c - 在用 C 编写的 DL 列表中出现段错误？

Question

我试图在 C 中实现一个 DLlist，当我向列表中添加第二个项目时出现了段错误......(int main 的最后一行)。这是我第一次用 C 编写，通常我使用 C++，所以我可能会做一些非常明显的错误而不知道。如果您看到我做错了什么，请告诉我。谢谢!

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

struct node {
    int value;
    struct node *next;
    struct node *previous;
};

int listSize = 0;
struct node *head;
struct node *tail;
struct node *cur;


/*
 * Function to add a process to the scheduler
 * @Param tid - the ID for the process/thread to be added to the 
 *      scheduler queue
 * @return true/false response for if the addition was successful
 */
int addProcess(int tid){
    if(tid)
    {
        cur->value = tid;
        if(listSize == 0)
        {
            cur->next = (struct node *) malloc( sizeof(struct node));
            cur->next->previous = cur;
            head = cur;
            tail = cur;
        }
        else
        {
            cur->next = (struct node *) malloc( sizeof(struct node));
            cur->next->previous = cur;
        }
        cur = cur->next;
        listSize++;
        return 1;    
    }
    return 0;
}
/*
 * Function to remove a process from the scheduler queue
 * @Param tid - the ID for the process/thread to be removed from the
 *      scheduler queue
 * @Return true/false response for if the removal was successful
 */
int removeProcess(int tid)
{
    cur = head;
     // handles list size of 1 removal
    if(cur == tail)
    {
        if(cur->value == tid)
        {
            free(cur->next);
            cur->next = NULL;
            free(cur);
            cur = NULL; 
            listSize--;
            return 1;
        }
    }
    // handles removal of the head node
    else if(head->value == tid)
    {
        cur->next->previous = NULL;
        head = cur->next;
        free(cur);
        listSize--;
        return 1;
    }
    else
    // all other cases
        while(cur->next != NULL)
        {
            // removal of a node with a next and a previous node
            if(cur->value == tid)
            {
                cur->previous->next = cur->next;
                cur->next->previous = cur->previous;
                free(cur);
                cur = NULL;
                listSize--;
                return 1;
            }   
            // removal of the tail
            else if(cur->next == tail)
            {
                if(tail->value == tid)
                {
                    free(cur->next);
                    cur->next = (struct node *) malloc( sizeof(struct node));
                    listSize--;
                    return 1;
                }
            }
            cur = cur->next;
        }
    return 0;
}
/*
 * Function to get the next process from the scheduler
 * @Return returns the thread id of the next process that should be 
 *      executed, returns -1 if there are no processes
 */
int nextProcess(){
    if(cur)    
    {
        cur = cur->next;
        return cur->previous->value;
    }
    return -1;
}

int main()
{
    cur = head = tail = (struct node *) malloc( sizeof(struct node));
    addProcess(5);
    while(cur)
    {
        printf("%d ", cur->value);
        cur=cur->next;
    }
    printf("\n");
    removeProcess(5);
    addProcess(5);
}

Answer 1

当程序试图用一个不存在的值做某事时，就会导致段错误，例如测试一个已经初始化但没有赋值的变量。编译器不会捕获段错误，这使它们成为最丑陋的错误。

我看到您能够使用 cur->next != NULL 行，它通常用于通过确保变量具有值来防止段错误。尝试使用诸如 if(cur != NULL) 之类的行来防止代码出现 fatal error 。

我还建议使用 gdb 调试器，它可以让您逐行检查代码并检查潜在问题。