c - 在结构中分配指针不起作用，为什么值没有改变？

Question

对于学校项目，我应该仅使用链表结构来实现 UNIX 文件系统的简化版本。我目前的 mkfs() 函数有问题，它应该只是初始化文件系统。

创建我正在使用的结构的头文件在这里:

typedef struct Lines {
  char line[82];
  struct Lines *next;
} Lines;

typedef struct Node {
  char *name;
  int id;
  struct Node *parent;
  struct Node *next;
  union {
    char line[82];
    struct Node *children;
  } contents;
} Node;

typedef struct Filesystem {
  char  *name;
  struct Node *root;
  struct Node *current;
} Filesystem;

这是我的单独文件中的方法，其中 #includes 这个头文件:

void mkfs(Filesystem *files) {
  Node *root = NULL; /* Creates a pointer to the directory we will use as
                      * the root directory for this filesystem*/

  files = (Filesystem *)malloc(sizeof(*files)); /* Allocates space for the the 
                             * filesystem structure */

  if(files == NULL){ /* If there is no memory available, prints error message
                      * and does nothing else */

    printf("Memory allocation failed!\n");

   } else {

    root = (Node *)malloc(sizeof(*root)); /* Allocates space for the root 
                   * directory of the filesystem. */


    if(root == NULL) { /* If there is no memory available, prints error
        * message and frees memory obtained thus far, but then 
        * does nothing else */

      printf("Memory allocation failed!\n");
      free(files);

    } else {

  /* Allocates space for the root directory's name string */
      root->name= (char *)malloc(sizeof(char)*(strlen("/")+1));

      if(root->name == NULL) { /* If there is no memory available, prints error
            * message and frees memory obtained thus far, 
            * but then does nothing else */

    printf("Memory allocation failed!\n");

    free(files);
    free(root);

      } else {

    root->name = "/"; /* Defines the root directory as being named by the
           * forward slash */ /* DO STR CPY HERE ITS CHANGING THE ADDRESS */
    root->contents.children = NULL;
    root->next = NULL;
    root->parent = NULL; /* UHH CHECK ON THIS NOOO CLUE IF ITS RIGHT FUUU*/

    files->root = root; /* The filesystems pointer to a directory is set 
             * to point to the root directory we just allocated
             * space for and set up */

    files->current = root; /* Sets the filesystems current directory to
            * point to the root directory as well, because 
            * it is the only directory in existence for this
            * filesystem at this point. */
      }
    }
  }
}

我遇到的问题是，当我运行 gdb 并逐步执行每一行时，最后两行分配行不会更改 file->root 和 file->current 的内容。
比如我这里打印files->root的内容，运行files->root = root这一行，然后再打印一次，可以看到地址没有变。但是，如果我只是打印 root，我试图将它分配给它，它显然具有不同的值，files->root 应该设置为:

(gdb) print files->root
$12 = (struct Node *) 0x400660
(gdb) step
(gdb) print files->root
$13 = (struct Node *) 0x400660
(gdb) print root
$14 = (Node *) 0x602030

有没有人知道为什么在这种情况下作业可能不起作用？这目前正在破坏我的整个项目，因此任何见解将不胜感激。谢谢!!!

Answer 1

它看起来像你的 mkfs函数正在接受一个指向已经存在的指针 Filesystem然后你试图为一个新的 Filesystem 分配内存在一个新的内存位置。像这样的函数有两种常见的约定:要么不接受参数并返回指向结构的指针，要么接受指向已分配结构的指针并填充该结构。看起来数据没有改变的原因是您实际上正在创建和填充第二个结构，而调用者的结构保持不变。

这是第一种情况的示例，将函数简化为仅内存分配部分:

Filesystem * mkfs() {
    Filesystem *files = (Filesystem *)malloc(sizeof(Filesystem));

    // (error handing omitted for brevity)

    // populate the files struct as appropriate...
    Node *root = (Node *)malloc(sizeof(Node));
    files->root = root;
    // etc, etc as you currently have

    return files;
}

// In this case you should also provide a way for the caller to free a filesystem,
// which will free everything you allocated during mkfs:
void freefs(Filessystem *files) {

    // first free any buffers you allocated inside the struct. For example:
    free(files->root);

    // then free the main filesystem struct
    free(files);
}

然后调用者使用这两个函数处理这个对象。例如:

int main() {
    Filesystem *files = mkfs();
    // now "files" is ready to use
    freefs(files); // free the objects when we're done with them.
}

这是第二种情况的示例，它假设调用者已经分配了适当的缓冲区，并且只需要填充它:

void mkfs(Filesystem *files) {

    // populate the files struct as appropriate...
    Node *root = (Node *)malloc(sizeof(Node));
    files->root = root;
    // etc, etc as you currently have

}

void freefs(Filesystem *files) {
    // still need to clean up all of the ancillary objects
    free(files->root);
    // etc, etc
}

在这种情况下，调用函数还有一些工作要做。例如:

int main() {
    Filesystem *files = (Filesystem *)malloc(sizeof(Filesystem));
    mkfs(files);
    // now "files" is ready to use
    freefs(files); // free the objects when we're done with them.
}

两种模式都是有效的；如果您希望调用者需要能够控制内存的分配方式，则前者很有用。例如，调用者可能决定在堆栈而不是堆上分配文件系统:

int main() {
    Filesystem files;
    mkfs(&files);
    // now "files" is ready to use
    freefs(&files); // free the ancillary objects when we're done with them.
    // "files" is still allocated here, but it's no longer valid
}

后者代表调用者处理分配。由于您的函数在堆上分配了更多结构，因此在这两种情况下都必须包含清理函数。