Most operating systems incorporate a powerful feature that allows a program to read and write files, even when such a capability has not been incorporated in the program. This is done through a process called ‘redirection’.
Normally a C program receives its input from the standard input device, which is assumed to be the keyboard, and sends its output to the standard output device, which is assumed to be the VDU. In other words, the OS makes certain assumptions about where input should come from and where output should go. Redirection permits us to change these assumptions
For example, using redirection the output of the program that normally goes to the VDU can be sent to the disk or the printer without really making a provision for it in the program. This is often a more convenient and flexible approach than providing a separate function in the program to write to the disk or printer. Similarly, redirection can be used to read information from disk file directly into a program, instead of receiving the input from keyboard.
To use redirection facility is to execute the program from the command prompt, inserting the redirection symbols at appropriate places. Let us understand this process with the help of a program.
Redirecting the Output
Let’s see how we can redirect the output of a program, from the screen to a file
/* File name: util.c */
#include "stdio.h"<+>
main( )
{
char ch ;
while ( ( ch = getc ( stdin ) ) != EOF )
putc ( ch, stdout ) ;
}
On compiling this program we would get an executable file UTIL.EXE. Normally, when we execute this file, the putc( ) function will cause whatever we type to be printed on screen, until we don’t type Ctrl-Z, at which point the program will terminate, as shown in the following sample run. The Ctrl-Z character is often called end of file character.
C>UTIL.EXE
perhaps I had a wicked childhood,
perhaps I had a miserable youth,
but somewhere in my wicked miserable past,
there must have been a moment of truth ^Z
C>