Standard and Special Files

You might think that programs start without any files open by default. This is not true. Linux programs usually have at least three open file descriptors when they begin. They are:

STDIN

• This is the standard input. It is a read-only file, and usually represents your keyboard. ^[3] This is always file descriptor 0.

STDOUT

• This is the standard output. It is a write-only file, and usually represents your screen display. This is always file descriptor 1.

STDERR

• This is your standard error. It is a write-only file, and usually represents your screen display. Most regular processing output goes to STDOUT, but any error messages that come up in the process go to STDERR. This way, if you want to, you can split them up into separate places. This is always file descriptor 2.

Any of these "files" can be redirected from or to a real file, rather than a screen or a keyboard. This is outside the scope of this book, but any good book on the UNIX command-line will describe it in detail. The program itself does not even need to be aware of this indirection - it can just use the standard file descriptors as usual.

Notice that many of the files you write to aren't files at all. UNIX-based operating systems treat all input/output systems as files. Network connections are treated as files, your serial port is treated like a file, even your audio devices are treated as files. Communication between processes is usually done through special files called pipes. Some of these files have different methods of opening and creating them than regular files (i.e. - they don't use the open system call), but they can all be read from and written to using the standard read and write system calls.

^[3]As we mentioned earlier, in Linux, almost everything is a "file". Your keyboard input is considered a file, and so is your screen display.