#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>
 
int main(void)
{
void *ptr = (char[]){'9', '8', '7', '6', '5', '4', '3', '2'};
uint64_t* a = ptr;
*a =((*a & 0x0F000F000F000F00)>>8) +
    ((*a & 0x000F000F000F000F)*10);
 
*a = 1000000 * ((*a >> 0 ) & 0xFF) +
       10000 * ((*a >> 16) & 0xFF) +
         100 * ((*a >> 32) & 0xFF) +
               ((*a >> 48) & 0xFF);
//little-endian only. Можно переделать под big-endian
printf("%"PRIu64, *a);
 
return 0;
}

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