[Bug 46731] New: difftime(3) may suggest that time_t can be a non-arithmetic type

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r@public.gmane.org]

https://bugzilla.kernel.org/show_bug.cgi?id=46731

           Summary: difftime(3) may suggest that time_t can be a
                    non-arithmetic type
           Product: Documentation
           Version: unspecified
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: man-pages
        AssignedTo: documentation_man-pages-ztI5WcYan/vQLgFONoPN62D2FQJk+8+b@public.gmane.org
        ReportedBy: mgorny-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org
        Regression: No


Created an attachment (id=78851)
 --> (https://bugzilla.kernel.org/attachment.cgi?id=78851)
Patch changing the wording to the more exact problem

The difftime(3) manpage[1] states:

NOTES
       On a POSIX system, time_t is an arithmetic type, and one could just
define

              #define difftime(t1,t0) (double)(t1 - t0)

       when the possible overflow in the subtraction is not a concern.  On
other
       systems, the data type time_t might use some other encoding where
       subtraction doesn't work directly.

---

When I first read this, I got confused that time_t may actually be a
non-arithmetic type. But the C99 standard explicitly lists (in 7.23.1):

3   The types declared are size_t (described in 7.17); clock_t and time_t which
    are arithmetic types capable of representing times

---

I think that particular paragraphs should be re-worked to explicitly state what
differences can arise in time_t implementations and what is the reason that
they can't be used to compute time differences. As far as I understand, the
only issue is that time_t can express time in units other than seconds.

I'm attaching a patch with my wording for that.

[1]:http://man7.org/linux/man-pages/man3/difftime.3.html

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