How should I print types like off_t and size_t?

Question

I'm trying to print types like off_t and size_t. What is the correct placeholder for printf() that is portable?

Or is there a completely different way to print those variables?

Answer 1

Which version of C are you using?

In C90, the standard practice is to cast to signed or unsigned long, as appropriate, and print accordingly. I've seen %z for size_t, but Harbison and Steele don't mention it under printf(), and in any case that wouldn't help you with ptrdiff_t or whatever.

In C99, the various _t types come with their own printf macros, so something like "Size is " FOO " bytes." I don't know details, but that's part of a fairly large numeric format include file.

Answer 2

As I recall, the only portable way to do it, is to cast the result to "unsigned long int" and use %lu.

printf("sizeof(int) = %lu", (unsigned long) sizeof(int));

Answer 3

Looking at man 3 printf on Linux, OS X, and OpenBSD all show support for %z for size_t and %t for ptrdiff_t (for C99), but none of those mention off_t. Suggestions in the wild usually offer up the %u conversion for off_t, which is "correct enough" as far as I can tell (both unsigned int and off_t vary identically between 64-bit and 32-bit systems).

Answer 4

You can use z for size_t and t for ptrdiff_t like in

printf("%zd", size);

But my manpage says some older library used a different character than z and discourages use of it. Nevertheless, it's standardized (by the C99 standard). For those intmax_t and int8_t of stdint.h and so on, there are macros you can use, like another answer said:

printf("value: %" PRId32, some_int32_t);
printf("value: %" PRIu16, some_uint16_t);

They are listed in the manpage of inttypes.h.

Personally, I would just cast the values to unsigned long or long like another answer recommends. If you use C99, then you can (and should, of course) cast to unsigned long long or long long and use the %llu or %lld formats respectively.

Answer 5

You'll want to use the formatting macros from inttypes.h.

See this question: http://stackoverflow.com/questions/174612/cross-platform-format-string-for-variables-of-type-sizet

Answer 6

use "%zo" for off_t. (octal) or "%zu" for decimal.