C链表-链接下一个链表节点

Question

我有一个应用程序，它接受多个命令行参数，在这些参数中，它接受一个目标主机文件，将对其执行某些操作。

173.194.40.225
157.55.152.112
200.49.185.230
195.95.178.226
98.158.27.203

每一行都是一个目标的IP地址，我使用下面的函数遍历文件;

// Open file read line by line to Linked List
ListNode* file(char *file, char *option) 
{
    FILE *ptr_file;
    char buffer[1000];
    char *destination;
    ListNode *linelist;
    // Create an empty line list
    linelist = NULL; 
    // Open a file
    if(strcmp(option, "r") == 0)
    {
        ptr_file = fopen(file, option);
        if(!ptr_file)
        {
            printf("Can't open filenn");
            exit(0);
        }
    }
    else
    {
        printf("File operation not implementednn");
        exit(0);
    }
    // Traverse through the file 
    while (fgets(buffer, LINE_MAX, ptr_file) != NULL)
    {
        printf("Line: %sn", buffer);
        // Add line to Linked List
        linelist = addtoList(linelist, buffer);
        printf("---------------------------------------nn");
    }
    printList(linelist);
    // Close the file
    fclose(ptr_file);
    // Return a pointer to linelist
    return linelist;
}

函数应该返回一个指向目标IP地址链表的指针，该链表可以在单独的套接字函数中使用。链表的设计如下:

// Define node structure
struct listnode 
{
    char *data;
    struct listnode *next;
};
// Define node as a type
typedef struct listnode ListNode;
// Add to List
ListNode* addtoList( ListNode *List, char *data );

file()函数使用while循环遍历传递的文件并使用fgets()将每一行加载到缓冲区后，缓冲区被传递给addtoList()函数，该函数接受两个变量，一个指向链表的指针和一个指向char的指针，实际上就是传递文件的每一行。该函数的作用是在传递的列表的末尾添加。

函数的工作方式如下:

1)创建一个ListNode &amp类型的临时指针;使用malloc()分配内存

2)检查malloc()是否返回NULL，如果返回则通知用户，否则用传入的数据填充临时ListNode 3)如果传递给函数的ListNode为NULL，只需将临时ListNode分配给传递的NULL ListNode并返回列表

4)如果传递给函数的ListNode不为NULL，并且传递的ListNode下一个值为NULL，则将临时ListNode分配给ListNode下一个值并返回列表

5)如果传递给函数的ListNode不为NULL，并且传递给函数的ListNode下一个值也不为NULL，则创建一个指向列表开头的指针的副本。使用while循环遍历列表，直到到达最后一个为NULL的ListNode。将临时ListNode分配给NULL ListNode

函数如下;

// Add to List
ListNode* addtoList(ListNode *List, char *data)
{
    printf("Adding: %sn", data);
    // Create pointer to allocated memory the size of a ListNode
    ListNode* temp = (ListNode*)malloc( sizeof( ListNode ) );
    // Check malloc didn't return NULL
    if(temp == NULL) 
    {
        printf( "Can't allocate memory for temp, failed to allocate the requested block of memory, a null pointer is returnednn" ); 
        exit(0);
    }
    else
    {
        printf("Memory allocated for temp, filling allocated memorynn");
        // Fill the allocated memory where data is a pointer to data
        temp->data = data;
        temp->next = NULL;
        printf("Allocated memory for temp filled with datann");
    }
    printf("temp->data = %snn", temp->data);
    int size = countList(List);
    printf("List size: %inn", size);
    if(List == NULL)
    {
        // If computer can't allocate memory let us know
        printf( "List is emptynn" ); 
        List = temp;
        printf( "List is now tempnn" ); 
        return List;
    }
    else
    if(List != NULL && List->next == NULL) 
    {
        printf("List isn't empty and List->next is NULLnn");
        List->next = temp;
        return List;
    }
    else
    if(List != NULL && List->next != NULL) 
    {
        printf("List isn't empty and List->next is not NULLnn");
        ListNode* Head = List;
        while(Head != NULL)
        {
            printf("List->next data: %s List->next pointer: %pnn", Head->data, Head->next);
            Head = Head->next;
        }
        if(Head == NULL)
        {
            printf("Head equals nulln");
            //problem here
            Head->next = temp;
        }
        return List;
    }
}

我的问题位于if(Head == NULL)条件语句中addtoList()函数的最后，将临时ListNode分配给Head->next;

if(Head == NULL)
{     
    printf("Head equals nulln");
    //problem here
    Head->next = temp;      
}

应用程序的输出如下:

######################################
Line: 173.194.40.225
Adding: 173.194.40.225
Memory allocated for temp, filling allocated memory
Allocated memory for temp filled with data
temp->data = 173.194.40.225

List size: 1
List is empty
List is now temp
---------------------------------------
Line: 157.55.152.112
Adding: 157.55.152.112
Memory allocated for temp, filling allocated memory
Allocated memory for temp filled with data
temp->data = 157.55.152.112

List size: 2
List isn't empty and List->next is NULL
---------------------------------------
Line: 200.49.185.230
Adding: 200.49.185.230
Memory allocated for temp, filling allocated memory
Allocated memory for temp filled with data
temp->data = 200.49.185.230

List size: 3
List isn't empty and List->next is not NULL
List->next data: 200.49.185.230
 List->next pointer: 0x8592180
List->next data: 200.49.185.230
 List->next pointer: (nil)
Head equals null
Segmentation fault

我遇到的问题是将临时ListNode分配到ListNode结构的NULL ListNode。

任何帮助都是非常感谢的……

编辑

所以我的addtoList()函数现在如下所示，在molbdnilo的&迭戈的建议;

// Add to end of List
ListNode* addtoList(ListNode *List, char *data)
{
    char* data_ptr = data;    // Create a pointer to passed data
    char* bkup_copy = NULL;   // Create a null pointer for backing up passed data
    bkup_copy = copyString(data_ptr); // Create a backup of data
    // Create pointer to allocated memory the size of a ListNode
    ListNode* temp = (ListNode*)malloc( sizeof( ListNode ) );
    // Check malloc didn't return NULL
    if(temp == NULL) 
    {
        printf("Can't allocate memory for temp, failed to allocate the requested block of memory, a null pointer returnednn" ); 
        exit(0);
    }
    else
    {
        // Fill the allocated memory where data is a pointer to data
        temp->data = bkup_copy;
        temp->next = NULL;       
    }
    if(List == NULL)
    {
        List = temp;
        return List;
    }
    else
    if(List != NULL && List->next == NULL) 
    {
        List->next = temp;
        return List;
    }
    else
    if (List != NULL && List->next != NULL) 
    {
        // Create a copy of pointer to passed list
        ListNode* Head = List;
        // Traverse through the list until last item
        while(Head->next != NULL)
        {
            Head = Head->next;
        }
        // Assign temp to the last list item next
        Head->next = temp;
        return List;
    }
    else
    {
        printf("Unknown state of Listnn");
        exit(0);
    }
}

使用新的copyString()函数返回字符串的副本，如下所示;

char* copyString(char* data)
{
    char* data_ptr = data;    // Create a pointer to passed data
    int orig_str_size = 0;    // Create an int to hold passed data size
    char* bkup_copy = NULL;   // Create a null pointer to backup passed data
    int bkup_index = 0;       // Create a index variable for traversal of passed data
    int length;
    // Count the number of characters in data_ptr
    while (*data_ptr++ != '')
    { 
        orig_str_size++; 
    }
    // Dynamically allocate space for a backup copy of data
    bkup_copy = (char*)malloc((orig_str_size+1) * sizeof(char));
    // Check malloc didn't return NULL
    if(bkup_copy == NULL) 
    {
        printf("Can't allocate memory for bkup_copy, failed to allocate the requested block of memory, a null pointer returnednn" ); 
        exit(0);
    }
    else // Copy data to separate allocated memory
    {
        // Place the '' character at the end of the backup string.
        bkup_copy[orig_str_size] = '';
        // Assign the pointer to data to the first pointer position in data
        data_ptr = &data[0]; 
        // The backup string is not the '' character copy data to bkup_copy
        while (*data_ptr != ''){ bkup_copy[bkup_index++] = *data_ptr++; }
    }
    return bkup_copy;
}

它现在像预期的那样工作…如果有人能看到任何问题的代码，请让我知道，这样我就可以修复它了…

Answer 1

问题是你不能解引用NULL，而你已经非常确定Head 是 NULL。

您需要提前停止遍历，以便将新节点链接到列表中的最后一个节点。

    // Find the last node - the node with NULL as its 'next'
    while (Head->next != NULL)
    {
        Head = Head->next;
    }
    // 'Head' is now the last node.
    // Replace the NULL at the end of the list with the new node.
    Head->next = temp;