CODEWITHSUNDEEP
cportabilityformat-specifiers
Georg Schölly
Georg Schölly

Reputation: 126115

How should I print types like off_t and size_t?

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?

Upvotes: 161

Views: 161962

Answers (10)

12431234123412341234123
12431234123412341234123

Reputation: 1

If you use C11 or C18, you can use _Generic.

#include <stdio.h>
#include <sys/types.h>

    #define TYPE_FORMAT(variable) _Generic \
      (                                    \
        (variable)                         \
        , unsigned char       : "%hhu"     \
        , unsigned short      : "%hu"      \
        , unsigned int        : "%u"       \
        , unsigned long       : "%lu"      \
        , unsigned long long  : "%llu"     \
        , signed   char       : "%hhi"     \
        , signed   short      : "%hi"      \
        , signed   int        : "%i"       \
        , signed   long       : "%li"      \
        , signed   long long  : "%lli"     \
      )
      
      
int main(void)
  {
    off_t a=3321;
    printf(TYPE_FORMAT(a), a);
  }

However, there is one major drawback: You can not combine strings this way. Means this:

printf("Foo: " TYPE_FORMAT(a), a);

will not work, which makes it useless for many situation. Yes you could combine the string during runtime but that is annoying.

Upvotes: 1

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 507065

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

printf("%zu %td", size, ptrdiff);

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.

Upvotes: 123

Elinx
Elinx

Reputation: 1204

I saw this post at least twice, because the accepted answer is hard to remeber for me(I rarely use z or j flags and they are seems not platform independant).

The standard never says clearly the exact data length of size_t, so I suggest you should first check the length size_t on your platform then select one of them:

if sizeof(size_t) == 4 use PRIu32
if sizeof(size_t) == 8 use PRIu64

And I suggest using stdint types instead of raw data types for consistancy.

Upvotes: -3

NoBrassRing
NoBrassRing

Reputation: 493

For Microsoft, the answer is different. VS2013 is largely C99 compliant but "[t]he hh, j, z, and t length prefixes are not supported." For size_t "that is, unsigned __int32 on 32-bit platforms, unsigned __int64 on 64-bit platforms" use prefix I (capital eye) with type specifier o, u, x, or X. See VS2013 Size specification

As for off_t, it is defined as long in VC\include\sys\types.h.

Upvotes: 5

Steve Jessop
Steve Jessop

Reputation: 279295

To print off_t:

printf("%jd\n", (intmax_t)x);

To print size_t:

printf("%zu\n", x);

To print ssize_t:

printf("%zd\n", x);

See 7.19.6.1/7 in the C99 standard, or the more convenient POSIX documentation of formatting codes:

http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html

If your implementation doesn't support those formatting codes (for example because you're on C89), then you have a bit of a problem since AFAIK there aren't integer types in C89 that have formatting codes and are guaranteed to be as big as these types. So you need to do something implementation-specific.

For example if your compiler has long long and your standard library supports %lld, you can confidently expect that will serve in place of intmax_t. But if it doesn't, you'll have to fall back to long, which would fail on some other implementations because it's too small.

Upvotes: 132

Shankar
Shankar

Reputation: 1

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

Upvotes: -9

James Curran
James Curran

Reputation: 103515

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));

Upvotes: -4

Michael Burr
Michael Burr

Reputation: 340248

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

See this question: Cross platform format string for variables of type size_t?

Upvotes: 4

dwc
dwc

Reputation: 24900

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).

Upvotes: 1

David Thornley
David Thornley

Reputation: 57046

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.

Upvotes: 3

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