[Bug c/77520] New: wrong value for extended ASCII characters in -Wformat message

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77520

            Bug ID: 77520
           Summary: wrong value for extended ASCII characters in -Wformat
                    message
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The argument_parser::find_format_char_info function is c-family/c-format.c
passes the value of an unexpected format character whose type is char as an
argument to a %x directive.  When char is a signed type and the value is in the
extended ASCII range ('\x80' and greater) it is sign-extended and yields a very
large value on output as in the test case below.  The character should be cast
to unsigned char to avoid the sign extension.  The following test case shows
the problem:

$ cat t.c && /home/msebor/build/gcc-49905/gcc/xgcc
-B/home/msebor/build/gcc-49905/gcc -S -Wformat t.c
void f (void)
{
  __builtin_printf ("%\x80");
}
t.c: In function ‘f’:
t.c:3:23: warning: unknown conversion type character 0xffffff80 in format
[-Wformat=]
   __builtin_printf ("%\x80");
                       ^~~~

Clang produces better output:

t.c:3:23: warning: invalid conversion specifier '\x80'