Question

I have a problem while using the printf function to print values of type unsigned long long int

I have no idea what’s wrong. I’m using Dev-Cpp 4.9.9.2 and Visual Studio 2010 Professional (I know that it’s not C compiler, but anyway, wanted to try) on Windows 7 Professional 64-bit. For displaying, I used %llu modifier (according to How do you printf an unsigned long long int(the format specifier for unsigned long long int)?) but I also tried I64d with no effect…

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Firstly, I just wanted to print minimum and maximum value of unsigned long long int (using ULONG_MAX from limits.h)

printf("unsigned long long int: \n%llu to %llu \n\n", 0, ULONG_MAX);

Returns:

unsigned long long int: 18446744069414584320 to 1580552164021 (Dev-Cpp)

unsigned long long int: 18446744069414584320 to 0 (Visual Studio)

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Then I tried to using printf to print two zeros

printf("unsigned long long int: \n%llu to %llu \n\n", 0, 0);

Returns:

unsigned long long int:
0 to 1580552164021 (Dev-Cpp)

unsigned long long int: 0 to 0 (Visual Studio)

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Also tried two ULONG_MAX values

printf("unsigned long long int: \n%llu to %llu \n\n", ULONG_MAX, ULONG_MAX);

Returns:

unsigned long long int: 18446744073709551615 to 1580552164021 (Dev-Cpp)

unsigned long long int:
18446744073709551615 to 0 (Visual Studio)

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Why does it behave like that? Could you explain it to me?

Answer 1

This is wrong:

printf("unsigned long long int: \n%llu to %llu \n\n", 0, ULONG_MAX);

You use a unsigned long long format specifier, but you pass an int and an unsigned long value. Promotion rules mean you can be sloppy for everything int-sized or smaller, which does not apply to long long.

Use casts:

printf("unsigned long long int: \n%llu to %llu \n\n",
       0ULL, (unsigned long long) ULONG_MAX);

Explanation: When passing arguments to printf, any type that can fit in an int is promoted to int, and then any type that can fit in an unsigned int is promoted to unsigned int. It is also okay to pass an unsigned type to a signed format specifier or vice versa as long as the value passed can be represented using the type specified by the format specifier.

So you must be careful with long and long long, but you can be sloppy with int, short, and char.

Most compilers have settings to make them warn you about this type of error, since it can be detected at compile-time fairly easily; GCC and Clang have -Wformat which results in the following warnings:

test.c:5: warning: format ‘%llu’ expects type ‘long long unsigned int’, but argument 2 has type ‘int’
test.c:5: warning: format ‘%llu’ expects type ‘long long unsigned int’, but argument 3 has type ‘long unsigned int’