format '%lld' expects type 'long long int', but argument 4 has type 'int64_t'

Question

I try to print a variable of type int64_t using %lld format specifier but get the following warning?

Warning: format '%lld' expects type 'long long int', but argument 4 has type 'int64_t'

In my opinion, under linux, the int64_t is always long long int, then:

1. Why does this warning occur?
2. How can I fix this?

Answer 1

how to fix?

Use PRId64: (Don't forget to include <inttypes.h>)

printf("var64 = %" PRId64 "\n", var64);

Use PRIx64 and proper cast if you want to print it as hex.

int64_t is always long long int, then why does this warning occur?

C99 onwards (link to draft, Page 22), C specs suggest the type long long int should be atleast 64 bits but it may be more also.

— minimum value for an object of type long long int
LLONG_MIN -9223372036854775807 // −(2⁶³−1)
— maximum value for an object of type long long int
LLONG_MAX +9223372036854775807 // 2⁶³− 1

On some platforms, long long int might be 128-bit and the print statement invokes UB on such platforms.

So treat this warning as a portability issue warning.