The math library used for PowerPC Mac OS and Windows (when optimized) is not fully compliant with the 1990 ANSI C standard. One way it deviates is that none of the math functions set errno.

The setting of errno is considered an obsolete mechanism because it is inefficient as well as un-informative. Further more various math facilities may set errno haphazardly for 68k Mac OS.

The MSL math libraries provide better means of error detection. Using fpclassify (which is fully portable) provides a better error reporting mechanism. "Example usage of error detection," shows an example code used for error detection that allows you to recover in your algorithm based on the value returned from fpclassify.