xnan

Xnan (often written xnan) is a term used in some computational contexts to denote an extended Not-a-Number value that carries additional diagnostic information within floating-point computations. It is not part of the IEEE 754 standard itself, but appears in certain libraries, research projects, and experimental language runtimes as a convention for encoding exceptional results or provenance beyond standard NaN.

An xnan extends the concept of NaN by reserving part of the NaN payload to encode a tag that identifies the error category and a variable-length diagnostic payload. In practice, xnan values are still NaNs at the hardware level, but software interprets the payload according to a predefined layout.

Xnan is distinct from standard quiet NaN and signaling NaN in that it emphasizes information content rather than signaling semantics. Because it relies on payload bits, cross-platform portability depends on consistent interpretation of the tag field and payload across compilers and runtimes.

Common use cases include long-running simulations, numerical instrumentation, and debugging pipelines, where restoring diagnostic context without raising exceptions is valuable. However, the lack of a standard means that interoperability is limited and misinterpretation can occur if different systems adopt incompatible payload schemes.

See also IEEE 754, NaN, qNaN, sNaN, NaN payload.