[Bug manual/14450] New: timeval_subtract example is erroneous

["guifo at wanadoo dot fr" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14450

             Bug #: 14450
           Summary: timeval_subtract example is erroneous
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: manual
        AssignedTo: unassigned@sourceware.org
        ReportedBy: guifo@wanadoo.fr
                CC: mtk.manpages@gmail.com, roland@gnu.org
    Classification: Unclassified


Created attachment 6572
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6572
timeval_subtract test case

The code given as an example in manual/examples/timeval_subtract.c is giving
erroneous results when the left operand is lower than the right operand.
It was previously embedded in manual/time.texi and has been as such since its
very first appearance (in 1995 according to git log).

It is quite surprising to see how many times this code has been blindly pasted
into various projects, most of the time untouched (a quick Google search shows
this).
Let's hope this has been made each time with due care...

The attached test_timeval_subtract.c source file exposes the error. This file
contains a copy of the function found in manual/examples/timeval_subtract.c
plus a new function with a proposed correction.

$ gcc -o test_timeval_subtract test_timeval_subtract.c
$ ./test_timeval_subtract 
Current timeval_subtract
1.500000 - 10.499998 => -9.000002   returned sign: -
1.500000 - 10.499999 => -9.000001   returned sign: -
1.500000 - 10.500000 => -9.000000   returned sign: -
1.500000 - 10.500001 => -10.999999   returned sign: -
1.500000 - 10.500002 => -10.999998   returned sign: -

New timeval_subtract
1.500000 - 10.499998 => 8.999998   returned sign: -
1.500000 - 10.499999 => 8.999999   returned sign: -
1.500000 - 10.500000 => 9.000000   returned sign: -
1.500000 - 10.500001 => 9.000001   returned sign: -
1.500000 - 10.500002 => 9.000002   returned sign: -

Values in the first block are obviously wrong, even showing a discontinuity,
while in the second block values computed with the corrected function are the
expected ones. 

A patch to apply the proposed correction on manual/examples/timeval_subtract.c
will be attached.

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