Re: Report bugs for variations in error handling in math functions?

["Michael Kerrisk" <mtk.manpages-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>]

On Fri, Jul 25, 2008 at 5:43 PM, Joseph S. Myers
<joseph-qD8j1LwMmJjtCj0u4l0SBw@public.gmane.org> wrote:
> On Fri, 25 Jul 2008, Michael Kerrisk wrote:
>
>> Currently, I'm revising all of the math pages in man-pages, and in the
>> process testing the error handling (glibc 2.8) for each function.
>>
>> I find the following:
>>
>> a) on error, many (probably a majority of) functions set errno AND
>> raise an exception (fetestexcept()).
>> b) on error, a very few functions DO set errno but DON"T raise an
>> exception (fetestexcept()).
>> c) on error, a few functions DON'T set errno but DO raise an exception
>> (fetestexcept()).
>> d) on error, a very few functions pursue a mixture of all of the
>> above, depending on the error.
>>
>> A math_error(7) page that I recently wrote (see
>> http://www.kernel.org/doc/man-pages/online/pages/man7/math_error.7.html
>> ) currently implies that all functions should do a).  Clearly I'll
>> need to amend that.
>>
>> But the main question is, should I raise glibc bugs for the functions
>> in cases b), c), and d)?
>
> I've run third-party C conformance tests on glibc that have shown similar
> issues.
>
> C90 requires errno to be set by various functions.  C99 allows it not to
> be set, in an incompatible quiet change from C90, if exceptions are used
> instead.  The correct handling under C99 depends on the value of
> math_errhandling.  Implementing math_errhandling requires compiler and
> linker support (see messages linked from the CONFORMANCE file):

Yes, I saw those already.

> if any
> translation unit is compiled with -fno-math-errno then math_errhandling &
> MATH_ERRNO must not be set.  (I'd suggest that the compiler set object
> attributes which the static linker then uses to provide the relevant
> information to libc.)
>
> I think the correct approach is to consider it a bug if functions do not
> set errno, or do not raise exceptions - that is, all of (b), (c) and (d)
> are bugs.  This would allow math_errhandling to be
> MATH_ERRNO|MATH_ERREXCEPT unless translation units are compiled with
> options preventing this, and make the error handling options available
> consistent across the math.h functions supported by glibc.  To conform
> with C99, at least one approach (errno or exceptions) must be consistently
> supported across all the functions, in any case.

Not even one approach is consistent.  Many functions don't set errno.
But there are one or two that don't raise exceptions: the Bessel
functions (j0(), y0 etc), for example.  It's all a bit of a mess,
unfortunately.

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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