Print a 64-bit integer using 32-bit integers

To print a 64-bit unsigned integer in a portable way, ideally you would do as follows:

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

int main()
{
    uint64_t val = 18369614221190020847;
    printf("%" PRIu64, val);
    return 0;
}

Where PRIu64 is a macro provided by inttypes.h, that typically expands to some string literal like llu. So upon compiling, the string will concatenate to form %llu, which formats an unsigned long long value in decimal:

18369614221190020847

However, if targeting an embedded platform with a stripped down version of libc¹, such as newlib-nano, this may silently fail! While the type uint64_t may exist, and the PRIu64 macro may also exist, if the version of printf (provided by libc) does not support the %llu macro, instead of printing 18369614221190020847, it will just print the string literal %llu²:

%llu

If you cannot change your version of libc, and need to print in decimal, there is no easy way around this.³

However, if you can tolerate printing in hexadecimal rather than decimal, you can seamlessly print the 64-bit unsigned integer as two 32-bit unsigned integers:

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

int main()
{
    uint64_t val = 18369614221190020847;
    printf("0x%08" PRIx32 "%08" PRIx32, (uint32_t)(val >> 32), (uint32_t)(val));
    return 0;
}

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