64-bit integer declaration in C

Question

I was trying to declare 64bit variable in C with unsigned long long datatype but it not working and behaving similar to uint32 bit variable. What is the solution to access / check the bit above 32 ?

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

int main() {
    // Write C code here
    printf("Hello world");
    unsigned long long temp = 0x1234567890123456ULL;
    unsigned long long temp2 = temp >> 1;
    
    uint64_t var = 0x1234567890123456; 
    
    printf("\n%x", (long long) temp); 
    printf("\n%x", (long long) temp2); 
    printf("\n%x", 1ULL << 31ULL);
    printf("\n%x", 1ULL << 32ULL);
    
    printf("\n%x", var); 
    
    return 0;
}

Output:

Hello world

90123456

48091a2b

80000000

0

90123456

You are telling `printf()` to print `unsigned int` and not `unsigned long long int`. — Jonathan Leffler, May 31 '21 at 13:53
Did the compiler not give you warnings when you compiled the code? Both clang and gcc warn about this by default and tell you how to fix it. — Paul Hankin, May 31 '21 at 14:37

score 8 · Answer 1 · answered May 31 '21 at 13:52

You used wrong format specifier for printf(). %x is for printing unsigned int. Mismatch in format specifier and actual data in printf() invokes undefined behavior.

To print integers in hexadecimal, you should use %llx for unsigned long long and PRIx64 (from inttypes.h) for uint64_t.

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

int main() {
    // Write C code here
    printf("Hello world");
    unsigned long long temp = 0x1234567890123456ULL;
    unsigned long long temp2 = temp >> 1;
    
    uint64_t var = 0x1234567890123456; 
    
    printf("\n%llx", (long long) temp); 
    printf("\n%llx", (long long) temp2); 
    printf("\n%llx", 1ULL << 31ULL);
    printf("\n%llx", 1ULL << 32ULL);
    
    printf("\n%" PRIx64, var); 
    
    return 0;
}

"To print integers in hexadecimal..." hmm... to me that's not clear/correct. Should it be "To print 64 bit integers...." — Support Ukraine, May 31 '21 at 13:55
The casts to `long long` should also be removed. The specifier expects `unsigned long long` so we shouldn't pass a `signed long long`. — Nate Eldredge, May 31 '21 at 14:42

64-bit integer declaration in C

1 Answers1