[Bug c/50154] New: attribute printf and scanf should imply attribute nonnull

["bruno at clisp dot org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50154

             Bug #: 50154
           Summary: attribute printf and scanf should imply attribute
                    nonnull
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bruno@clisp.org


Created attachment 25076
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25076
test case

When a function is declared as taking a printf or scanf format string, gcc
gives a warning when a format string with invalid syntax is passed. It should
also give a warning when a NULL format string is passed.

How to reproduce:
Save the test case file as foo.c.
$ gcc -Wformat=2 -c foo.c
[no warning]
$ gcc -Wformat=2 -c foo.c -DDECLARE_NONNULL
foo.c: In function 'foo':
foo.c:10:3: warning: null argument where non-null required (argument 3)
[-Wnonnull]

Rationale:
1) ISO C 99 specifies that the printf and scanf format strings must be
"multibyte
character sequences"; that excludes NULL.
See ISO C 99 sections 7.19.6.1 paragraph 3 and 7.19.6.2 paragraph 3.
2) Passing NULL as first argument to printf crashes the program on FreeBSD,
Solaris, and other platforms.