链表头不能跨越函数调用进行更新 [英] Linked List head not being updated across function calls

问题描述

我想实现我自己的链接列表，并已与code学习有关动态内存分配和指针和这样瞎搞。当我尝试的东西添加到我的链表，我收到了段错误，并且在使用我意识到调试器，这是因为最初我链表的头指针并没有指向空，然后我的附加功能是不承认的头部是空的。但我有一个设置链表的头指针为NULL的初始化函数，但由于某种原因，我一旦退出初始化函数的出来，到add函数，头不再指向NULL。

I trying to implement my own linked list and have been messing around with the code learning about dynamic memory allocation and pointers and such. When I try to add something to my linked list I get a segfault, and upon using the debugger I realized that it was because initially my linked list's head pointer was not pointing to null and then my add function was not recognizing the head as being empty. But I have an initialize function that is setting the linked list's head pointer to NULL but for some reason once I exit out of the initialize function and into the add function, the head is no longer pointing to NULL.

下面是我的code：

list.h

typedef struct node{
    int value;
    struct node *next;
} Node;

typedef struct list{
    Node *head;
} List;

list.c

list.c

void initialize(List *l){
    if(l != NULL){
        l = malloc(sizeof(List));
        l->head = NULL;
    }
}

void add(List *l, int a){
    //Code
}

int main(){
    List l;
    initialize(&l)
    add(&l, 2);
}

当我踏进add函数，并打印出* L，我看到头不指向为0x0。我一直在抓我的头，为什么它不是。我认为这是一件由价值做通，但我不认为它是。我在做什么错在这里？

As soon as I step into the add function and print out *l, I see that the head is not pointing to 0x0. And I've been scratching my head as to why it's not. I thought it was something to do with pass by value but I don't think it is. What am I doing wrong here?

推荐答案

是的，通过按值就是你的罪魁祸首。你是按值传递的指针。

Yes, pass-by-value is your culprit. You are passing a pointer by value.

假设→在的main（）在地址 0xABCD 。那么你的的main（）被编译到

Suppose l in your main() is at address 0xABCD. Then your main() gets compiled to

int main(void) {
    List l;
    initialize(0xABCD);
    add(0xABCD, 2);
}

和您的初始化（）调用看起来像这样（假设的malloc（）成功，并在地址分配内存 0xCDEF ：

and your initialize() call looks like this (suppose malloc() succeeds and allocates memory at address 0xCDEF:

void initialize(List *l) {
    if(l != 0x0) {
        l = 0xCDEF;    // malloc()
        l->head = 0x0;
    }
}

这是 L = 0xCDEF 不传播到的main（），因为→是按值传递。

That l = 0xCDEF does not propagate to main(), because l was passed by value.

您想要做的是什么

void initialize(List **l) {
    if(l != NULL) {
        *l = malloc(sizeof(List)); // note dereferencing the passed-by-value pointer
        (*l)->head = NULL;
    }
}

int main(void) {
    List * l;
    initialize(&l);
    add(l, 2);
}

这将传递指针的指针的列表（在的main（），它允许在code 初始化（）更改→变量的main（）。

which will pass pointer to pointer to list (actually the address of the pointer in your main(). It allows the code in initialize() to change the l variable in main().

另外，你可以使用

List * list_init() {
    List * retval = malloc(sizeof(List));
    if(retval == NULL) { // you should check malloc return value
        // abort(), print warning or just
        return NULL;
    }
    retval->head = NULL;
    return retval;
}

int main(void) {
    List * l = list_init();
    if(l == NULL) {
        // handle the error
    }
    add(l, 2);
}

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