[轉]文件IO詳解(二)---文件描述符(fd)和inode號的關系

原文：https://www.cnblogs.com/frank-yxs/p/5925563.html

文件IO詳解(二)---文件描述符(fd)和inode號的關系

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1、文件描述符和inode號碼是不同的兩個東西。

 

2、對於每個進程，系統會建立一個進程控制塊（PCB）來保存相關的信息，而這個PCB在內核中的表現其實就是一個稱為task_struct的結構體，這個結構體的成員用來保存與此進程有關的相關信息，其中有個成員是struct file_struct  *files，它是用來找到此進程所有打開的文件列表的，files變量指向的是struct file_struct類型的結構體，這個結構體中有一個成員是一個指針數組struct file *fd_array[]，這個數組中的每個元素會指向一個struct file的結構體，這個結構體是用來描述一個打開的文件的，有很多的成員，其中有個成員是指向struct file_operation的指針變量，而struct file_operation結構體中包括了很多的函數指針，也就是open，write，realease，read等等對文件進行操作的函數，具體這些函數的實現就是驅動開發需要做的事情。如下圖所示。

3、在對文件操作前，必須要先打開文件，打開文件過程其實就是讓程序能通過某個路徑能夠找到此文件，關閉文件就是將此路徑斷開，找不到文件。

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13.13 Duplicating Descriptors

You can duplicate a file descriptor, or allocate another file descriptor that refers to the same open file as the original. Duplicate descriptors share one file position and one set of file status flags (see File Status Flags), but each has its own set of file descriptor flags (see Descriptor Flags).

The major use of duplicating a file descriptor is to implement redirection of input or output: that is, to change the file or pipe that a particular file descriptor corresponds to.

You can perform this operation using the fcntl function with the F_DUPFD command, but there are also convenient functions dup and dup2 for duplicating descriptors.

The fcntl function and flags are declared in fcntl.h, while prototypes for dup and dup2 are in the header file unistd.h.

Function:  int  dup  (int old)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function copies descriptor old to the first available descriptor number (the first number not currently open). It is equivalent to fcntl (old, F_DUPFD, 0).

Function:  int  dup2  (int old, int new)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function copies the descriptor old to descriptor number new.

If old is an invalid descriptor, then dup2 does nothing; it does not close new. Otherwise, the new duplicate of old replaces any previous meaning of descriptor new, as if new were closed first.

If old and new are different numbers, and old is a valid descriptor number, then dup2 is equivalent to:

close (new);
fcntl (old, F_DUPFD, new)

However, dup2 does this atomically; there is no instant in the middle of calling dup2 at which new is closed and not yet a duplicate of old.

Macro:  int  F_DUPFD

This macro is used as the command argument to fcntl, to copy the file descriptor given as the first argument.

The form of the call in this case is:

fcntl (old, F_DUPFD, next-filedes)

The next-filedes argument is of type int and specifies that the file descriptor returned should be the next available one greater than or equal to this value.

The return value from fcntl with this command is normally the value of the new file descriptor. A return value of -1 indicates an error. The following errno error conditions are defined for this command:

EBADF

The old argument is invalid.

EINVAL

The next-filedes argument is invalid.

EMFILE

There are no more file descriptors available—your program is already using the maximum. In BSD and GNU, the maximum is controlled by a resource limit that can be changed; see Limits on Resources, for more information about the RLIMIT_NOFILE limit.

ENFILE is not a possible error code for dup2 because dup2 does not create a new opening of a file; duplicate descriptors do not count toward the limit which ENFILE indicates. EMFILE is possible because it refers to the limit on distinct descriptor numbers in use in one process.

Here is an example showing how to use dup2 to do redirection. Typically, redirection of the standard streams (like stdin) is done by a shell or shell-like program before calling one of the exec functions (see Executing a File) to execute a new program in a child process. When the new program is executed, it creates and initializes the standard streams to point to the corresponding file descriptors, before its main function is invoked.

So, to redirect standard input to a file, the shell could do something like:

pid = fork ();
if (pid == 0)
  {
    char *filename;
    char *program;
    int file;
    …
    file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY));
    dup2 (file, STDIN_FILENO);
    TEMP_FAILURE_RETRY (close (file));
    execv (program, NULL);
  }

There is also a more detailed example showing how to implement redirection in the context of a pipeline of processes in Launching Jobs.