[Bug c/67956] New: gcc's printf attribute accepts %m as a format character

["christos at zoulas dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67956

            Bug ID: 67956
           Summary: gcc's printf attribute accepts %m as a format
                    character
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: christos at zoulas dot com
  Target Milestone: ---

Created attachment 36503
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36503&action=edit
patch

According to TOG
(http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html), %m is
not a legal formatting character for printf(3) like functions. Some
implementations (glibc) accept this formatting character as an extension and
expand it to the error string via strerror(3) that errno currently points to.
Most other c library implementations do not expand %m. 

Attached is a patch that makes printf warn when %m is present and adds a
"syslog" attribute that accepts %m. I recognize that this is an incompatible
change, and I welcome suggestions how to do this in a less intrusive way.

I tried to mininize the size of the patch; there are better ways to do this,
but require more refactoring.