Sams Teach Yourself C in 21 Days

If your DOS system has a printer connected to port LPT1:, you can compile and run this

program. It prints three lines on the page. Line 14 sends an “\f”to the printer. \fis the

escape sequence for a form feed, the command that causes the printer to advance a page

(or, in the case of a laser printer, to eject the current page).

372 Day 14

This output is printed by the printer. It won’t appear on-screen.

Note

DOusefprintf()to create programs

that can send output to stdout,stderr,

stdprn, or any other stream.

DOusefprintf()withstderrto print

error messages to the screen.

DOcreate functions such as

error_messageto make your code more

structured and maintainable.

DON’Tever try to redirect stderr.

DON’Tusestderrfor purposes other

than printing error messages or warn-

ings.

DO DON’T

Summary ............................................................................................................

This was a long day full of important information on program input and output. You

learned how C uses streams, treating all input and output as a sequence of bytes. You also

learned that ASNI C has three predefined streams:

stdin The keyboard

stdout The screen

stderr The screen

Input from the keyboard arrives from the stream stdin. Using C’s standard library func-

tions, you can accept keyboard input character by character, a line at a time, or as for-

matted numbers and strings. Character input can be buffered or unbuffered, echoed or

unechoed.

Output to the display screen is normally done with the stdoutstream. Like input, pro-

gram output can be by character, by line, or as formatted numbers and strings. For output

to the printer, you use fprintf()to send data to the stream stdprn.

22 448201x-CH14 8/13/02 11:12 AM Page 372