Most operating systems, including DOS, provide a means to redirect program input and output to and from different devices. This means that rather than your program output (stdout) going to the screen; it can be redirected to a file or printer port. Similarly, your program?s input (stdin) can come from a file rather than the keyboard. In DOS, this task is accomplished using the redirection characters, < and >. For example, if you wanted a program named PRINTIT.EXE to receive its input (stdin) from a file named
STRINGS.TXT, you would enter the following command at the DOS prompt:

C:>PRINTIT < STRINGS.TXT

Notice that the name of the executable file always comes first. The less-than sign (<) tells DOS to take the strings contained in STRINGS.TXT and use them as input for the PRINTIT program.

The following example would redirect the program?s output to the prn device, usually the printer attached on LPT1:

C :> REDIR > PRN

Alternatively, you might want to redirect the program?s output to a file, as the following example shows:

C :> REDIR > REDIR.OUT

In this example, all output that would have normally appeared on-screen will be written to the file
REDIR.OUT.

Redirection of standard streams does not always have to occur at the operating system. You can redirect a standard stream from within your program by using the standard C library function named freopen(). For example, if you wanted to redirect the stdout standard stream within your program to a file named OUTPUT.TXT, you would implement the freopen() function as shown here:

...
freopen(?output.txt?, ?w?, stdout);
...

Now, every output statement (printf(), puts(), putch(), and so on) in your program will appear in the file
OUTPUT.TXT.

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