sNaN

sNaN, short for signaling NaN, is a type of NaN defined in the IEEE 754 floating-point standard. It is distinguished from a quiet NaN (qNaN) by a signaling bit in the NaN’s significand. When a floating-point value is NaN, the exponent is all ones and the significand is nonzero; the signaling bit determines whether the NaN is meant to signal an invalid operation (sNaN) or simply propagate as a quiet result (qNaN). In binary formats, the most significant bit of the significand acts as this quiet/signaling bit: if that bit is 0, the NaN is signaling; if it is 1, the NaN is quiet.

The primary purpose of an sNaN is to trigger a floating-point exception, typically an invalid operation, when it participates in arithmetic or other IEEE 754 operations. The exact behavior depends on the platform and the floating-point environment: with exceptions enabled, using an sNaN usually raises an invalid-operation fault; the resulting value is typically NaN. If exceptions are masked or disabled, the operation may complete and yield a NaN, potentially a qNaN, without signaling.

In practice, platform and language implementations vary. Some hardware and software environments convert sNaNs to qNaNs to maintain performance and compatibility, often masking the signaling exception. Others retain signaling semantics for debugging or numerical analysis. As a result, the presence of sNaNs and their diagnostic value can be inconsistent across systems.

History and use of sNaNs have been shaped by the evolution of IEEE 754 revisions. While the concept remains defined, real-world support is heterogeneous, making sNaNs less relied upon for portable error signaling in modern software.

See also: NaN, IEEE 754, quiet NaN, signaling.