使用列表的堆栈操作

Question

我想使用链接列表在C上编程堆栈。我有一个CreateStack函数的问题。当我创建一个堆栈时，一切似乎都很好，但当我尝试创建另一个堆栈时，该程序只会崩溃。这里是我的代码的一部分

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

struct StackData{ 
    int data; 
    struct StackData* prev; 
    struct StackPoint* point; 
}; 

struct StackPoint{ 
    struct StackData* basePointer; 
    struct StackData* stackPointer; 
    char exists; 
}; 

int CreateNewStack(struct StackData** newPointer){ 
    if((*newPointer)->point->exists=='Y') //i'm trying to check whether the list with the same adress has been already created and i think this is the part which causes error 
     return 1; //if error 
    else{ 
     (*newPointer)=(struct StackData*)malloc(sizeof(struct StackData)); 
     (*newPointer)->data=NULL; 
     (*newPointer)->prev=NULL; 
     (*newPointer)->point=(struct StackPoint*)malloc(sizeof(struct StackPoint)); 
     (*newPointer)->point->basePointer=newPointer; 
     (*newPointer)->point->stackPointer=newPointer; 
     (*newPointer)->point->exists='Y'; 
     return 0; 
    } 
} 

int main() 
{ 
    struct StackData *pointeris, *temp; 
    int state; 
    state=CreateNewStack(&pointeris); //state 0 

    state=CreateNewStack(&pointeris); //state 1 


    CreateNewStack(&temp);   //crash 
    return 0; 
} 

Answer 1

此代码有编译器警告。

test.c:21:28: warning: incompatible pointer to integer conversion assigning to 'int' from 'void *' 
     [-Wint-conversion] 
     (*newPointer)->data=NULL; 
          ^~~~~ 
test.c:24:42: warning: incompatible pointer types assigning to 'struct StackData *' from 
     'struct StackData **'; dereference with * [-Wincompatible-pointer-types] 
     (*newPointer)->point->basePointer=newPointer; 
             ^~~~~~~~~~~ 
              * 
test.c:25:43: warning: incompatible pointer types assigning to 'struct StackData *' from 
     'struct StackData **'; dereference with * [-Wincompatible-pointer-types] 
     (*newPointer)->point->stackPointer=newPointer; 
              ^~~~~~~~~~~ 
              * 

解决了那些后眼前的问题是这一行：

if((*newPointer)->point->exists=='Y') 

*newPointer可能为空。 *newPointer->point可能是垃圾。 *newPointer->point->exists可能是垃圾。

问题的一部分是CreateNewStack真的“也许创建一个新的堆栈，如果堆栈不存在”。如果您将“创建新堆栈”与“如果尚未存在的情况下创建新堆栈”分开，则您的代码将更加简单。实际上，每个结构都应该有自己的创建和销毁功能。

我也冒昧为您的结构添加typedef。我已经从malloc切换到calloc，以确保分配的结构在创建时为零。这意味着，即使您忘记初始化每个成员，结构成员中也不会有任何垃圾。

typedef struct StackData { 
    int data; 
    struct StackData* prev; 
    struct StackPoint* point; 
} StackData; 

typedef struct StackPoint { 
    struct StackData* basePointer; 
    struct StackData* stackPointer; 
    char exists; 
} StackPoint; 

StackPoint *CreateStackPoint() { 
    StackPoint *point = calloc(1, sizeof(StackPoint)); 

    point->exists = 'Y'; 

    return point; 
} 

StackData *CreateStackData() { 
    StackData *data = calloc(1, sizeof(StackData)); 

    data->point = CreateStackPoint(); 
    data->point->basePointer = data; 
    data->point->stackPointer = data; 

    return data; 
} 

现在CreateNewStack变得MaybeCreateStackData和简单得多。

int MaybeCreateStackData(StackData **maybeStack_p) { 
    if(maybeStack_p == NULL) { 
     fprintf(stderr, "The stack double pointer cannot be null"); 
     exit(1); 
    } 

    StackData *maybeStack = *maybeStack_p; 

    if(maybeStack && maybeStack->point && maybeStack->point->exists == 'Y') { 
     return 1; 
    } 
    else { 
     *maybeStack_p = CreateStackData(); 
     return 0; 
    } 
} 

老实说这个方案太复杂了。在C中，很难知道指针何时被正确初始化，而只是指向随机存储器。您的exists成员会尝试解决该问题，但垃圾仍可能变为Y。相反，我会放弃整个想法，让调用者决定他们是否有一个有效的结构或不。现在你有了可靠的初始化函数，很少有人可能会有一个初始化不好的指针。