在Linux中，所有对设备和文件的操作都使用文件描述符来进行。

　　Linux中一个进程启动时，都会打开3个文件：标准输入、标准输出和标准出错处理。这三个文件分别对应文件描述符0、1、2。

　　在C语言中，在程序开始运行时，系统自动打开3个标准文件：标准输入、标准输出、标准出错输出。通常这3个文件都与终端相联系。因此，以前我们所用到的从终端输入或输出都不需要打开终端文件。系统自定义了3个文件指针 stdin、stdout、stderr，分别指向终端输入、终端输出和标准出错输出（也从终端输出）。

　　标准输入流：stdin

　　标准输出流：stdout

　　标准错误输出流：stderr

　　stdin

　　object FILE * stdin;

　　Standard input stream

　　The standard input stream is the default source of data for applications. It is usually directed to the input device of the standard console (generally, a keyboard).

　　stdin can be used as an argument for any function that expects an input stream as one of its parameters, like fgets or fscanf.

　　Although it is generally safe to assume that the source of data for stdin is going to be a keyboard, bear in mind that this may not be the case even in regular console systems, since stdin can be redirected at the operating system level. For example, many systems, among them DOS/Windows and most UNIX shells, support the following command syntax:

　　myapplication < example.txt

　　to use the content of the file example.txt as the primary source of data for myapplication instead of the console keyboard.

　　It is also possible to redirect stdin to some other source of data from within a program using the freopen function.

　　stdout

　　object FILE * stdout;