#include <stdio.h>
#include <stdint.h>
#include <string.h>
 
uint64_t numberize(const uint8_t from[8]) {
    uint64_t r = 0;
    uint8_t *p = &r;
 
    #if '01' == 0x4849 // big endian
        memcpy(p, from, 8);
    #else // little endian
        for (int i=7; i >= 0; --i)
            *p++ = from[i];     
    #endif
 
    return r;
}
 
int main() {
    const uint8_t data[8] = { 0x02, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07 };
 
    printf("result is %016llx\n", numberize(data));
 
    return 0;
}

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