Bug 6785 - erf() does not set errno for underflow
Summary: erf() does not set errno for underflow
Status: RESOLVED INVALID
Alias: None
Product: glibc
Classification: Unclassified
Component: math (show other bugs)
Version: unspecified
: P2 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2008-07-30 14:50 UTC by Michael Kerrisk
Modified: 2014-11-28 13:00 UTC (History)
2 users (show)

See Also:
Host:
Target:
Build:
Last reconfirmed:
fweimer: security-


Attachments
test program (2.71 KB, text/plain)
2008-07-31 08:49 UTC, Michael Kerrisk
Details

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Description Michael Kerrisk 2008-07-30 14:50:51 UTC
When erf() is given a subnormal argument, it raises an underflow exception. 
However errno is not set.  It should be set to ERANGE.

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).
Comment 1 Michael Kerrisk 2008-07-31 08:49:27 UTC
Created attachment 2850 [details]
test program

Sample run showing problem:

$ /tmp/mt_erf subnormal:20
errno == 0
fetestexcept() says:  FE_UNDERFLOW FE_INEXACT
erf(2.12199579096527232e-314)=2.39441584310907145e-314
0 FE_UNDERFLOW subnormal
Comment 2 Joseph Myers 2012-02-29 21:04:45 UTC
Confirmed with current sources on both x86 and x86_64.
Comment 3 Joseph Myers 2013-11-28 16:57:35 UTC
It's now documented that errno is only intended to be set to ERANGE for underflow to 0, not for underflow to subnormals, and erf can't underflow to 0 (the result for small x is about x * 2/sqrt(pi)).
Comment 4 Nadezhda Viukova 2014-11-28 13:00:59 UTC
IMHO there is a related bug in gcc. See 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64101