See also

• Sentinel node

Notes

1. ↑ McConnell, Steve. "Code Complete" Edition 2 Pg. 621 ISBN - 0-7356-1967-0