[Linux] [阅读] [APUE] - 第三章 文件I/O

本章需要关注的是

• 文件共享的描述（File Sharing），如下。

The UNIX System supports the sharing of open files among different processes. Before describing the dup function, we need to describe this sharing. To do this, we'll examine the data structures used by the kernel for all I/O. The following description is conceptual. It may or may not match a particular implementation. Refer to Bach [1986] for a discussion of these structures in System V. McKusick et al. [1996] describes these structures in 4.4BSD. McKusick and Neville-Neil [2005] cover FreeBSD 5.2. For a similar discussion of Solaris, see Mauro and McDougall [2001]. The kernel uses three data structures to represent an open file, and the relationships among them determine the effect one process has on another with regard to file sharing. Every process has an entry in the process table. Within each process table entry is a table of open file descriptors, which we can think of as a vector, with one entry per descriptor. Associated with each file descriptor are The file descriptor flags (close-on-exec; refer to Figure 3.6 and Section 3.14) A pointer to a file table entry The kernel maintains a file table for all open files. Each file table entry contains The file status flags for the file, such as read, write, append, sync, and nonblocking; more on these in Section 3.14 The current file offset A pointer to the v-node table entry for the file Each open file (or device) has a v-node structure that contains information about the type of file and pointers to functions that operate on the file. For most files, the v-node also contains the i-node for the file. This information is read from disk when the file is opened, so that all the pertinent information about the file is readily available. For example, the i-node contains the owner of the file, the size of the file, pointers to where the actual data blocks for the file are located on disk, and so on. (We talk more about i-nodes in Section 4.14 when we describe the typical UNIX file system in more detail.) Linux has no v-node. Instead, a generic i-node structure is used. Although the implementations differ, the v-node is conceptually the same as a generic i-node. Both point to an i-node structure specific to the file system.

• dup and dup2 Functions

#include <unistd.h> int dup(int filedes); int dup2(int filedes, int filedes2); Both return: new file descriptor if OK, 1 on error

说明：dup返回的新文件描述符一定是当前可用文件描述符中的最小值。用dup2则可以用filedes2参数指定新描述符的数值。如果filedes2已经打开，则先将其关闭。若filedes等于filedes2，则dup2返回filedes2，而不关闭它。另外需注意，这两个函数返回的新文件描述符与参数filedes共享同一个文件表项（file table）。例如

newfd = dup(1); // newfd和1指向同一个文件表项，如下表：