There are four different versions of the math library routines: IEEE,
POSIX, X/Open, or SVID. The version may be selected at runtime by
setting the global variable _LIB_VERSION, defined in
math.h. It may be set to one of the following constants defined
in math.h: _IEEE_, _POSIX_, _XOPEN_, or
_SVID_. The _LIB_VERSION variable is not specific to any
thread, and changing it will affect all threads.
The versions of the library differ only in how errors are handled.
In IEEE mode, the matherr function is never called, no warning
messages are printed, and errno is never set.
In POSIX mode, errno is set correctly, but the matherr
function is never called and no warning messages are printed.
In X/Open mode, errno is set correctly, and matherr is
called, but warning message are not printed.
In SVID mode, functions which overflow return 3.40282346638528860e+38,
the maximum single-precision floating-point value, rather than infinity.
Also, errno is set correctly, matherr is called, and, if
matherr returns 0, warning messages are printed for some errors.
For example, by default ‘log(-1.0)’ writes this message on standard
error output:
log: DOMAIN error
The library is set to X/Open mode by default.
The aforementioned error reporting is the supported Newlib libm error handling method. However, the majority of the functions are written so as to produce the floating-point exceptions (e.g. "invalid", "divide-by-zero") as required by the C and POSIX standards, for floating-point implementations that support them. Newlib does not provide the floating-point exception access routines defined in the standards for fenv.h, though, which is why they are considered unsupported. It is mentioned in case you have separately-provided access routines so that you are aware that they can be caused.
Most of the individual function descriptions describe the standards to which each function complies. However, these descriptions are mostly out of date, having been written before C99 was released. One of these days we'll get around to updating the rest of them. (If you'd like to help, please let us know.)
“C99” refers to ISO/IEC 9899:1999, “Programming languages–C”. “POSIX” refers to IEEE Standard 1003.1. POSIX® is a registered trademark of The IEEE.