What is PRIu64 in C?

I am new to C and I am confronted with:

#include <stdio.h>
#include <inttypes.h>

int main(void)
{
uint64_t foo = 10;
printf("foo is equal to %" PRIu64 "!\n", foo);

return 0;
}
And it works! I don't understand why. Can somebody help me about this?

[`PRIu64`](

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) 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

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.