How can one print a size_t variable portably using the printf family?

Question

I have a variable of type size_t, and I want to print it using printf(). What format specifier do I use to print it portably?

In 32-bit ma

Answer 1

As AraK said, the c++ streams interface will always work portably.

std::size_t s = 1024; std::cout << s; // or any other kind of stream like stringstream!

If you want C stdio, there is no portable answer to this for certain cases of "portable." And it gets ugly since as you've seen, picking the wrong format flags may yield a compiler warning or give incorrect output.

C99 tried to solve this problem with inttypes.h formats like "%"PRIdMAX"\n". But just as with "%zu", not everyone supports c99 (like MSVS prior to 2013). There are "msinttypes.h" files floating around to deal with this.

If you cast to a different type, depending on flags you may get a compiler warning for truncation or a change of sign. If you go this route pick a larger relevant fixed size type. One of unsigned long long and "%llu" or unsigned long "%lu" should work, but llu may also slow things down in a 32bit world as excessively large. (Edit - my mac issues a warning in 64 bit for %llu not matching size_t, even though %lu, %llu, and size_t are all the same size. And %lu and %llu are not the same size on my MSVS2012. So you may need to cast + use a format that matches.)

For that matter, you can go with fixed size types, such as int64_t. But wait! Now we're back to c99/c++11, and older MSVS fails again. Plus you also have casts (e.g. map.size() is not a fixed size type)!

You can use a 3rd party header or library such as boost. If you're not already using one, you may not want to inflate your project that way. If you're willing to add one just for this issue, why not use c++ streams, or conditional compilation?

So you're down to c++ streams, conditional compilation, 3rd party frameworks, or something sort of portable that happens to work for you.