子函数返回值数组赋值给指针，调试监控指针显示无法读取内存

Question

子函数返回值数组赋值给指针，调试监控指针显示无法读取内存
定义的全局变量

unsigned long long ll_a[5] = { 1844674409709551615,1844674409709551615,1844674409709551615,1844674409709551615 ,0 };
unsigned long long ll_b[5] = { 1844674409709551615,115292150606846975,115292150606846975,115292150606846970 ,0 };
long long ll_p[5] = { 4294967293 ,4294967293 ,4294967293 ,4294967293 ,0 };
long long ll_k[5] = { 4294967293 ,4294967293 ,4294967293 ,4294967293 ,0 };
long long* p;
unsigned short us_datalength = 4;
unsigned short us_bitlenght = 64;
unsigned short us_carrybit = 0;
unsigned long long* l;
unsigned long long ll_z[5] = {0};

main.c文件

int main()
{      
       unsigned short i;
        l= sub(ll_a, ll_b);
    
        for (i = 0; i < 5; i++)
        {
            printf("a[]= %llu\n", *(l+i));
        }
    return 0;
}

子函数sub在sub.c文件

unsigned long long sub(unsigned long long a[], unsigned long long b[])
{
    unsigned short i;
    static unsigned long long ll_z[5];

    us_carrybit = 0;
    for (i = 0; i < us_datalength; i++)
    {
        if (a[i] < (b[i] + us_carrybit))
        {
            ll_z[i] = (unsigned long long) ((0x01 << us_bitlenght) - 1) + a[i] - b[i] - us_carrybit;
            us_carrybit = 1;
        }
        else
        {
            ll_z[i] = a[i] - b[i] - us_carrybit;
            us_carrybit = 0;
        }    
    } ;
    return ll_z;
}

运行结果及报错内容

调试时在printf会报如下异常。监控指针l会显示无法读取内存
0x00007FF68E6118BE 处(位于 ECC Project.exe 中)引发的异常: 0xC0000005: 读取位置 0xFFFFFFFF8E61C248 时发生访问冲突。

我的解答思路和尝试过的方法

同样的main函数，我放到sub.c文件执行，就完全没有问题。
我上面的程序是否存在指针越界？所以才出现这个问题？

我想要达到的结果

请问，如果我的主函数和子函数是两个.c文件，为什么 报错，怎么解决呀？

Answer 1

你可以把代码改成如下形式

main.c

#include <stdio.h>

extern unsigned long long ll_a[5];
extern unsigned long long ll_b[5];

unsigned long long *sub(unsigned long long a[], unsigned long long b[]);

int main() {
  unsigned short i;
  unsigned long long *l = sub(ll_a, ll_b);

  for (i = 0; i < 5; i++)
    printf("a[%d]= %llu\n", i, *(l + i));

  return 0;
}

sub.c

unsigned long long ll_a[5] = {1844674409709551615, 1844674409709551615,
                              1844674409709551615, 1844674409709551615, 0};
unsigned long long ll_b[5] = {1844674409709551615, 115292150606846975,
                              115292150606846975, 115292150606846970, 0};
unsigned short us_datalength = 4;
unsigned short us_bitlenght = 64;
unsigned short us_carrybit = 0;

unsigned long long *sub(unsigned long long a[], unsigned long long b[]) {
  unsigned short i;
  static unsigned long long ll_z[5];

  us_carrybit = 0;
  for (i = 0; i < us_datalength; i++) {
    if (a[i] < (b[i] + us_carrybit)) {
      ll_z[i] = (unsigned long long)((0x01 << us_bitlenght) - 1) + a[i] - b[i] -
                us_carrybit;
      us_carrybit = 1;
    } else {
      ll_z[i] = a[i] - b[i] - us_carrybit;
      us_carrybit = 0;
    }
  }
  return ll_z;
}

$ gcc -Wall main.c sub.c
$ ./a.out
a[0]= 0
a[1]= 1729382259102704640
a[2]= 1729382259102704640
a[3]= 1729382259102704645
a[4]= 0