通用堆栈库的问题

Question

我正在尝试编写一个通用堆栈库[使用数组]，它可以为任何元素大小和元素数量分配一个堆栈。我正在尝试下面的代码。 首先，我收到一条警告，说我在 memcpy 中使用时取消引用 void 指针。

其次，我在尝试打印的输出中遇到了一些问题。请帮忙。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct _stk_
{
    int top;
    int elemsize;
    int numElem;
    void *array;
}stack;

stack *create_stack(int elemsize, int numElem)
{
    stack *stk_handle = malloc(sizeof(stack));

    if(!stk_handle)
    {
        printf("Malloc failed for stack handle\r\n");
        return NULL;
    }
    stk_handle->top = -1;
    stk_handle->elemsize = elemsize;
    stk_handle->numElem = numElem;
    stk_handle->array = calloc(elemsize, numElem);

    if(!(stk_handle->array))
    {
        printf("Malloc failed for stack\r\n");
        free(stk_handle);
        return NULL;
    }

    return stk_handle;
}

void push (stack *stk_handle, void *element)
{
    memcpy(&(stk_handle->array[++stk_handle->top]), element, stk_handle->elemsize);
    printf("Pushed element %s to stack. Top is %d\r\n", element, stk_handle->top);
}

void pop (stack *stk_handle, void *element)
{
    memcpy(element, &(stk_handle->array[stk_handle->top--]), stk_handle->elemsize);
    printf("Popped element %s from stack. Top is %d\r\n", element, stk_handle->top);
}

int main()
{
    stack *stk_handle = NULL;
    char string1[] = "close";
    char string2[] = "the";
    char string3[] = "door";
    char string4[6], string5[6],string6[6];

    stk_handle = create_stack(6, 5);
    push(stk_handle, &(string1));
    push(stk_handle, &(string2));
    push(stk_handle, &(string3));
    pop(stk_handle, string4);
    pop(stk_handle, string5);
    pop(stk_handle, string6);
    return 1;
}

当我编译时，我得到以下编译警告。

$ gcc stack_lib.c
stack_lib.c: In function âpushâ:
stack_lib.c:39: warning: dereferencing âvoid *â pointer
stack_lib.c: In function âpopâ:
stack_lib.c:45: warning: dereferencing âvoid *â pointer

当我运行程序时，我得到以下输出：

Pushed element close to stack. Top is 0
Pushed element the to stack. Top is 1
Pushed element door to stack. Top is 2
Popped element door from stack. Top is 1
Popped element tdoor from stack. Top is 0
Popped element ctdoortdoor from stack. Top is -1

请告诉我哪里出错了。

Answer 1

stk_handle->array[...] 是 void pointer 取消引用。由于 array 成员是 void 指针，因此您不能直接取消引用它（使用取消引用运算符 * 或像您一样使用数组语法）而不强制转换为具体类型。

请记住，对于数组索引，编译器使用索引乘以基类型来获得正确条目的偏移量。当您有void* 时，基本类型是什么？ void 本身不是有效的基类型，那么编译器应该将索引与什么相乘？它根本行不通。

我的建议是使用int8_t 指针，然后将top 与元素大小相乘：

typedef struct _stk_
{
    int top;
    int elemsize;
    int numElem;
    int8_t *array;
}stack;

...

memcpy(stk_handle->array + (++stk_handle->top * stk_handle->elmsize), ...);

Answer 2

1. 那些dereferencing void * pointer 警告可以通过使用适当的指针算法来修复：

   void push (stack *stk_handle, void *element)
   {
           memcpy((char *)stk_handle->array + ++stk_handle->top * stk_handle->elemsize, element, stk_handle->elemsize);
           printf("Pushed element %s to stack. Top is %d\r\n", (char *)element, stk_handle->top);
   }
   
   void pop (stack *stk_handle, void *element)
   {
           memcpy(element, (char *)stk_handle->array + stk_handle->top-- * stk_handle->elemsize, stk_handle->elemsize);
           printf("Popped element %s from stack. Top is %d\r\n", (char *)element, stk_handle->top);
   }
   

   如果您使用gcc，则不需要在stk_handle 之前进行类型转换，请参阅Pointer arithmetic for void pointer in C。

2. push(stk_handle, &(string1));中的&操作是不必要的，你可以写

   push(stk_handle, string1);
   push(stk_handle, string2);
   push(stk_handle, string3);