一个循环赋值的问题 - C语言论坛

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一个循环赋值的问题

我用的是Visual Studio 2017。

正常情况下代码如下

程序代码：

#include<stdio.h>
#include<conio.h>
int main()
{
    char f1_name[4096], f2_name[4096];
    unsigned long long *ip1, *ip2, *ipe1 = (unsigned long long*)(f1_name + 4096);

    ip1 = (unsigned long long*)f1_name;
    ip2 = (unsigned long long*)f2_name;
    for (; ip1 < ipe1; ip1++, ip2++)
        *ip1 = *ip2 = 0;
    ip1 = (unsigned long long*)f1_name;
    ip2 = (unsigned long long*)f2_name;
    for (; ip1 < ipe1; ip1++, ip2++)
        printf("%llX %llX\n", *ip1, *ip2);
    return 0;
}

输出是两列0。

在赋值前加入输出代码后代码如下

程序代码：

#include<stdio.h>
#include<conio.h>
int main()
{
    char f1_name[4096], f2_name[4096];
    unsigned long long *ip1, *ip2, *ipe1 = (unsigned long long*)(f1_name + 4096);

    ip1 = (unsigned long long*)f1_name;
    ip2 = (unsigned long long*)f2_name;
    for (; ip1 < ipe1; ip1++, ip2++)
    {
        printf("%llX %llX\n", *ip1, *ip2);
    };
    getchar();

    ip1 = (unsigned long long*)f1_name;
    ip2 = (unsigned long long*)f2_name;
    for (; ip1 < ipe1; ip1++, ip2++)
        *ip1 = *ip2 = 0;
    ip1 = (unsigned long long*)f1_name;
    ip2 = (unsigned long long*)f2_name;
    for (; ip1 < ipe1; ip1++, ip2++)
    {
        printf("%llX %llX\n", *ip1, *ip2);
    }
    return 0;
}

为什么在加入代码后输出的第二列不是0，而是一列相同的数字（每次运行不一定一样），例如63006200610060、67006600650064、6B006A00690068？

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#include<stdio.h>
#include<conio.h>
int main()
{
    char f1_name[4096], f2_name[4096];
    unsigned long long *ip1, *ip2, *ipe1 = (unsigned long long*)(f1_name + 4096);

    ip1 = (unsigned long long*)f1_name;
    ip2 = (unsigned long long*)f2_name;
    for (; ip1 < ipe1; ip1++, ip2++)
    {
        printf("%llX %llX\n", *ip1, *ip2);    //默认是输出字符串f1_name，f2_name,  他们没有初始化，所以出现的是乱码
    };
    getchar();

    ip1 = (unsigned long long*)f1_name;
    ip2 = (unsigned long long*)f2_name;
    for (; ip1 < ipe1; ip1++, ip2++)
        *ip1 = *ip2 = 0;            //这是给字符串f1_name，f2_name赋值，所有的值都设置为0
    ip1 = (unsigned long long*)f1_name;
    ip2 = (unsigned long long*)f2_name;
    for (; ip1 < ipe1; ip1++, ip2++)
    {
        printf("%llX %llX\n", *ip1, *ip2);        //这里的输出字符串f1_name，f2_name，即输出0
    }
    return 0;
}