Created attachment 25076 [details]
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.