[Bug c/40960] New: POSIX requires that option -D have a lower precedence than -U

[vincent at vinc17 dot org]

[This concerns the POSIX c99 utility, but gcc should probably behave in the
same way, as on some platforms, c99 is gcc.]

In http://www.opengroup.org/onlinepubs/9699919799/utilities/c99.html POSIX
specifies:

  -D  name[=value]
    Define name as if by a C-language #define directive. If no = value
    is given, a value of 1 shall be used. The -D option has lower
    precedence than the -U option. That is, if name is used in both a
    -U and a -D option, name shall be undefined regardless of the
    order of the options.

However, gcc doesn't take the precedence rule into account:

$ cat tst.c
int main(void)
{
#ifdef FOO
  return 1;
#else
  return 0;
#endif
}
$ c99 tst.c -UFOO -DFOO=1
$ ./a.out
zsh: exit 1     ./a.out

whereas FOO should be undefined and the return value should be 0, not 1.

I could reproduce this with various GCC versions, including:
gcc-snapshot (Debian 20090718-1) 4.5.0 20090718 (experimental) [trunk revision
149777]


-- 
           Summary: POSIX requires that option -D have a lower precedence
                    than -U
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vincent at vinc17 dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40960