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* [Bug c/94615] New: -Wstringop-truncation warns on strncpy() with struct lastlog (or utmp)
@ 2020-04-16  9:42 allison.karlitskaya at redhat dot com
  2020-04-16 10:12 ` [Bug c/94615] " rguenth at gcc dot gnu.org
                   ` (4 more replies)
  0 siblings, 5 replies; 6+ messages in thread
From: allison.karlitskaya at redhat dot com @ 2020-04-16  9:42 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94615

            Bug ID: 94615
           Summary: -Wstringop-truncation warns on strncpy() with struct
                    lastlog (or utmp)
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: allison.karlitskaya at redhat dot com
  Target Milestone: ---

Consider this code:

{
  struct lastlog entry;

  strncpy (entry.ll_host, rhost, sizeof entry.ll_host);
  /* fill other fields */

  /* write record to the lastlog */
}

strncpy() turns out to be exactly what you want in this case because the
ll_host field in struct lastlog is only *optionally* nul terminated, in the
case of the hostname being shorter than the field width.  If the hostname is
equal to or longer than the field length then you should write the whole thing
in, without including a nul.  Consequently, any reader code needs to be more
careful about checking for both conditions (which it ought to be, in any case).

GCC 8 gives this warning, though:

/usr/include/bits/string_fortified.h:106:10: warning: '__builtin_strncpy'
specified bound 256 equals destination size [-Wstringop-truncation]

... which on its face seems kinda funny, because the bound *should* equal the
destination size.  Particularly in this case.


Perhaps there is some attribute or other hint that can be added to the struct
in question that the character array is intended to be only optionally nul
terminated, and in that case, we could skip the warning?  Otherwise, it would
be nice if there were a way to avoid this warning that didn't involve a mess of
#if and #pragma.

^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2020-04-16 17:58 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-04-16  9:42 [Bug c/94615] New: -Wstringop-truncation warns on strncpy() with struct lastlog (or utmp) allison.karlitskaya at redhat dot com
2020-04-16 10:12 ` [Bug c/94615] " rguenth at gcc dot gnu.org
2020-04-16 10:17 ` fw at gcc dot gnu.org
2020-04-16 17:38 ` allison.karlitskaya at redhat dot com
2020-04-16 17:44 ` allison.karlitskaya at redhat dot com
2020-04-16 17:58 ` allison.karlitskaya at redhat dot com

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