Print string with `%016llx`

Question

Sorry for the probably basic question, but I'm trying to understand some code and I can't understand what this (%016llx) placeholder means.

I understand that %x is HEX coding, and to the best of my knowledge the numbers in between should be the number of digits and the 0 padding; but I cannot decode the 016ll sequence, and googling didn't helped so much more than showing an example in which it was somehow related to gcc.

Given that I'm a nearly-zero-experience programmer, can you help me?

Answer 1

The description of fprintf() in the C99 Standard tells us that the %016llx conversion specification is made up of

1. the mandatory % character
2. a 0 flag for padding
3. the 16 as "minimum field width"
4. the ll as "length modifiers"
5. the x conversion specifier

So, in whole it means to write a unsigned long long int in hexadecimal notation occupying a minimum of 16 positions, padded with 0.