Re: C portability question

["Greg A. Woods; Planix, Inc." <woods%planix.ca@localhost>]

On 11-May-08, at 7:49 PM, Steven M. Bellovin wrote:

> Working on a package, I had problems with (in effect)
>
>         off_t var;
>         printf("%ld\n", var);
>
> This crashes on netbsd-i386, since our longs are 4 bytes.  (I didn't
> quite see what off_t is, but it's 8 bytes long.)
>
> Anyway -- what is the portable way to printf an off_t, which is (of
> course) an opaque type.  I could cast (var) to (long), but that's
> clearly wrong (and in fact loses data here for us).

C99 defines printf (and scanf) format specifiers for various integer  
types (which you'll find actually defined in <machine/int_fmtio.h> on  
NetBSD since 1.6):

        #include <sys/types.h> /* for off_t */
        #include <inttypes.h>
        #include <limits.h>
        #include <stdint.h>
        #include <stdio.h>

        off_t var = INT64_MAX;

        /* assert(sizeof(off_t) <= sizeof(int64_t)); */

        printf("%" PRId64 "\n", (int64_t) var);

Without C99 though (on at least I32/LP32 architectures) you may be  
stuck with trying to figure out if the compiler supports "long long"  
or similar and what printf() offers for printing "long long" (eg.  
"%q", assuming of course sizeof(off_t) is also actually greater than 4  
on the target platform.

--
                                        Greg A. Woods; Planix, Inc.
                                        <woods%planix.ca@localhost>