For a domain error, tan() does not set errno. It should set errno to EDOM. NOTE: the inverse trig functions do correctly set errno on error! Background: On error, many glibc math functions both set errno and raise an exception (fetestexcept(3)). For example, the following function all do this: acos(), asin(), cosh(), sinh(), acosh(), asinh(), exp(), exp2(), ldexp(), log(), log10(), log2(). However, there is much inconsistency. Some functions raise an exception, but don't set errno. Some functions set errno for some errors, but not others. A few set errno, but don't raise an exception. This series of bug reports documents deviations from what I consider the ideal: all functions should BOTH set errno AND raise an exception for all errors. All of these reports relate to tests on glibc 2.8 (as provided by SUSE 11.0).
Created attachment 2847 [details] test program test run showing problem $ /tmp/mt_tan inf errno == 0 fetestexcept() says: FE_INVALID tan(inf)=nan 0 FE_INVALID nan
This bug was fixed in glibc 2.10. The man page is updated for man-pages-2.37.
This was fixed in 2009.