Linux – format ‘%lld’ expects type ‘long long int’, but argument 4 has type ‘int64_t’

clinux

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:

Why does this warning occur?
How can I fix this?

Best Answer

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.

Related Solutions

How to one print a size_t variable portably using the printf family

Use the z modifier:

size_t x = ...;
ssize_t y = ...;
printf("%zu\n", x);  // prints as unsigned decimal
printf("%zx\n", x);  // prints as hex
printf("%zd\n", y);  // prints as signed decimal

How to print a int64_t type in C

For int64_t type:

#include <inttypes.h>
int64_t t;
printf("%" PRId64 "\n", t);

for uint64_t type:

#include <inttypes.h>
uint64_t t;
printf("%" PRIu64 "\n", t);

you can also use PRIx64 to print in hexadecimal.

cppreference.com has a full listing of available macros for all types including intptr_t (PRIxPTR). There are separate macros for scanf, like SCNd64.

A typical definition of PRIu16 would be "hu", so implicit string-constant concatenation happens at compile time.

For your code to be fully portable, you must use PRId32 and so on for printing int32_t, and "%d" or similar for printing int.

Best Answer

Related Solutions

How to one print a size_t variable portably using the printf family

How to print a int64_t type in C

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