指向指针和指针数组的指针

Question

gcc 4.4.4 c89

我理解指针好了。 但是，我正在加紧指针数组和指针指针。

我一直在搞乱这个代码片段，并留下了我认为我理解的内容的评论。

非常感谢任何建议，如果我的评论与代码行一致吗？

void increment_ptr()
{
    /* Static char array */
    char src[] = "rabbit";
    /* pointer to array of pointers to char's - create 6 pointers in this array */
    char *dest[sizeof(src)];
    size_t i = 0;

    /* pointer to a char */
    char* chr_ptr = NULL;
    /* pointer to pointer that points to a char */
    char** ptr_ptr = NULL;

    /* chr_ptr pointer now points to the memory location where 'rabbit' is stored. */
    chr_ptr = src;
    /* ptr_ptr points to the first memory address of the pointer array of where dest is stored */
    ptr_ptr = dest;

    /* Deference chr_ptr and keep going until nul is reached 'rabbit\0'  */
    while(*chr_ptr != '\0')
        /* deference ptr_ptr and assign the address of each letter to the momory location where
           ptr_ptr is currently pointing to. */
        *ptr_ptr++ = chr_ptr++;

    /* reset the ptr_ptr to point to the first memory location 'rabbit' */
    ptr_ptr = dest;

    /* Keep going until NULL is found - However, my program never finds it, ends in UB */
    while(ptr_ptr != NULL) {
        /* Dereference what the pointer to pointer is pointing at the memory lcoation */
        printf("[ %s ]\n", *ptr_ptr++);
    }
}

Answer 1

每个部分下面的评论（我没有提到的位是正确的）：

/* Static char array */
char src[] = "rabbit";

此数组不是静态的 - 它具有auto存储持续时间。

/* pointer to array of pointers to char's - create 6 pointers in this array */
char *dest[sizeof(src)];

这是一个指向char的指针数组，而不是指向数组的指针。 数组的长度是7，因为sizeof(src)是7（它包括nul字符串终止符）。

/* chr_ptr pointer now points to the memory location where 'rabbit' is stored. */
chr_ptr = src;

更准确地说，它指向src中的第一个字符，即"rabbit"的'r' "rabbit" 。

/* ptr_ptr points to the first memory address of the pointer array of where dest is stored */
ptr_ptr = dest;

它指向dest数组中的第一个指针。

/* Keep going until NULL is found - However, my program never finds it, ends in UB */
while(ptr_ptr != NULL) {

正确 - 因为你从未初始化dest 。 您可以将dest的声明更改为：

char *dest[sizeof(src)] = { 0 };

......它会起作用。

Answer 2

我建议您阅读在线C-FAQ的第6部分： 6。数组和指针

Answer 3

错误是当你将dest分配给ptr_ptr时 ，它实际上是一个未初始化的字符指针数组，通过它会导致while循环失败。

/* reset the ptr_ptr to point to the first memory location 'rabbit' */
ptr_ptr = dest;

/* Keep going until NULL is found - However, my program never finds it, ends in UB */
while(ptr_ptr != NULL) {
    /* Dereference what the pointer to pointer is pointing at the memory lcoation */
    printf("[ %s ]\n", *ptr_ptr++);
}