On 05/31/2018 08:26 PM, Keno Fischer wrote:
Comparisons of mode_t with -1 require an explicit cast, since mode_t
is unsigned on Darwin.
It's not JUST that mode_t is unsigned (an unsigned int compares just
fine against -1), but ALSO that mode_t has unspecified width. That is,
you cannot portably assume whether mode_t is smaller, equivalent, or
larger rank than int. If it is smaller, then you can't use mode_t in
va_arg(), and mode_t will promote to signed int, whether or not mode_t
is unsigned; but '((mode_t)-1) == -1' is going to be false if mode_t is
unsigned (because the cast truncates the sign extension bits into a
positive value). Conversely, since mode_t can be larger than int
(although I know of no such platform that does so), blindly using 'int'
when trying to parse a mode_t argument through va_arg() will truncate bits.
So, for example, to portably write a wrapper around open(), you HAVE to
use hairy constructions like:
mode = (sizeof (mode_t) < sizeof (int)
? va_arg (ap, int)
: va_arg (ap, mode_t));
or, to avoid spurious compiler warnings on the branch not taken, define
a macro learned at configure time. This is what gnulib does, for example:
AC_CACHE_CHECK([for promoted mode_t type], [gl_cv_promoted_mode_t], [
dnl Assume mode_t promotes to 'int' if and only if it is smaller
than 'int',
dnl and to itself otherwise. This assumption is not guaranteed by
the ISO C
dnl standard, but we don't know of any real-world counterexamples.
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/types.h>]],
[[typedef int array[2 * (sizeof (mode_t) < sizeof (int)) - 1];]])],
[gl_cv_promoted_mode_t='int'],
[gl_cv_promoted_mode_t='mode_t'])
])
AC_DEFINE_UNQUOTED([PROMOTED_MODE_T], [$gl_cv_promoted_mode_t],
[Define to the type that is the result of default argument
promotions of type mode_t.])
+++ b/hw/9pfs/9p-local.c
@@ -310,7 +310,7 @@ update_map_file:
if (credp->fc_gid != -1) {
gid = credp->fc_gid;
}
- if (credp->fc_mode != -1) {
+ if (credp->fc_mode != (mode_t)-1) {
At any rate, this is the correct portability fix for this code.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
