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
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
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