在Linux中，所有對設備和文件的操作都使用文件描述符來進行。

　　Linux中一個進程啟動時，都會打開3個文件：標準輸入、標準輸出和標準出錯處理。這三個文件分別對應文件描述符0、1、2。

　　在C語言中，在程序開始運行時，系統(tǒng)自動打開3個標準文件：標準輸入、標準輸出、標準出錯輸出。通常這3個文件都與終端相聯(lián)系。因此，以前我們所用到的從終端輸入或輸出都不需要打開終端文件。系統(tǒng)自定義了3個文件指針 stdin、stdout、stderr，分別指向終端輸入、終端輸出和標準出錯輸出(也從終端輸出)。

　　標準輸入流：stdin

　　標準輸出流：stdout

　　標準錯誤輸出流：stderr

　　stdin

　　objectFILE * 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

　　objectFILE * stdout;

　　Standard output stream

　　The standard output stream is the default destination of regular output for applications. It is usually directed to the output device of the standard console (generally, the screen).

　　stdout can be used as an argument for any function that expects an output stream as one of its parameters, like fputs or fprintf.

　　Although it is generally safe to assume that the default destination for stdout is going to be the screen, bear in mind that this may not be the case even in regular console systems, since stdout 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 redirect the output of myapplication to the file example.txt instead of the screen.

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