From http://pubs.opengroup.org/onlinepubs/009695399/functions/printf.html:
- z
Specifies that a following [...] conversion specifier applies to asize_t
or the corresponding signed integer type argument; - t
Specifies that a following [...] conversion specifier applies to aptrdiff_t
or the corresponding unsigned type argument;
And from http://en.wikipedia.org/wiki/Size_t#Size_and_pointer_difference_types:
size_t
is used to represent the size of any object (including arrays) in the particular implementation. It is used as the return type of thesizeof
operator.ptrdiff_t
is used to represent the difference between pointers.
On the current OS X and iOS platforms we have
typedef __SIZE_TYPE__ size_t;
typedef __PTRDIFF_TYPE__ ptrdiff_t;
where __SIZE_TYPE__
and __PTRDIFF_TYPE__
are predefined by the
compiler. For 32-bit the compiler defines
#define __SIZE_TYPE__ long unsigned int
#define __PTRDIFF_TYPE__ int
and for 64-bit the compiler defines
#define __SIZE_TYPE__ long unsigned int
#define __PTRDIFF_TYPE__ long int
(This may have changed between Xcode versions. Motivated by @user102008's comment, I have checked this with Xcode 6.2 and updated the answer.)
So ptrdiff_t
and NSInteger
are both typedef'd to the same type:
int
on 32-bit and long
on 64-bit. Therefore
NSLog(@"%td", i);
NSLog(@"%tu", u);
work correctly and compile without warnings on all current iOS and OS X platforms.
size_t
and NSUInteger
have the same size on all platforms, but
they are not the same type, so
NSLog(@"%zu", u);
actually gives a warning when compiling for 32-bit.
But this relation is not fixed in any standard (as far as I know), therefore I would
not consider it safe (in the same sense as assuming that long
has the same size
as a pointer is not considered safe). It might break in the future.
The only alternative to type casting that I know of is from the answer to "Foundation types when compiling for arm64 and 32-bit architecture", using preprocessor macros:
// In your prefix header or something
#if __LP64__
#define NSI "ld"
#define NSU "lu"
#else
#define NSI "d"
#define NSU "u"
#endif
NSLog(@"i=%"NSI, i);
NSLog(@"u=%"NSU, u);