Question

Mountain Lion’s implementation of libmath (located at /usr/lib/system/libsystem_m.dylib) has all the standard libmath functions, plus, for each of them, a $fenv_access_off variation. For instance, there’s acos, and acos$fenv_access_off. (I do not have any other version of Mac OS installed available to check if it was the case before 10.8.)

What does $fenv_access_off mean?

I understand that I won’t ever be calling directly any of these, but I’m still curious about them.

Answer 1

There is a mailing list thread between some of Apple’s engineers that contains a discussion about fenv_access_off. Stephen Canon, an Apple engineer, explains:

C99 knows whether you are looking the IEEE flags / changing rounding
modes or not based on whether or not you’ve included fenv.h and done
#pragma STDC FENV_ACCESS on or not, as required by standard. So in principle, we could actually have two parallel math libraries, one
that spends time diligently setting flags and defending against
rounding mode changes and a faster one that does not. In practice,
this can be just implemented by having a separate set of symbols
decorated with $fenv_access_off for all the stuff in the math library.

From the man page of fenv:

The header declares types, macros, and functions to provide
access to the floating-point environment, consisting of any
floating-point status flags and control modes supported by the
implementation.

…

The FENV_ACCESS pragma provides a means to inform the compiler that
the program might access the floating-point environment to test status
flags or change the control modes.