K＆安培; R接口阅读目录：多余DIR结构？ [英] K&R interface for reading directories: superfluous DIR structure?

问题描述

在第2版的C程序设计语言的由Kernighan和Ritchie他们实现了UNIX命令 LS （段的简化版本8.6的示例 - 列出目录的，第179页）。为此，他们建立以下界面，提供对存储在目录中的文件名和inode号的系统无关的访问。

 的#define NAME_MAX 14 / *最长的文件名部分; * /
                              / *系统相关的* /typedef结构{/ *便携式主任入境* /
    长井野; / *索引节点号* /
    焦炭名[NAME_MAX + 1]; / *名称+'\\ 0'终止* /
}的dirent;typedef结构{/ *最小的DIR：无缓冲等* /
    INT的fd; / *文件描述符目录* /
    的dirent D组; / *目录项* /
} DIR;DIR *执行opendir（字符*目录名）;
*的dirent READDIR（DIR * DFD）;
无效closedir（DIR * DFD）;
 

然后，他们实现了这个接口的版本7和System V UNIX系统。

• 执行opendir（）基本上都采用该系统
  叫的open（）来打开一个目录，
  的malloc（）来的分配空间
  DIR 结构。文件描述符
  通过返回的open（）然后存储
  在的变量 FD DIR 。
  没有什么是存储在的dirent
  组件。




• READDIR（）使用系统调用
  阅读（）来得到下一个
  （系统相关的）的目录项
  一个打开的目录和副本等等
  获得inode编号和文件名
  为静态的dirent 结构（以
  这是返回指针）。该
  只有需要的信息
  READDIR（）是文件描述符
  存储在 DIR 结构。

现在我的问题：什么是具有 DIR 结构的意义呢？如果我的这个计划的理解是正确的， DIR 的的dirent 分量从来没有使用过，那么为什么不更换整个结构文件描​​述符并直接使用的open（）和的close（）？

感谢。

诗：据我所知，在现代Unix系统阅读（）不能再上的目录（我已经尝试了这个程序在Ubuntu 10.04）使用，但我还是想确保我没有忽略一些重要的东西在这个例子中。

解决方案

从K＆安培; R：

令人遗憾的是，一个目录的格式和precise内容不是在所有同
该系统的版本。因此，我们将划分任务分成两部分来尝试隔离
非便携式部件。外部级别定义的结构称为的dirent 和三个程序的执行opendir 的 READDIR 和 closedir 的提供系统 - 无关的访问的名称和节点号码的目录条目。

所以，原因是便携性。他们希望能够定义上有不同的统计结构或系统的非标准的open（）和的close（）。他们去建立一堆围绕它可重复使用的工具，如果他们是一个类Unix系统上都不在乎。这是包装的地步。

也许这不是使用，因为他们开始了通过定义数据结构（带的dirent内DIR），但最终不使用它。保持数据结构的分组喜欢就是好的设计。

In the 2nd edition of "The C Programming Language" by Kernighan and Ritchie they implement a simplified version of the UNIX command ls (section 8.6 "Example - Listing Directories", p. 179). For this purpose they create the following interface which provides a system-independent access to the name and inode number of the files stored in a directory.

#define NAME_MAX 14   /* longest filename component; */
                              /* system dependent */

typedef struct {      /* portable director-entry */
    long ino;                 /* inode number */
    char name[NAME_MAX+1];    /* name + '\0' terminator */
} Dirent;

typedef struct {      /* minimal DIR: no buffering, etc. */
    int fd;                   /* file descriptor for directory */
    Dirent d;                 /* the directory entry */
} DIR;

DIR *opendir(char *dirname);
Dirent *readdir(DIR *dfd);
void closedir(DIR *dfd);

Then they implement this interface for Version 7 and System V UNIX systems.

• opendir() basically uses the system call open() to open a directory and malloc() to allocate space for a DIR structure. The file descriptor returned by open() is then stored in the variable fd of that DIR. Nothing is stored in the Dirent component.

• readdir() uses the system call read() to get the next (system-dependent) directory entry of an opened directory and copies the so obtained inode number and filename into a static Dirent structure (to which a pointer is returned). The only information needed by readdir() is the file descriptor stored in the DIR structure.

Now to my question: What is the point of having a DIR structure? If my understanding of this program is correct, the Dirent component of DIR is never used, so why not replace the whole structure with a file descriptor and directly use open() and close()?

Thanks.

Ps: I am aware that on modern UNIX systems read() can no longer be used on directories (I have tried out this program on Ubuntu 10.04), but I still want to make sure that I have not overlooked something important in this example.

解决方案

From K&R:

Regrettably, the format and precise contents of a directory are not the same on all versions of the system. So we will divide the task into two pieces to try to isolate the non-portable parts. The outer level defines a structure called a Dirent and three routines opendir, readdir, and closedir to provide system-independent access to the name and inode number in a directory entry.

So the reason is portability. They want to define an interface that can survive on systems that have different stat structs or nonstandard open() and close(). They go on to build a bunch of reusable tools around it, which don't even care if they're on a Unix-like system. That's the point of wrappers.

Maybe it's not used because they started out by defining their data structures (with a Dirent inside DIR) but ended up not using it. Keeping data structures grouped like that is good design.

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