指向双精度指针的指针算术

Question

我试图了解执行此操作时会发生什么

#include <stdio.h>

const int MAX = 3;

int main () {

   //double  var[] = {10.0, 100.1, 200.2};
   double  var[3];
   int  i;
   double *ptr;
   double **ptr2;


   for(i=0;i<MAX;i++)var[i] = 10.0*(double)(i+1);
   /* let us have array address in pointer */
   ptr = var;
   ptr2 = &ptr;

   for ( i = 0; i < MAX; i++) {

      printf("Address of var[%d] = %x\n", i, *ptr2 );
      printf("Value of var[%d] = %f\n", i, **ptr2 );

      /* move to the next location */
      ptr2++;
   }

   return 0;
}

此代码段在第二次迭代中给出了段错误

Address of var[0] = b0e04370                                                                                                                                                                 
Value of var[0] = 10.000000                                                                                                                                                                  
Address of var[1] = 0  

但是，如果我们不调用ptr2而是调用ptr这将起作用

for ( i = 0; i < MAX; i++) {

 printf("Address of var[%d] = %x\n", i, ptr );
 printf("Value of var[%d] = %f\n", i, *ptr );

  /* move to the next location */
  ptr++;
}

给

Address of var[0] = 3e9e89a0                                                                                                                                                                 
Value of var[0] = 10.000000                                                                                                                                                                  
Address of var[1] = 3e9e89a8                                                                                                                                                                 
Value of var[1] = 20.000000                                                                                                                                                                  
Address of var[2] = 3e9e89b0                                                                                                                                                                 
Value of var[2] = 30.000000 

Answer 1

之所以会出现段错误，是因为下一个地址实际上没有容纳任何内容。

我将循环更改为此，现在它可以按您期望的方式工作：

/* let us have array address in pointer */
ptr = var;
ptr2 = &ptr;

for (i = 0; i < MAX; i++) 
{
    printf("Address of var[%d] = 0x%x\n", i, (void*)*ptr2 );
    printf("Value of var[%d] = %f -- using ptr\n", i, *ptr);
    printf("Value of var[%d] = %f -- using ptr2\n", i, **ptr2);
    ptr++;
    // This updates ptr2, which was missed out
    ptr2 = &ptr;
}

以此打印以下内容：

Address of var[0] = a313c9b0
Value of var[0] = 10.000000 -- using ptr
Value of var[0] = 10.000000 -- using ptr2
Address of var[1] = a313c9b8
Value of var[1] = 20.000000 -- using ptr
Value of var[1] = 20.000000 -- using ptr2
Address of var[2] = a313c9c0
Value of var[2] = 30.000000 -- using ptr
Value of var[2] = 30.000000 -- using ptr2

---

您的代码在第二次迭代（而不是第一次）中出现段错误的原因是

ptr = var;
ptr2 = &ptr;

在这两行中，当ptr指向var[0] ， ptr2不保存ptr的地址。

现在让我们看一下更新后的代码的第二次迭代：

ptr++;

这使ptr指向var[1]

ptr2 = &ptr;

这使ptr2保留ptr的地址，该地址现在指向var[1] 。
我想这可以解释。 也许您可以自己尝试第三次迭代。