In C, how to set the MSB of an uint64_t according to a variable

Question

In C, I have a variable of the type uint64 ( x ) and a variable of type int ( i ). I need to change the MSB of x to the value of i (which will vary). How can I achieve that. Please help!

int i;
//
.. some code here that will set i to 0 or to 1. 
//

uint64_t x = 0xbeefcafebabecab1;

The binary representation of x is: 1011111011101111110010101111111010111010101111101100101010110001. I need to change the MSB (the leftmost 1 in this case) to the current value of i (say one or zero) How can i achieve that? I have some ideas but i'm getting more confused. Any suggestions will be really helpful.

Answer 1

Pretty much like this:

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

int main() {
    uint64_t x = 0x0f9c432a673750a1;

    for (int i = 0; i < 2; ++i) {
        if ( i )
            x |= ((uint64_t) 1 << 63);
        else
            x &= ~((uint64_t) 1 << 63);

        printf ("i: %d x: %lx\n", i, x);
    }

}

John

Answer 2

As you are interesting only on MSB, it is importante to take care of the endiness. To do so, I've adapted the JohnRowe code for this:

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

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
    #define _MSB_BIT_MASK_UINT64    (63)
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
    #define _MSB_BIT_MASK_UINT64    (0)
#else
    #error "Unknown endiness"
#endif

#define _MSB_MASK_UINT64        ((uint64_t) 1 << _MSB_BIT_MASK_UINT64)

#define _SET_MSB_UINT64(x)      (x | _MSB_MASK_UINT64)
#define _CLEAR_MSB_UINT64(x)    (x & ~_MSB_MASK_UINT64)

void printMessage(int i, uint64_t x)
{
    printf ("i: %d x: %llx\n", i, x);
}

int main() {
    uint64_t x1 = 0x0123456789ABCDEF;
    uint64_t x2 = 0x8FEDCBA987654321;

    printMessage(0, _CLEAR_MSB_UINT64(x1));
    printMessage(1, _SET_MSB_UINT64(x1));

    printMessage(0, _CLEAR_MSB_UINT64(x2));
    printMessage(1, _SET_MSB_UINT64(x2));
}

The macro BYTE_ORDER is GCC dependent.