Question

I’m trying to fit 16 hex values into an unsigned long long in visual C

unsigned long long test = 0x1A2A00DABABA7890;
printf("long long value %X\n", test);
printf("%d", sizeof(test));

My output shows it is 8 bytes but only storing the first 4 bytes as
it outputs

long long value BABA7890
8

Am I misunderstanding how this works? Thanks for any help.

Answer 1

Your printf format specifier doesn’t match your type, which causes undefined behaviour. Try:

printf("long long value %llX\n", test);

%d is the wrong format for a sizeof result too. You should be using %zu there.

Your compiler may warn you if you turn up some warning settings (clang does by default, for example):

example.c:6:30: warning: format specifies type 'unsigned int' but the argument
      has type 'unsigned long long' [-Wformat]
    printf("long long value %X\n", test);
                            ~^     ~~~~
                            %llX
example.c:7:14: warning: format specifies type 'int' but the argument has type
      'unsigned long' [-Wformat]
    printf("%d", sizeof(test));
            ~^   ~~~~~~~~~~~~
            %ld
2 warnings generated.

Edit: I notice in your question that you’re using Visual C. %z is a C99 feature and might not be supported by your compiler. In that case you should check the documentation to see the right format to use.