C上的链表返回NULL

Question

我一直在尝试制作一个创建列表列表的程序(列表组包含两个列表:transform和obj)。所有的代码都完成了，但是当我试图打印列表的元素时，我得到了一个NULL。我想问题出在链表节点之间的链接上。下面是将转换(或对象，语法相同)插入到转换列表的函数和将组插入到主列表的函数。有什么建议吗?

void insertTransform (Transform* transform, char *command, char *command2)
{
     Transform * newTransform = (Transform*) malloc(sizeof(Transform));
     newTransform->k = findKey(command);
     newTransform->ttype = command;
     newTransform->param = command2;
     if (transform==NULL)
     {
         transform=newTransform;
         transform->next=NULL;
     }
     else
     {
         newTransform->next=transform;
         transform=newTransform;
     }
}
void insertGroup (Group* group, Transform *transform, Obj *obj)
{
     Group * newGroup = (Group*) malloc(sizeof(Group));
     newGroup->t = transform;
     newGroup->obj = obj;
     newGroup->next=group;
     group=newGroup;
}

Answer 1

您必须通过地址传递列表头(指针指向指针)或从函数返回新的列表头。我更喜欢前者，因为它可以让我从函数返回真正的错误代码，而不仅仅是NULL。

以相反的顺序处理:

int insertGroup (Group** group, Transform *transform, Obj *obj)
{
     Group * newGroup = malloc(sizeof(Group));
     if (newGroup == NULL)
     {
         perror("Failed to allocate new group");
         return -1;
     }
     newGroup->t = transform;
     newGroup->obj = obj;
     newGroup->next=*group;
     *group=newGroup;
     return 0;
}

调用

Group *grp = NULL;
if (insertGroup(&grp, transform, obj) != 0)
{
     // error condition
}

转换的处理类似:

int insertTransform (Transform** transform, char *command, char *command2)
{
     Transform * newTransform = malloc(sizeof(Transform));
     if (newTransform == NULL)
     {
         perror("Failed to allocate new transform");
         return -1;
     }
     newTransform->k = findKey(command);
     newTransform->ttype = command;
     newTransform->param = command2;
     newTransform->next = *transform;
     *transform = newTransform;
     return 0;
}

再次调用为:

Transform *tfrm = NULL;
if (insertTransform(&tfrm, cmd1, cmd2) != 0)
{
    // error condition
}

Answer 2

试试这个:

void insertGroup (Group* group, Transform *transform, Obj *obj)
{
     Group * newGroup = (Group*) malloc(sizeof(Group));
     newGroup->t = transform;
     newGroup->obj = obj;
     newGroup->next=NULL;
     group->next=newGroup;
}

假设传入的组是当前头部，并且您试图在末尾添加新节点。

或者在开头插入:

void insertGroup (Group* group, Transform *transform, Obj *obj)
{
     Group * newGroup = (Group*) malloc(sizeof(Group));
     newGroup->t = transform;
     newGroup->obj = obj;
     newGroup->next= group->next;
     group->next=newGroup;
}