big.c

/*
 *  Such as num is:12345678910, 'from_big1' funcation will be wrong
 *  since the sign extension: 'cltq' instruction will full fill upper 32 bits with 0xffffffff
 */
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <inttypes.h>

void to_big(unsigned char *buf, uint64_t v)
{
    buf[0] = (v >> 56) & 0xff;
    buf[1] = (v >> 48) & 0xff;
    buf[2] = (v >> 40) & 0xff;
    buf[3] = (v >> 32) & 0xff;
    buf[4] = (v >> 24) & 0xff;
    buf[5] = (v >> 16) & 0xff;
    buf[6] = (v >> 8) & 0xff;
    buf[7] = v & 0xff;
}

uint64_t from_big1(unsigned char *buf)
{
    uint64_t val = 0;

    val |= (uint64_t) buf[0] << 56;
    val |= (uint64_t) buf[1] << 48;
    val |= (uint64_t) buf[2] << 40;
    val |= (uint64_t) buf[3] << 32;
    val |= buf[4] << 24;
    val |= buf[5] << 16;
    val |= buf[6] << 8;
    val |= buf[7];
    return val;
}

/*
 * this is OK
 */
uint64_t from_big2(unsigned char *buf)
{
    uint64_t val = 0;

    val = buf[0];
    val = (val<<8) | buf[1];
    val = (val<<8) | buf[2];
    val = (val<<8) | buf[3];
    val = (val<<8) | buf[4];
    val = (val<<8) | buf[5];
    val = (val<<8) | buf[6];
    val = (val<<8) | buf[7];
    return val;
}

/*
 * this is OK
 */
uint64_t from_big3(unsigned char *buf)
{
    uint64_t val = 0;

    val |= (uint64_t) buf[0] << 56;
    val |= (uint64_t) buf[1] << 48;
    val |= (uint64_t) buf[2] << 40;
    val |= (uint64_t) buf[3] << 32;
    val |= (uint64_t) buf[4] << 24;
    val |= (uint64_t) buf[5] << 16;
    val |= (uint64_t) buf[6] << 8;
    val |= (uint64_t) buf[7];
    return val;
}



int main(int argc, char *argv[])
{
    if (argc != 3) {
        printf("./xx [num] [method:1 or 2]");
        return -1;
    }

    uint64_t v1, v2;
    unsigned char xx[8];
    int method = atoi(argv[2]);

    v1 = strtoull(argv[1], NULL, 0);
    to_big(xx, v1);

    switch (method) {
        case 1:
        v2 = from_big1(xx);
        break;
        case 2:
        v2 = from_big2(xx);
        break;
        case 3:
        v2 = from_big3(xx);
        break;
        default:break;
    }

    printf("v2 is %" PRIu64 "\n", v2);
    return 0;
}