Files and Devices

In the C standard library there are two basic functions which read and write respectively from/to the user's terminal :
getchar() returns one character every time it is called from the so called standard input
putchar(c) sends c to the standard output.
In most environments a file may be substituted for the terminal using the symbol > for the output and < for the input. If the program myprog for example uses getchar() to read from the terminal then the command

myprog < infile

will read input from infile instead of the keyboard.
This is called redirection.

The keyboard and the screen which are the peripheral devices related to the above standard input and standard output (logical) devices respectively plus two or three other standard devices which are explained below are treated for the first time under UNIX (and of course "C") and later under DOS and other console oriented operating systems as files. Although file functions are not discussed here it must be mentioned that when opening a file in C using one of the two existing standard functions a File Descriptor is returned. This is a value which is used as the reference to the opened file. The functions previously mentioned getchar() and putchar() are actually file functions which read and write respectively from two special files opened automatically when a program starts.
In other words these standard devices are treated as files. This is a great feature since it is related with other concepts which give a great flexibity to a programmer's strategic. For example a program made to write text to a file can be used to write to the standard printer file i.e. to print instead of storing the text on disk. Here is a table showing all the standard devices and other important names related to them. It must be mentioned that UNIX and DOS do not support all common features of standard devices.
  - there are differences in redirection :
     UNIX commands can redirect the error device also
  - the name at the command line are not the same (here DOS names are shown)

Device	Name in C (ANSI descriptor)	DOS Device	Mode	System file descriptor
Keyboard	stdin	CON	Input	0
Screen	stdout	CON	Output	1
Error	stderr	( N/A)	Output	2
T/C port	stdaux	COM1	I/O	3
Printer	stdprn	LPT1	Output	4

A function called getc( fd) similar to getchar() reads one character from file fd everytime it is called.
This means that getchar() is identical to getc(stdio)

Comments
COM1 and LPT1 are names which are used also as file names, in which case they are not used as standard devices.

The ANSI descriptors are pointers of type FILE (a special structure predefined in C) . So while file pointers are used to access files (or devices) using ANSI file functions , we can see that using system file functions the descriptors are non-negative integers.
We call such a file pointer a stream since using the basic file functions which read or write a character everytime other functions read/write a sequence of characters for example one line terminated with CarriageReturn and LineFeed

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