What are escape characters? in C programming

What are escape characters?

Escape characters are characters designed to perform a command or task instead of being printed on the computer screen. For example, an escape character could be a character sent to a device that tells the computer screen to draw the next line in red rather than the normal white. The escape character is sent to the device that draws the red line along with the actual characters the device is supposed to draw in red. So how does the device know that the character is an escape character? Typically, the Escape key (decimal 27, octal /033) is sent just before the escape character so that the device knows that an escape character is next to arrive. After the device has the escape character, it acts on the command that the escape character represents, then resumes normal operation—taking characters and printing them on-screen. Because it usually takes two or more characters to pull off the desired command (the Escape key plus the command character itself ), this is often referred to as an escape sequence.

I know that this sounds confusing (the Escape key, followed by the escape character), but that is preciselywhy these are called escape characters. The Escape key is used to inform whoever wants to know that the next character is a command, not an ordinary character. The escape character itself (the one that comes after the Escape key) can be any character—it could even be another Escape key. The actual character that represents the desired command to occur is up to the program that is reading these characters and waiting for such commands.

An example of this is the ANSI.SYS device driver. This driver, loaded in your CONFIG.SYS file, intercepted all characters that were printed on-screen and processed these characters for escape sequences. The purpose of ANSI.SYS was to provide a way to print colored, underlined, or blinking text, or to perform higher-level commands such as clear the screen. The advantage to ANSI.SYS was that you didn’t have to know what type of monitor or display card you had—ANSI.SYS took care of that for you. All you had to do was embed the escape characters into the appropriate places in the character strings you displayed on-screen, and ANSI.SYS would take care of the rest. For example, if you printed “\033H4Hello there” ANSI.SYS would print “Hello there” on-screen in red. ANSI.SYS would see the Escape key (\033), read the command (which in this case is H4—print remaining characters in red), and print what was left (“Hello there”). 

Before ANSI.SYS, escape characters were used in the old centralized computing environments (one mainframe computer with a bunch of dumb terminals connected to it). Back in those days, the terminals had no computing power of their own and could not display graphics, and many were monochrome, unable to display color. However, each monitor did have a series of escape characters that the mainframe could send to the monitor to make it do such things as clear the screen, underline, or blink. Programmers would embed the escape characters into their character strings just as you do with ANSI.SYS, and the monitor would perform the desired command.

Today, this type of escape sequence usage has all but died out. On the other hand, many other types of character sequences could be described as escape characters that have been around for just as long, and they are still used heavily today. For example, in the section where I describe how to assign an octal or a hex value to a variable, I am using an escape character (the pair “0x” in the case of hex, and the single \ character in octal). Note that these characters do not actually use the Escape key as the “hey, here comes something special” notifier, but nonetheless they are used to denote something special about what is to immediately follow. In fact, the backslash character (\) is used quite frequently as an escape character. In C, you use \n to tell the computer to “perform a linefeed operation here.” You can use \t to perform a tab advance, and so on.

Cross Reference:

XX.23: What is octal?
XX.24: What is hexadecimal?