I've noticed quite a few hits to my printf formats specifications reference page,
looking for information on how to print time_t, off_t and size_t types etc. However I realised the appropriate
formats weren't immediately obvious from that page, and going by lots of code I've reviewed, it's a common confusion.
The main point to note is that the size of these types depend of various things, so outputting them portably is tricky. For example the size of off_t depends on the _FILE_OFFSET_BITS definition, and so one must cast it to the maximum int type supported to print it portably. Similarly time_t is usually dependent on the native integer size of the platform, but may even be represented as a float, so again one must cast it to the max int type before printing.
Now the C99 standard helps here as it defines new %z, %j, and %t formats for printing size_t, intmax_t, and ptrdiff_t sizes respectively. Previously one would have to do ugly casts to long long for example. C99 also specifies types and formats for a specific number of bits, like uint32_t and PRIu32 for example. The example program below shows how to portably output all these types and should work on all but obsolete systems. See also getlimits in coreutils.
#define __USE_MINGW_ANSI_STDIO 1 /* So mingw uses its printf not msvcrt */
#define _FILE_OFFSET_BITS 64
#include <inttypes.h>
#include <limits.h>
#include <stdio.h>
#include <time.h> /* for time_t */
#include <sys/types.h> /* for off_t */
#define TYPE_SIGNED(t) (! ((t) 0 < (t) -1))
#define TYPE_MAX(t) \
((t) (! TYPE_SIGNED (t) \
? (t) -1 \
: ~ (~ (t) 0 << (sizeof (t) * CHAR_BIT - 1))))
int main(void)
{
uint32_t uint32=0xffffFFFF;
uintmax_t uintmax=UINTMAX_MAX;
off_t offset=TYPE_MAX(off_t); /* Depends on _FILE_OFFSET_BITS */
time_t time=TYPE_MAX(time_t); /* May be float! */
size_t size=TYPE_MAX(size_t); /* Depends on int size */
printf("native int bits %20zu %16x\n"
"native long bits%20zu %16lx\n"
"uint32_t max %20"PRIu32" %16"PRIx32"\n"
"uintmax_t max %20ju %16jx\n" /* try PRIuMAX if %ju unsupported */
"off_t max %20jd %16jx\n" /* try PRIdMAX if %jd unsupported */
"time_t max %20jd %16jx\n"
"size_t max %20zu %16zx\n",
sizeof(int)*CHAR_BIT, UINT_MAX,
sizeof(long)*CHAR_BIT, ULONG_MAX,
uint32, uint32,
uintmax, uintmax,
(intmax_t)offset, (intmax_t)offset,
(intmax_t)time, (intmax_t)time,
size, size);
#if defined __GNUC__ && defined _LP64
unsigned __int128 ui128=TYPE_MAX(unsigned __int128);
/* Note no way to specify constants > UINTMAX_MAX.
Also printing is awkward and only feasible really for hex. */
printf("__int128 max %20s %016"PRIx64"%016"PRIx64"\n", "",
(uint64_t)(ui128>>64),(uint64_t)ui128);
#endif
return 0;
}
On my 64 bit linux system, the output from the above program is:
native int bits 32 ffffffff native long bits 64 ffffffffffffffff uint32_t max 4294967295 ffffffff uintmax_t max 18446744073709551615 ffffffffffffffff off_t max 9223372036854775807 7fffffffffffffff time_t max 9223372036854775807 7fffffffffffffff size_t max 18446744073709551615 ffffffffffffffff __int128 max ffffffffffffffffffffffffffffffffCompiling on the same system in -m32 mode gives:
native int bits 32 ffffffff native long bits 32 ffffffff uint32_t max 4294967295 ffffffff uintmax_t max 18446744073709551615 ffffffffffffffff off_t max 9223372036854775807 7fffffffffffffff time_t max 2147483647 7fffffff size_t max 4294967295 ffffffff
© Jun 27 2008