#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>
 
int main(void)
{
 
int8_t n[8] = "98765432";
uint64_t* a = (uint64_t *)n;
*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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