PRIu64 is a format specifier, introduced in C99, for printing uint64_t, where uint64_t is (from linked reference page):
unsigned integer type with width of … 64 bits respectively
(provided only if the implementation directly supports the type)
PRIu64 is a string (literal), for example the following:
printf("%s\n", PRIu64);
prints llu on my machine. Adjacent string literals are concatenated, from section 6.4.5 String literals of the C99 standard:
In translation phase 6, the multibyte character sequences specified by any sequence of adjacent character and wide string literal tokens are concatenated into a single multibyte character sequence. If any of the tokens are wide string literal tokens, the resulting multibyte character sequence is treated as a wide string literal; otherwise, it is treated as a character string literal.
This means:
printf("foo is equal to %" PRIu64 "!\n", foo);
(on my machine) is the same as:
printf("foo is equal to %llu!\n", foo);
See http://ideone.com/jFvKR9 .