How to printf a 64-bit integer as hex?

Question

With the following code I am trying to output the value of a unit64_t variable using printf(). Compiling the code with gcc, returns the following warning:

warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 2 has type ‘uint64_t’ [-Wformat=]

The code:

#include <stdio.h>
#include <stdint.h>

int main ()
{
    uint64_t val = 0x1234567890abcdef;
    printf("val = 0x%x\n", val);

    return 0;
}

The output:

val = 0x90abcdef

Expected output:

val = 0x1234567890abcdef

How can I output a 64bit value as a hexadecimal integer using printf()? The x specifier seems to be wrong in this case.

Solution 1

The warning from your compiler is telling you that your format specifier doesn't match the data type you're passing to it.

Try using %lx or %llx. For more portability, include inttypes.h and use the PRIx64 macro.

For example: printf("val = 0x%" PRIx64 "\n", val); (note that it's string concatenation)

Solution 2

Edit: Use printf("val = 0x%" PRIx64 "\n", val); instead.

Try printf("val = 0x%llx\n", val);. See the printf manpage:

ll (ell-ell). A following integer conversion corresponds to a long long int or unsigned long long int argument, or a following n conversion corresponds to a pointer to a long long int argument.

Edit: Even better is what @M_Oehm wrote: There is a specific macro for that, because unit64_t is not always a unsigned long long: PRIx64 see also this stackoverflow answer

Comments

[M Oehm]

See here. basically, use the PRIx64 macro from <inttypes.h>.

[Daniel]

Use printf("val = %#018"PRIx64"\n", val); to print leading zeros. Don't forget to #include <inttypes.h>.

[David Ljung Madison Stellar]

I don't believe your edit is correct - it's not "%PRIx64" - it's a string concat with the value defined by the macro PRIx64

[Superlokkus]

@DavidLjungMadison You were right, fixed it

[Top-Master]

also %016llx could be used to ensure output has padding with zero (at the beginning) to reach minimum field width of 16 characters

[user2023370]

Why does 0000000000000000 not get a 0x at the front?

[Welgriv]

For the record, PRIx64 is #defined to "ll" "x"

[tangrs]

@Welgriv on which platform and compiler though? There's no guarantee that the macro will be defined to "ll" "x" on all platforms, and that's the whole point of the macro.

[Welgriv]

Yes, but inttypes.h looks the same in any platform. At least I have the same on Win10 (MINGW), Linux (gcc) and embed environment too (armcc).

[tangrs]

@Welgriv I'm not sure what point you're trying to make. I don't doubt that many popular, modern platforms share the same definition of PRIx64.

[tangrs]

@Welgriv It is certainly a mistake to assume that this is necessarily true on "any platform", though. I'm sure there's a counterexample out there, if one looks hard enough. Whether this is a real problem in practice is a different matter, however.

[swineone]

PRIX64 also works, and prints uppercase rather than lowercase letters (A-F).