Open System Services Library Calls Reference Manual (G06.27+, H06.04+)

OSS Library Calls (a - d) acos(3)
The XPG4 Version 2 specification leaves some features to the implementing vendor to define.
The following feature is affected in the HP implementation:
• For IEEE floating-point values, errno is not set to [EDOM] when the value of x is NaN.
527187-004 Hewlett-Packard Company 1−7